This is make.info, produced by makeinfo version 5.2 from make.texi. This file documents the GNU 'make' utility, which determines automatically which pieces of a large program need to be recompiled, and issues the commands to recompile them. This is Edition 0.73, last updated 5 October 2014, of 'The GNU Make Manual', for GNU 'make' version 4.1. Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled "GNU Free Documentation License." (a) The FSF's Back-Cover Text is: "You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom." INFO-DIR-SECTION Software development START-INFO-DIR-ENTRY * Make: (make). Remake files automatically. END-INFO-DIR-ENTRY  File: make.info, Node: Top, Next: Overview, Prev: (dir), Up: (dir) GNU 'make' ********** This file documents the GNU 'make' utility, which determines automatically which pieces of a large program need to be recompiled, and issues the commands to recompile them. This is Edition 0.73, last updated 5 October 2014, of 'The GNU Make Manual', for GNU 'make' version 4.1. Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled "GNU Free Documentation License." (a) The FSF's Back-Cover Text is: "You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom." * Menu: * Overview:: Overview of 'make'. * Introduction:: An introduction to 'make'. * Makefiles:: Makefiles tell 'make' what to do. * Rules:: Rules describe when a file must be remade. * Recipes:: Recipes say how to remake a file. * Using Variables:: You can use variables to avoid repetition. * Conditionals:: Use or ignore parts of the makefile based on the values of variables. * Functions:: Many powerful ways to manipulate text. * Invoking make: Running. How to invoke 'make' on the command line. * Implicit Rules:: Use implicit rules to treat many files alike, based on their file names. * Archives:: How 'make' can update library archives. * Extending make:: Using extensions to 'make'. * Features:: Features GNU 'make' has over other 'make's. * Missing:: What GNU 'make' lacks from other 'make's. * Makefile Conventions:: Conventions for writing makefiles for GNU programs. * Quick Reference:: A quick reference for experienced users. * Error Messages:: A list of common errors generated by 'make'. * Complex Makefile:: A real example of a straightforward, but nontrivial, makefile. * GNU Free Documentation License:: License for copying this manual. * Concept Index:: Index of Concepts. * Name Index:: Index of Functions, Variables, & Directives. -- The Detailed Node Listing -- Overview of 'make' * Preparing:: Preparing and running 'make'. * Reading:: On reading this text. * Bugs:: Problems and bugs. An Introduction to Makefiles * Rule Introduction:: What a rule looks like. * Simple Makefile:: A simple makefile. * How Make Works:: How 'make' processes this makefile. * Variables Simplify:: Variables make makefiles simpler. * make Deduces:: Letting 'make' deduce the recipes. * Combine By Prerequisite:: Another style of makefile. * Cleanup:: Rules for cleaning the directory. Writing Makefiles * Makefile Contents:: What makefiles contain. * Makefile Names:: How to name your makefile. * Include:: How one makefile can use another makefile. * MAKEFILES Variable:: The environment can specify extra makefiles. * Remaking Makefiles:: How makefiles get remade. * Overriding Makefiles:: How to override part of one makefile with another makefile. * Reading Makefiles:: How makefiles are parsed. * Secondary Expansion:: How and when secondary expansion is performed. What Makefiles Contain * Splitting Lines:: Splitting long lines in makefiles Writing Rules * Rule Example:: An example explained. * Rule Syntax:: General syntax explained. * Prerequisite Types:: There are two types of prerequisites. * Wildcards:: Using wildcard characters such as '*'. * Directory Search:: Searching other directories for source files. * Phony Targets:: Using a target that is not a real file's name. * Force Targets:: You can use a target without a recipe or prerequisites to mark other targets as phony. * Empty Targets:: When only the date matters and the files are empty. * Special Targets:: Targets with special built-in meanings. * Multiple Targets:: When to make use of several targets in a rule. * Multiple Rules:: How to use several rules with the same target. * Static Pattern:: Static pattern rules apply to multiple targets and can vary the prerequisites according to the target name. * Double-Colon:: How to use a special kind of rule to allow several independent rules for one target. * Automatic Prerequisites:: How to automatically generate rules giving prerequisites from source files themselves. Using Wildcard Characters in File Names * Wildcard Examples:: Several examples. * Wildcard Pitfall:: Problems to avoid. * Wildcard Function:: How to cause wildcard expansion where it does not normally take place. Searching Directories for Prerequisites * General Search:: Specifying a search path that applies to every prerequisite. * Selective Search:: Specifying a search path for a specified class of names. * Search Algorithm:: When and how search paths are applied. * Recipes/Search:: How to write recipes that work together with search paths. * Implicit/Search:: How search paths affect implicit rules. * Libraries/Search:: Directory search for link libraries. Static Pattern Rules * Static Usage:: The syntax of static pattern rules. * Static versus Implicit:: When are they better than implicit rules? Writing Recipes in Rules * Recipe Syntax:: Recipe syntax features and pitfalls. * Echoing:: How to control when recipes are echoed. * Execution:: How recipes are executed. * Parallel:: How recipes can be executed in parallel. * Errors:: What happens after a recipe execution error. * Interrupts:: What happens when a recipe is interrupted. * Recursion:: Invoking 'make' from makefiles. * Canned Recipes:: Defining canned recipes. * Empty Recipes:: Defining useful, do-nothing recipes. Recipe Syntax * Splitting Recipe Lines:: Breaking long recipe lines for readability. * Variables in Recipes:: Using 'make' variables in recipes. Recipe Execution * One Shell:: One shell for all lines in a recipe. * Choosing the Shell:: How 'make' chooses the shell used to run recipes. Parallel Execution * Parallel Output:: Handling output during parallel execution * Parallel Input:: Handling input during parallel execution Recursive Use of 'make' * MAKE Variable:: The special effects of using '$(MAKE)'. * Variables/Recursion:: How to communicate variables to a sub-'make'. * Options/Recursion:: How to communicate options to a sub-'make'. * -w Option:: How the '-w' or '--print-directory' option helps debug use of recursive 'make' commands. How to Use Variables * Reference:: How to use the value of a variable. * Flavors:: Variables come in two flavors. * Advanced:: Advanced features for referencing a variable. * Values:: All the ways variables get their values. * Setting:: How to set a variable in the makefile. * Appending:: How to append more text to the old value of a variable. * Override Directive:: How to set a variable in the makefile even if the user has set it with a command argument. * Multi-Line:: An alternate way to set a variable to a multi-line string. * Undefine Directive:: How to undefine a variable so that it appears as if it was never set. * Environment:: Variable values can come from the environment. * Target-specific:: Variable values can be defined on a per-target basis. * Pattern-specific:: Target-specific variable values can be applied to a group of targets that match a pattern. * Suppressing Inheritance:: Suppress inheritance of variables. * Special Variables:: Variables with special meaning or behavior. Advanced Features for Reference to Variables * Substitution Refs:: Referencing a variable with substitutions on the value. * Computed Names:: Computing the name of the variable to refer to. Conditional Parts of Makefiles * Conditional Example:: Example of a conditional * Conditional Syntax:: The syntax of conditionals. * Testing Flags:: Conditionals that test flags. Functions for Transforming Text * Syntax of Functions:: How to write a function call. * Text Functions:: General-purpose text manipulation functions. * File Name Functions:: Functions for manipulating file names. * Conditional Functions:: Functions that implement conditions. * Foreach Function:: Repeat some text with controlled variation. * File Function:: Write text to a file. * Call Function:: Expand a user-defined function. * Value Function:: Return the un-expanded value of a variable. * Eval Function:: Evaluate the arguments as makefile syntax. * Origin Function:: Find where a variable got its value. * Flavor Function:: Find out the flavor of a variable. * Make Control Functions:: Functions that control how make runs. * Shell Function:: Substitute the output of a shell command. * Guile Function:: Use GNU Guile embedded scripting language. How to Run 'make' * Makefile Arguments:: How to specify which makefile to use. * Goals:: How to use goal arguments to specify which parts of the makefile to use. * Instead of Execution:: How to use mode flags to specify what kind of thing to do with the recipes in the makefile other than simply execute them. * Avoiding Compilation:: How to avoid recompiling certain files. * Overriding:: How to override a variable to specify an alternate compiler and other things. * Testing:: How to proceed past some errors, to test compilation. * Options Summary:: Summary of Options Using Implicit Rules * Using Implicit:: How to use an existing implicit rule to get the recipes for updating a file. * Catalogue of Rules:: A list of built-in rules. * Implicit Variables:: How to change what predefined rules do. * Chained Rules:: How to use a chain of implicit rules. * Pattern Rules:: How to define new implicit rules. * Last Resort:: How to define a recipe for rules which cannot find any. * Suffix Rules:: The old-fashioned style of implicit rule. * Implicit Rule Search:: The precise algorithm for applying implicit rules. Defining and Redefining Pattern Rules * Pattern Intro:: An introduction to pattern rules. * Pattern Examples:: Examples of pattern rules. * Automatic Variables:: How to use automatic variables in the recipe of implicit rules. * Pattern Match:: How patterns match. * Match-Anything Rules:: Precautions you should take prior to defining rules that can match any target file whatever. * Canceling Rules:: How to override or cancel built-in rules. Using 'make' to Update Archive Files * Archive Members:: Archive members as targets. * Archive Update:: The implicit rule for archive member targets. * Archive Pitfalls:: Dangers to watch out for when using archives. * Archive Suffix Rules:: You can write a special kind of suffix rule for updating archives. Implicit Rule for Archive Member Targets * Archive Symbols:: How to update archive symbol directories. Extending GNU 'make' * Guile Integration:: Using Guile as an embedded scripting language. * Loading Objects:: Loading dynamic objects as extensions. GNU Guile Integration * Guile Types:: Converting Guile types to 'make' strings. * Guile Interface:: Invoking 'make' functions from Guile. * Guile Example:: Example using Guile in 'make'. Loading Dynamic Objects * load Directive:: Loading dynamic objects as extensions. * Remaking Loaded Objects:: How loaded objects get remade. * Loaded Object API:: Programmatic interface for loaded objects. * Loaded Object Example:: Example of a loaded object  File: make.info, Node: Overview, Next: Introduction, Prev: Top, Up: Top 1 Overview of 'make' ******************** The 'make' utility automatically determines which pieces of a large program need to be recompiled, and issues commands to recompile them. This manual describes GNU 'make', which was implemented by Richard Stallman and Roland McGrath. Development since Version 3.76 has been handled by Paul D. Smith. GNU 'make' conforms to section 6.2 of 'IEEE Standard 1003.2-1992' (POSIX.2). Our examples show C programs, since they are most common, but you can use 'make' with any programming language whose compiler can be run with a shell command. Indeed, 'make' is not limited to programs. You can use it to describe any task where some files must be updated automatically from others whenever the others change. * Menu: * Preparing:: Preparing and running 'make'. * Reading:: On reading this text. * Bugs:: Problems and bugs.  File: make.info, Node: Preparing, Next: Reading, Prev: Overview, Up: Overview Preparing and Running Make ========================== To prepare to use 'make', you must write a file called the "makefile" that describes the relationships among files in your program and provides commands for updating each file. In a program, typically, the executable file is updated from object files, which are in turn made by compiling source files. Once a suitable makefile exists, each time you change some source files, this simple shell command: make suffices to perform all necessary recompilations. The 'make' program uses the makefile data base and the last-modification times of the files to decide which of the files need to be updated. For each of those files, it issues the recipes recorded in the data base. You can provide command line arguments to 'make' to control which files should be recompiled, or how. *Note How to Run 'make': Running.  File: make.info, Node: Reading, Next: Bugs, Prev: Preparing, Up: Overview 1.1 How to Read This Manual =========================== If you are new to 'make', or are looking for a general introduction, read the first few sections of each chapter, skipping the later sections. In each chapter, the first few sections contain introductory or general information and the later sections contain specialized or technical information. The exception is the second chapter, *note An Introduction to Makefiles: Introduction, all of which is introductory. If you are familiar with other 'make' programs, see *note Features of GNU 'make': Features, which lists the enhancements GNU 'make' has, and *note Incompatibilities and Missing Features: Missing, which explains the few things GNU 'make' lacks that others have. For a quick summary, see *note Options Summary::, *note Quick Reference::, and *note Special Targets::.  File: make.info, Node: Bugs, Prev: Reading, Up: Overview 1.2 Problems and Bugs ===================== If you have problems with GNU 'make' or think you've found a bug, please report it to the developers; we cannot promise to do anything but we might well want to fix it. Before reporting a bug, make sure you've actually found a real bug. Carefully reread the documentation and see if it really says you can do what you're trying to do. If it's not clear whether you should be able to do something or not, report that too; it's a bug in the documentation! Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest possible makefile that reproduces the problem. Then send us the makefile and the exact results 'make' gave you, including any error or warning messages. Please don't paraphrase these messages: it's best to cut and paste them into your report. When generating this small makefile, be sure to not use any non-free or unusual tools in your recipes: you can almost always emulate what such a tool would do with simple shell commands. Finally, be sure to explain what you expected to occur; this will help us decide whether the problem was really in the documentation. Once you have a precise problem you can report it in one of two ways. Either send electronic mail to: bug-make@gnu.org or use our Web-based project management tool, at: http://savannah.gnu.org/projects/make/ In addition to the information above, please be careful to include the version number of 'make' you are using. You can get this information with the command 'make --version'. Be sure also to include the type of machine and operating system you are using. One way to obtain this information is by looking at the final lines of output from the command 'make --help'.  File: make.info, Node: Introduction, Next: Makefiles, Prev: Overview, Up: Top 2 An Introduction to Makefiles ****************************** You need a file called a "makefile" to tell 'make' what to do. Most often, the makefile tells 'make' how to compile and link a program. In this chapter, we will discuss a simple makefile that describes how to compile and link a text editor which consists of eight C source files and three header files. The makefile can also tell 'make' how to run miscellaneous commands when explicitly asked (for example, to remove certain files as a clean-up operation). To see a more complex example of a makefile, see *note Complex Makefile::. When 'make' recompiles the editor, each changed C source file must be recompiled. If a header file has changed, each C source file that includes the header file must be recompiled to be safe. Each compilation produces an object file corresponding to the source file. Finally, if any source file has been recompiled, all the object files, whether newly made or saved from previous compilations, must be linked together to produce the new executable editor. * Menu: * Rule Introduction:: What a rule looks like. * Simple Makefile:: A simple makefile. * How Make Works:: How 'make' processes this makefile. * Variables Simplify:: Variables make makefiles simpler. * make Deduces:: Letting 'make' deduce the recipes. * Combine By Prerequisite:: Another style of makefile. * Cleanup:: Rules for cleaning the directory.  File: make.info, Node: Rule Introduction, Next: Simple Makefile, Prev: Introduction, Up: Introduction 2.1 What a Rule Looks Like ========================== A simple makefile consists of "rules" with the following shape: TARGET ... : PREREQUISITES ... RECIPE ... ... A "target" is usually the name of a file that is generated by a program; examples of targets are executable or object files. A target can also be the name of an action to carry out, such as 'clean' (*note Phony Targets::). A "prerequisite" is a file that is used as input to create the target. A target often depends on several files. A "recipe" is an action that 'make' carries out. A recipe may have more than one command, either on the same line or each on its own line. *Please note:* you need to put a tab character at the beginning of every recipe line! This is an obscurity that catches the unwary. If you prefer to prefix your recipes with a character other than tab, you can set the '.RECIPEPREFIX' variable to an alternate character (*note Special Variables::). Usually a recipe is in a rule with prerequisites and serves to create a target file if any of the prerequisites change. However, the rule that specifies a recipe for the target need not have prerequisites. For example, the rule containing the delete command associated with the target 'clean' does not have prerequisites. A "rule", then, explains how and when to remake certain files which are the targets of the particular rule. 'make' carries out the recipe on the prerequisites to create or update the target. A rule can also explain how and when to carry out an action. *Note Writing Rules: Rules. A makefile may contain other text besides rules, but a simple makefile need only contain rules. Rules may look somewhat more complicated than shown in this template, but all fit the pattern more or less.  File: make.info, Node: Simple Makefile, Next: How Make Works, Prev: Rule Introduction, Up: Introduction 2.2 A Simple Makefile ===================== Here is a straightforward makefile that describes the way an executable file called 'edit' depends on eight object files which, in turn, depend on eight C source and three header files. In this example, all the C files include 'defs.h', but only those defining editing commands include 'command.h', and only low level files that change the editor buffer include 'buffer.h'. edit : main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o cc -o edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o main.o : main.c defs.h cc -c main.c kbd.o : kbd.c defs.h command.h cc -c kbd.c command.o : command.c defs.h command.h cc -c command.c display.o : display.c defs.h buffer.h cc -c display.c insert.o : insert.c defs.h buffer.h cc -c insert.c search.o : search.c defs.h buffer.h cc -c search.c files.o : files.c defs.h buffer.h command.h cc -c files.c utils.o : utils.c defs.h cc -c utils.c clean : rm edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o We split each long line into two lines using backslash/newline; this is like using one long line, but is easier to read. *Note Splitting Long Lines: Splitting Lines. To use this makefile to create the executable file called 'edit', type: make To use this makefile to delete the executable file and all the object files from the directory, type: make clean In the example makefile, the targets include the executable file 'edit', and the object files 'main.o' and 'kbd.o'. The prerequisites are files such as 'main.c' and 'defs.h'. In fact, each '.o' file is both a target and a prerequisite. Recipes include 'cc -c main.c' and 'cc -c kbd.c'. When a target is a file, it needs to be recompiled or relinked if any of its prerequisites change. In addition, any prerequisites that are themselves automatically generated should be updated first. In this example, 'edit' depends on each of the eight object files; the object file 'main.o' depends on the source file 'main.c' and on the header file 'defs.h'. A recipe may follow each line that contains a target and prerequisites. These recipes say how to update the target file. A tab character (or whatever character is specified by the '.RECIPEPREFIX' variable; *note Special Variables::) must come at the beginning of every line in the recipe to distinguish recipes from other lines in the makefile. (Bear in mind that 'make' does not know anything about how the recipes work. It is up to you to supply recipes that will update the target file properly. All 'make' does is execute the recipe you have specified when the target file needs to be updated.) The target 'clean' is not a file, but merely the name of an action. Since you normally do not want to carry out the actions in this rule, 'clean' is not a prerequisite of any other rule. Consequently, 'make' never does anything with it unless you tell it specifically. Note that this rule not only is not a prerequisite, it also does not have any prerequisites, so the only purpose of the rule is to run the specified recipe. Targets that do not refer to files but are just actions are called "phony targets". *Note Phony Targets::, for information about this kind of target. *Note Errors in Recipes: Errors, to see how to cause 'make' to ignore errors from 'rm' or any other command.  File: make.info, Node: How Make Works, Next: Variables Simplify, Prev: Simple Makefile, Up: Introduction 2.3 How 'make' Processes a Makefile =================================== By default, 'make' starts with the first target (not targets whose names start with '.'). This is called the "default goal". ("Goals" are the targets that 'make' strives ultimately to update. You can override this behavior using the command line (*note Arguments to Specify the Goals: Goals.) or with the '.DEFAULT_GOAL' special variable (*note Other Special Variables: Special Variables.). In the simple example of the previous section, the default goal is to update the executable program 'edit'; therefore, we put that rule first. Thus, when you give the command: make 'make' reads the makefile in the current directory and begins by processing the first rule. In the example, this rule is for relinking 'edit'; but before 'make' can fully process this rule, it must process the rules for the files that 'edit' depends on, which in this case are the object files. Each of these files is processed according to its own rule. These rules say to update each '.o' file by compiling its source file. The recompilation must be done if the source file, or any of the header files named as prerequisites, is more recent than the object file, or if the object file does not exist. The other rules are processed because their targets appear as prerequisites of the goal. If some other rule is not depended on by the goal (or anything it depends on, etc.), that rule is not processed, unless you tell 'make' to do so (with a command such as 'make clean'). Before recompiling an object file, 'make' considers updating its prerequisites, the source file and header files. This makefile does not specify anything to be done for them--the '.c' and '.h' files are not the targets of any rules--so 'make' does nothing for these files. But 'make' would update automatically generated C programs, such as those made by Bison or Yacc, by their own rules at this time. After recompiling whichever object files need it, 'make' decides whether to relink 'edit'. This must be done if the file 'edit' does not exist, or if any of the object files are newer than it. If an object file was just recompiled, it is now newer than 'edit', so 'edit' is relinked. Thus, if we change the file 'insert.c' and run 'make', 'make' will compile that file to update 'insert.o', and then link 'edit'. If we change the file 'command.h' and run 'make', 'make' will recompile the object files 'kbd.o', 'command.o' and 'files.o' and then link the file 'edit'.  File: make.info, Node: Variables Simplify, Next: make Deduces, Prev: How Make Works, Up: Introduction 2.4 Variables Make Makefiles Simpler ==================================== In our example, we had to list all the object files twice in the rule for 'edit' (repeated here): edit : main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o cc -o edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o Such duplication is error-prone; if a new object file is added to the system, we might add it to one list and forget the other. We can eliminate the risk and simplify the makefile by using a variable. "Variables" allow a text string to be defined once and substituted in multiple places later (*note How to Use Variables: Using Variables.). It is standard practice for every makefile to have a variable named 'objects', 'OBJECTS', 'objs', 'OBJS', 'obj', or 'OBJ' which is a list of all object file names. We would define such a variable 'objects' with a line like this in the makefile: objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o Then, each place we want to put a list of the object file names, we can substitute the variable's value by writing '$(objects)' (*note How to Use Variables: Using Variables.). Here is how the complete simple makefile looks when you use a variable for the object files: objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) main.o : main.c defs.h cc -c main.c kbd.o : kbd.c defs.h command.h cc -c kbd.c command.o : command.c defs.h command.h cc -c command.c display.o : display.c defs.h buffer.h cc -c display.c insert.o : insert.c defs.h buffer.h cc -c insert.c search.o : search.c defs.h buffer.h cc -c search.c files.o : files.c defs.h buffer.h command.h cc -c files.c utils.o : utils.c defs.h cc -c utils.c clean : rm edit $(objects)  File: make.info, Node: make Deduces, Next: Combine By Prerequisite, Prev: Variables Simplify, Up: Introduction 2.5 Letting 'make' Deduce the Recipes ===================================== It is not necessary to spell out the recipes for compiling the individual C source files, because 'make' can figure them out: it has an "implicit rule" for updating a '.o' file from a correspondingly named '.c' file using a 'cc -c' command. For example, it will use the recipe 'cc -c main.c -o main.o' to compile 'main.c' into 'main.o'. We can therefore omit the recipes from the rules for the object files. *Note Using Implicit Rules: Implicit Rules. When a '.c' file is used automatically in this way, it is also automatically added to the list of prerequisites. We can therefore omit the '.c' files from the prerequisites, provided we omit the recipe. Here is the entire example, with both of these changes, and a variable 'objects' as suggested above: objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) main.o : defs.h kbd.o : defs.h command.h command.o : defs.h command.h display.o : defs.h buffer.h insert.o : defs.h buffer.h search.o : defs.h buffer.h files.o : defs.h buffer.h command.h utils.o : defs.h .PHONY : clean clean : rm edit $(objects) This is how we would write the makefile in actual practice. (The complications associated with 'clean' are described elsewhere. See *note Phony Targets::, and *note Errors in Recipes: Errors.) Because implicit rules are so convenient, they are important. You will see them used frequently.  File: make.info, Node: Combine By Prerequisite, Next: Cleanup, Prev: make Deduces, Up: Introduction 2.6 Another Style of Makefile ============================= When the objects of a makefile are created only by implicit rules, an alternative style of makefile is possible. In this style of makefile, you group entries by their prerequisites instead of by their targets. Here is what one looks like: objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) $(objects) : defs.h kbd.o command.o files.o : command.h display.o insert.o search.o files.o : buffer.h Here 'defs.h' is given as a prerequisite of all the object files; 'command.h' and 'buffer.h' are prerequisites of the specific object files listed for them. Whether this is better is a matter of taste: it is more compact, but some people dislike it because they find it clearer to put all the information about each target in one place.  File: make.info, Node: Cleanup, Prev: Combine By Prerequisite, Up: Introduction 2.7 Rules for Cleaning the Directory ==================================== Compiling a program is not the only thing you might want to write rules for. Makefiles commonly tell how to do a few other things besides compiling a program: for example, how to delete all the object files and executables so that the directory is 'clean'. Here is how we could write a 'make' rule for cleaning our example editor: clean: rm edit $(objects) In practice, we might want to write the rule in a somewhat more complicated manner to handle unanticipated situations. We would do this: .PHONY : clean clean : -rm edit $(objects) This prevents 'make' from getting confused by an actual file called 'clean' and causes it to continue in spite of errors from 'rm'. (See *note Phony Targets::, and *note Errors in Recipes: Errors.) A rule such as this should not be placed at the beginning of the makefile, because we do not want it to run by default! Thus, in the example makefile, we want the rule for 'edit', which recompiles the editor, to remain the default goal. Since 'clean' is not a prerequisite of 'edit', this rule will not run at all if we give the command 'make' with no arguments. In order to make the rule run, we have to type 'make clean'. *Note How to Run 'make': Running.  File: make.info, Node: Makefiles, Next: Rules, Prev: Introduction, Up: Top 3 Writing Makefiles ******************* The information that tells 'make' how to recompile a system comes from reading a data base called the "makefile". * Menu: * Makefile Contents:: What makefiles contain. * Makefile Names:: How to name your makefile. * Include:: How one makefile can use another makefile. * MAKEFILES Variable:: The environment can specify extra makefiles. * Remaking Makefiles:: How makefiles get remade. * Overriding Makefiles:: How to override part of one makefile with another makefile. * Reading Makefiles:: How makefiles are parsed. * Secondary Expansion:: How and when secondary expansion is performed.  File: make.info, Node: Makefile Contents, Next: Makefile Names, Prev: Makefiles, Up: Makefiles 3.1 What Makefiles Contain ========================== Makefiles contain five kinds of things: "explicit rules", "implicit rules", "variable definitions", "directives", and "comments". Rules, variables, and directives are described at length in later chapters. * An "explicit rule" says when and how to remake one or more files, called the rule's "targets". It lists the other files that the targets depend on, called the "prerequisites" of the target, and may also give a recipe to use to create or update the targets. *Note Writing Rules: Rules. * An "implicit rule" says when and how to remake a class of files based on their names. It describes how a target may depend on a file with a name similar to the target and gives a recipe to create or update such a target. *Note Using Implicit Rules: Implicit Rules. * A "variable definition" is a line that specifies a text string value for a variable that can be substituted into the text later. The simple makefile example shows a variable definition for 'objects' as a list of all object files (*note Variables Make Makefiles Simpler: Variables Simplify.). * A "directive" is an instruction for 'make' to do something special while reading the makefile. These include: * Reading another makefile (*note Including Other Makefiles: Include.). * Deciding (based on the values of variables) whether to use or ignore a part of the makefile (*note Conditional Parts of Makefiles: Conditionals.). * Defining a variable from a verbatim string containing multiple lines (*note Defining Multi-Line Variables: Multi-Line.). * '#' in a line of a makefile starts a "comment". It and the rest of the line are ignored, except that a trailing backslash not escaped by another backslash will continue the comment across multiple lines. A line containing just a comment (with perhaps spaces before it) is effectively blank, and is ignored. If you want a literal '#', escape it with a backslash (e.g., '\#'). Comments may appear on any line in the makefile, although they are treated specially in certain situations. You cannot use comments within variable references or function calls: any instance of '#' will be treated literally (rather than as the start of a comment) inside a variable reference or function call. Comments within a recipe are passed to the shell, just as with any other recipe text. The shell decides how to interpret it: whether or not this is a comment is up to the shell. Within a 'define' directive, comments are not ignored during the definition of the variable, but rather kept intact in the value of the variable. When the variable is expanded they will either be treated as 'make' comments or as recipe text, depending on the context in which the variable is evaluated. * Menu: * Splitting Lines:: Splitting long lines in makefiles  File: make.info, Node: Splitting Lines, Prev: Makefile Contents, Up: Makefile Contents 3.1.1 Splitting Long Lines -------------------------- Makefiles use a "line-based" syntax in which the newline character is special and marks the end of a statement. GNU 'make' has no limit on the length of a statement line, up to the amount of memory in your computer. However, it is difficult to read lines which are too long to display without wrapping or scrolling. So, you can format your makefiles for readability by adding newlines into the middle of a statement: you do this by escaping the internal newlines with a backslash ('\') character. Where we need to make a distinction we will refer to "physical lines" as a single line ending with a newline (regardless of whether it is escaped) and a "logical line" being a complete statement including all escaped newlines up to the first non-escaped newline. The way in which backslash/newline combinations are handled depends on whether the statement is a recipe line or a non-recipe line. Handling of backslash/newline in a recipe line is discussed later (*note Splitting Recipe Lines::). Outside of recipe lines, backslash/newlines are converted into a single space character. Once that is done, all whitespace around the backslash/newline is condensed into a single space: this includes all whitespace preceding the backslash, all whitespace at the beginning of the line after the backslash/newline, and any consecutive backslash/newline combinations. If the '.POSIX' special target is defined then backslash/newline handling is modified slightly to conform to POSIX.2: first, whitespace preceding a backslash is not removed and second, consecutive backslash/newlines are not condensed.  File: make.info, Node: Makefile Names, Next: Include, Prev: Makefile Contents, Up: Makefiles 3.2 What Name to Give Your Makefile =================================== By default, when 'make' looks for the makefile, it tries the following names, in order: 'GNUmakefile', 'makefile' and 'Makefile'. Normally you should call your makefile either 'makefile' or 'Makefile'. (We recommend 'Makefile' because it appears prominently near the beginning of a directory listing, right near other important files such as 'README'.) The first name checked, 'GNUmakefile', is not recommended for most makefiles. You should use this name if you have a makefile that is specific to GNU 'make', and will not be understood by other versions of 'make'. Other 'make' programs look for 'makefile' and 'Makefile', but not 'GNUmakefile'. If 'make' finds none of these names, it does not use any makefile. Then you must specify a goal with a command argument, and 'make' will attempt to figure out how to remake it using only its built-in implicit rules. *Note Using Implicit Rules: Implicit Rules. If you want to use a nonstandard name for your makefile, you can specify the makefile name with the '-f' or '--file' option. The arguments '-f NAME' or '--file=NAME' tell 'make' to read the file NAME as the makefile. If you use more than one '-f' or '--file' option, you can specify several makefiles. All the makefiles are effectively concatenated in the order specified. The default makefile names 'GNUmakefile', 'makefile' and 'Makefile' are not checked automatically if you specify '-f' or '--file'.  File: make.info, Node: Include, Next: MAKEFILES Variable, Prev: Makefile Names, Up: Makefiles 3.3 Including Other Makefiles ============================= The 'include' directive tells 'make' to suspend reading the current makefile and read one or more other makefiles before continuing. The directive is a line in the makefile that looks like this: include FILENAMES... FILENAMES can contain shell file name patterns. If FILENAMES is empty, nothing is included and no error is printed. Extra spaces are allowed and ignored at the beginning of the line, but the first character must not be a tab (or the value of '.RECIPEPREFIX')--if the line begins with a tab, it will be considered a recipe line. Whitespace is required between 'include' and the file names, and between file names; extra whitespace is ignored there and at the end of the directive. A comment starting with '#' is allowed at the end of the line. If the file names contain any variable or function references, they are expanded. *Note How to Use Variables: Using Variables. For example, if you have three '.mk' files, 'a.mk', 'b.mk', and 'c.mk', and '$(bar)' expands to 'bish bash', then the following expression include foo *.mk $(bar) is equivalent to include foo a.mk b.mk c.mk bish bash When 'make' processes an 'include' directive, it suspends reading of the containing makefile and reads from each listed file in turn. When that is finished, 'make' resumes reading the makefile in which the directive appears. One occasion for using 'include' directives is when several programs, handled by individual makefiles in various directories, need to use a common set of variable definitions (*note Setting Variables: Setting.) or pattern rules (*note Defining and Redefining Pattern Rules: Pattern Rules.). Another such occasion is when you want to generate prerequisites from source files automatically; the prerequisites can be put in a file that is included by the main makefile. This practice is generally cleaner than that of somehow appending the prerequisites to the end of the main makefile as has been traditionally done with other versions of 'make'. *Note Automatic Prerequisites::. If the specified name does not start with a slash, and the file is not found in the current directory, several other directories are searched. First, any directories you have specified with the '-I' or '--include-dir' option are searched (*note Summary of Options: Options Summary.). Then the following directories (if they exist) are searched, in this order: 'PREFIX/include' (normally '/usr/local/include' (1)) '/usr/gnu/include', '/usr/local/include', '/usr/include'. If an included makefile cannot be found in any of these directories, a warning message is generated, but it is not an immediately fatal error; processing of the makefile containing the 'include' continues. Once it has finished reading makefiles, 'make' will try to remake any that are out of date or don't exist. *Note How Makefiles Are Remade: Remaking Makefiles. Only after it has tried to find a way to remake a makefile and failed, will 'make' diagnose the missing makefile as a fatal error. If you want 'make' to simply ignore a makefile which does not exist or cannot be remade, with no error message, use the '-include' directive instead of 'include', like this: -include FILENAMES... This acts like 'include' in every way except that there is no error (not even a warning) if any of the FILENAMES (or any prerequisites of any of the FILENAMES) do not exist or cannot be remade. For compatibility with some other 'make' implementations, 'sinclude' is another name for '-include'. ---------- Footnotes ---------- (1) GNU Make compiled for MS-DOS and MS-Windows behaves as if PREFIX has been defined to be the root of the DJGPP tree hierarchy.  File: make.info, Node: MAKEFILES Variable, Next: Remaking Makefiles, Prev: Include, Up: Makefiles 3.4 The Variable 'MAKEFILES' ============================ If the environment variable 'MAKEFILES' is defined, 'make' considers its value as a list of names (separated by whitespace) of additional makefiles to be read before the others. This works much like the 'include' directive: various directories are searched for those files (*note Including Other Makefiles: Include.). In addition, the default goal is never taken from one of these makefiles (or any makefile included by them) and it is not an error if the files listed in 'MAKEFILES' are not found. The main use of 'MAKEFILES' is in communication between recursive invocations of 'make' (*note Recursive Use of 'make': Recursion.). It usually is not desirable to set the environment variable before a top-level invocation of 'make', because it is usually better not to mess with a makefile from outside. However, if you are running 'make' without a specific makefile, a makefile in 'MAKEFILES' can do useful things to help the built-in implicit rules work better, such as defining search paths (*note Directory Search::). Some users are tempted to set 'MAKEFILES' in the environment automatically on login, and program makefiles to expect this to be done. This is a very bad idea, because such makefiles will fail to work if run by anyone else. It is much better to write explicit 'include' directives in the makefiles. *Note Including Other Makefiles: Include.  File: make.info, Node: Remaking Makefiles, Next: Overriding Makefiles, Prev: MAKEFILES Variable, Up: Makefiles 3.5 How Makefiles Are Remade ============================ Sometimes makefiles can be remade from other files, such as RCS or SCCS files. If a makefile can be remade from other files, you probably want 'make' to get an up-to-date version of the makefile to read in. To this end, after reading in all makefiles, 'make' will consider each as a goal target and attempt to update it. If a makefile has a rule which says how to update it (found either in that very makefile or in another one) or if an implicit rule applies to it (*note Using Implicit Rules: Implicit Rules.), it will be updated if necessary. After all makefiles have been checked, if any have actually been changed, 'make' starts with a clean slate and reads all the makefiles over again. (It will also attempt to update each of them over again, but normally this will not change them again, since they are already up to date.) If you know that one or more of your makefiles cannot be remade and you want to keep 'make' from performing an implicit rule search on them, perhaps for efficiency reasons, you can use any normal method of preventing implicit rule look-up to do so. For example, you can write an explicit rule with the makefile as the target, and an empty recipe (*note Using Empty Recipes: Empty Recipes.). If the makefiles specify a double-colon rule to remake a file with a recipe but no prerequisites, that file will always be remade (*note Double-Colon::). In the case of makefiles, a makefile that has a double-colon rule with a recipe but no prerequisites will be remade every time 'make' is run, and then again after 'make' starts over and reads the makefiles in again. This would cause an infinite loop: 'make' would constantly remake the makefile, and never do anything else. So, to avoid this, 'make' will *not* attempt to remake makefiles which are specified as targets of a double-colon rule with a recipe but no prerequisites. If you do not specify any makefiles to be read with '-f' or '--file' options, 'make' will try the default makefile names; *note What Name to Give Your Makefile: Makefile Names. Unlike makefiles explicitly requested with '-f' or '--file' options, 'make' is not certain that these makefiles should exist. However, if a default makefile does not exist but can be created by running 'make' rules, you probably want the rules to be run so that the makefile can be used. Therefore, if none of the default makefiles exists, 'make' will try to make each of them in the same order in which they are searched for (*note What Name to Give Your Makefile: Makefile Names.) until it succeeds in making one, or it runs out of names to try. Note that it is not an error if 'make' cannot find or make any makefile; a makefile is not always necessary. When you use the '-t' or '--touch' option (*note Instead of Executing Recipes: Instead of Execution.), you would not want to use an out-of-date makefile to decide which targets to touch. So the '-t' option has no effect on updating makefiles; they are really updated even if '-t' is specified. Likewise, '-q' (or '--question') and '-n' (or '--just-print') do not prevent updating of makefiles, because an out-of-date makefile would result in the wrong output for other targets. Thus, 'make -f mfile -n foo' will update 'mfile', read it in, and then print the recipe to update 'foo' and its prerequisites without running it. The recipe printed for 'foo' will be the one specified in the updated contents of 'mfile'. However, on occasion you might actually wish to prevent updating of even the makefiles. You can do this by specifying the makefiles as goals in the command line as well as specifying them as makefiles. When the makefile name is specified explicitly as a goal, the options '-t' and so on do apply to them. Thus, 'make -f mfile -n mfile foo' would read the makefile 'mfile', print the recipe needed to update it without actually running it, and then print the recipe needed to update 'foo' without running that. The recipe for 'foo' will be the one specified by the existing contents of 'mfile'.  File: make.info, Node: Overriding Makefiles, Next: Reading Makefiles, Prev: Remaking Makefiles, Up: Makefiles 3.6 Overriding Part of Another Makefile ======================================= Sometimes it is useful to have a makefile that is mostly just like another makefile. You can often use the 'include' directive to include one in the other, and add more targets or variable definitions. However, it is invalid for two makefiles to give different recipes for the same target. But there is another way. In the containing makefile (the one that wants to include the other), you can use a match-anything pattern rule to say that to remake any target that cannot be made from the information in the containing makefile, 'make' should look in another makefile. *Note Pattern Rules::, for more information on pattern rules. For example, if you have a makefile called 'Makefile' that says how to make the target 'foo' (and other targets), you can write a makefile called 'GNUmakefile' that contains: foo: frobnicate > foo %: force @$(MAKE) -f Makefile $@ force: ; If you say 'make foo', 'make' will find 'GNUmakefile', read it, and see that to make 'foo', it needs to run the recipe 'frobnicate > foo'. If you say 'make bar', 'make' will find no way to make 'bar' in 'GNUmakefile', so it will use the recipe from the pattern rule: 'make -f Makefile bar'. If 'Makefile' provides a rule for updating 'bar', 'make' will apply the rule. And likewise for any other target that 'GNUmakefile' does not say how to make. The way this works is that the pattern rule has a pattern of just '%', so it matches any target whatever. The rule specifies a prerequisite 'force', to guarantee that the recipe will be run even if the target file already exists. We give the 'force' target an empty recipe to prevent 'make' from searching for an implicit rule to build it--otherwise it would apply the same match-anything rule to 'force' itself and create a prerequisite loop!  File: make.info, Node: Reading Makefiles, Next: Secondary Expansion, Prev: Overriding Makefiles, Up: Makefiles 3.7 How 'make' Reads a Makefile =============================== GNU 'make' does its work in two distinct phases. During the first phase it reads all the makefiles, included makefiles, etc. and internalizes all the variables and their values, implicit and explicit rules, and constructs a dependency graph of all the targets and their prerequisites. During the second phase, 'make' uses these internal structures to determine what targets will need to be rebuilt and to invoke the rules necessary to do so. It's important to understand this two-phase approach because it has a direct impact on how variable and function expansion happens; this is often a source of some confusion when writing makefiles. Here we will present a summary of the phases in which expansion happens for different constructs within the makefile. We say that expansion is "immediate" if it happens during the first phase: in this case 'make' will expand any variables or functions in that section of a construct as the makefile is parsed. We say that expansion is "deferred" if expansion is not performed immediately. Expansion of a deferred construct is not performed until either the construct appears later in an immediate context, or until the second phase. You may not be familiar with some of these constructs yet. You can reference this section as you become familiar with them, in later chapters. Variable Assignment ------------------- Variable definitions are parsed as follows: IMMEDIATE = DEFERRED IMMEDIATE ?= DEFERRED IMMEDIATE := IMMEDIATE IMMEDIATE ::= IMMEDIATE IMMEDIATE += DEFERRED or IMMEDIATE IMMEDIATE != IMMEDIATE define IMMEDIATE DEFERRED endef define IMMEDIATE = DEFERRED endef define IMMEDIATE ?= DEFERRED endef define IMMEDIATE := IMMEDIATE endef define IMMEDIATE ::= IMMEDIATE endef define IMMEDIATE += DEFERRED or IMMEDIATE endef define IMMEDIATE != IMMEDIATE endef For the append operator, '+=', the right-hand side is considered immediate if the variable was previously set as a simple variable (':=' or '::='), and deferred otherwise. For the shell assignment operator, '!=', the right-hand side is evaluated immediately and handed to the shell. The result is stored in the variable named on the left, and that variable becomes a simple variable (and will thus be re-evaluated on each reference). Conditional Directives ---------------------- Conditional directives are parsed immediately. This means, for example, that automatic variables cannot be used in conditional directives, as automatic variables are not set until the recipe for that rule is invoked. If you need to use automatic variables in a conditional directive you _must_ move the condition into the recipe and use shell conditional syntax instead. Rule Definition --------------- A rule is always expanded the same way, regardless of the form: IMMEDIATE : IMMEDIATE ; DEFERRED DEFERRED That is, the target and prerequisite sections are expanded immediately, and the recipe used to construct the target is always deferred. This general rule is true for explicit rules, pattern rules, suffix rules, static pattern rules, and simple prerequisite definitions.  File: make.info, Node: Secondary Expansion, Prev: Reading Makefiles, Up: Makefiles 3.8 Secondary Expansion ======================= In the previous section we learned that GNU 'make' works in two distinct phases: a read-in phase and a target-update phase (*note How 'make' Reads a Makefile: Reading Makefiles.). GNU make also has the ability to enable a _second expansion_ of the prerequisites (only) for some or all targets defined in the makefile. In order for this second expansion to occur, the special target '.SECONDEXPANSION' must be defined before the first prerequisite list that makes use of this feature. If that special target is defined then in between the two phases mentioned above, right at the end of the read-in phase, all the prerequisites of the targets defined after the special target are expanded a _second time_. In most circumstances this secondary expansion will have no effect, since all variable and function references will have been expanded during the initial parsing of the makefiles. In order to take advantage of the secondary expansion phase of the parser, then, it's necessary to _escape_ the variable or function reference in the makefile. In this case the first expansion merely un-escapes the reference but doesn't expand it, and expansion is left to the secondary expansion phase. For example, consider this makefile: .SECONDEXPANSION: ONEVAR = onefile TWOVAR = twofile myfile: $(ONEVAR) $$(TWOVAR) After the first expansion phase the prerequisites list of the 'myfile' target will be 'onefile' and '$(TWOVAR)'; the first (unescaped) variable reference to ONEVAR is expanded, while the second (escaped) variable reference is simply unescaped, without being recognized as a variable reference. Now during the secondary expansion the first word is expanded again but since it contains no variable or function references it remains the value 'onefile', while the second word is now a normal reference to the variable TWOVAR, which is expanded to the value 'twofile'. The final result is that there are two prerequisites, 'onefile' and 'twofile'. Obviously, this is not a very interesting case since the same result could more easily have been achieved simply by having both variables appear, unescaped, in the prerequisites list. One difference becomes apparent if the variables are reset; consider this example: .SECONDEXPANSION: AVAR = top onefile: $(AVAR) twofile: $$(AVAR) AVAR = bottom Here the prerequisite of 'onefile' will be expanded immediately, and resolve to the value 'top', while the prerequisite of 'twofile' will not be full expanded until the secondary expansion and yield a value of 'bottom'. This is marginally more exciting, but the true power of this feature only becomes apparent when you discover that secondary expansions always take place within the scope of the automatic variables for that target. This means that you can use variables such as '$@', '$*', etc. during the second expansion and they will have their expected values, just as in the recipe. All you have to do is defer the expansion by escaping the '$'. Also, secondary expansion occurs for both explicit and implicit (pattern) rules. Knowing this, the possible uses for this feature increase dramatically. For example: .SECONDEXPANSION: main_OBJS := main.o try.o test.o lib_OBJS := lib.o api.o main lib: $$($$@_OBJS) Here, after the initial expansion the prerequisites of both the 'main' and 'lib' targets will be '$($@_OBJS)'. During the secondary expansion, the '$@' variable is set to the name of the target and so the expansion for the 'main' target will yield '$(main_OBJS)', or 'main.o try.o test.o', while the secondary expansion for the 'lib' target will yield '$(lib_OBJS)', or 'lib.o api.o'. You can also mix in functions here, as long as they are properly escaped: main_SRCS := main.c try.c test.c lib_SRCS := lib.c api.c .SECONDEXPANSION: main lib: $$(patsubst %.c,%.o,$$($$@_SRCS)) This version allows users to specify source files rather than object files, but gives the same resulting prerequisites list as the previous example. Evaluation of automatic variables during the secondary expansion phase, especially of the target name variable '$$@', behaves similarly to evaluation within recipes. However, there are some subtle differences and "corner cases" which come into play for the different types of rule definitions that 'make' understands. The subtleties of using the different automatic variables are described below. Secondary Expansion of Explicit Rules ------------------------------------- During the secondary expansion of explicit rules, '$$@' and '$$%' evaluate, respectively, to the file name of the target and, when the target is an archive member, the target member name. The '$$<' variable evaluates to the first prerequisite in the first rule for this target. '$$^' and '$$+' evaluate to the list of all prerequisites of rules _that have already appeared_ for the same target ('$$+' with repetitions and '$$^' without). The following example will help illustrate these behaviors: .SECONDEXPANSION: foo: foo.1 bar.1 $$< $$^ $$+ # line #1 foo: foo.2 bar.2 $$< $$^ $$+ # line #2 foo: foo.3 bar.3 $$< $$^ $$+ # line #3 In the first prerequisite list, all three variables ('$$<', '$$^', and '$$+') expand to the empty string. In the second, they will have values 'foo.1', 'foo.1 bar.1', and 'foo.1 bar.1' respectively. In the third they will have values 'foo.1', 'foo.1 bar.1 foo.2 bar.2', and 'foo.1 bar.1 foo.2 bar.2 foo.1 foo.1 bar.1 foo.1 bar.1' respectively. Rules undergo secondary expansion in makefile order, except that the rule with the recipe is always evaluated last. The variables '$$?' and '$$*' are not available and expand to the empty string. Secondary Expansion of Static Pattern Rules ------------------------------------------- Rules for secondary expansion of static pattern rules are identical to those for explicit rules, above, with one exception: for static pattern rules the '$$*' variable is set to the pattern stem. As with explicit rules, '$$?' is not available and expands to the empty string. Secondary Expansion of Implicit Rules ------------------------------------- As 'make' searches for an implicit rule, it substitutes the stem and then performs secondary expansion for every rule with a matching target pattern. The value of the automatic variables is derived in the same fashion as for static pattern rules. As an example: .SECONDEXPANSION: foo: bar foo foz: fo%: bo% %oo: $$< $$^ $$+ $$* When the implicit rule is tried for target 'foo', '$$<' expands to 'bar', '$$^' expands to 'bar boo', '$$+' also expands to 'bar boo', and '$$*' expands to 'f'. Note that the directory prefix (D), as described in *note Implicit Rule Search Algorithm: Implicit Rule Search, is appended (after expansion) to all the patterns in the prerequisites list. As an example: .SECONDEXPANSION: /tmp/foo.o: %.o: $$(addsuffix /%.c,foo bar) foo.h @echo $^ The prerequisite list printed, after the secondary expansion and directory prefix reconstruction, will be '/tmp/foo/foo.c /tmp/bar/foo.c foo.h'. If you are not interested in this reconstruction, you can use '$$*' instead of '%' in the prerequisites list.  File: make.info, Node: Rules, Next: Recipes, Prev: Makefiles, Up: Top 4 Writing Rules *************** A "rule" appears in the makefile and says when and how to remake certain files, called the rule's "targets" (most often only one per rule). It lists the other files that are the "prerequisites" of the target, and the "recipe" to use to create or update the target. The order of rules is not significant, except for determining the "default goal": the target for 'make' to consider, if you do not otherwise specify one. The default goal is the target of the first rule in the first makefile. If the first rule has multiple targets, only the first target is taken as the default. There are two exceptions: a target starting with a period is not a default unless it contains one or more slashes, '/', as well; and, a target that defines a pattern rule has no effect on the default goal. (*Note Defining and Redefining Pattern Rules: Pattern Rules.) Therefore, we usually write the makefile so that the first rule is the one for compiling the entire program or all the programs described by the makefile (often with a target called 'all'). *Note Arguments to Specify the Goals: Goals. * Menu: * Rule Example:: An example explained. * Rule Syntax:: General syntax explained. * Prerequisite Types:: There are two types of prerequisites. * Wildcards:: Using wildcard characters such as '*'. * Directory Search:: Searching other directories for source files. * Phony Targets:: Using a target that is not a real file's name. * Force Targets:: You can use a target without a recipe or prerequisites to mark other targets as phony. * Empty Targets:: When only the date matters and the files are empty. * Special Targets:: Targets with special built-in meanings. * Multiple Targets:: When to make use of several targets in a rule. * Multiple Rules:: How to use several rules with the same target. * Static Pattern:: Static pattern rules apply to multiple targets and can vary the prerequisites according to the target name. * Double-Colon:: How to use a special kind of rule to allow several independent rules for one target. * Automatic Prerequisites:: How to automatically generate rules giving prerequisites from source files themselves.  File: make.info, Node: Rule Example, Next: Rule Syntax, Prev: Rules, Up: Rules 4.1 Rule Example ================ Here is an example of a rule: foo.o : foo.c defs.h # module for twiddling the frobs cc -c -g foo.c Its target is 'foo.o' and its prerequisites are 'foo.c' and 'defs.h'. It has one command in the recipe: 'cc -c -g foo.c'. The recipe starts with a tab to identify it as a recipe. This rule says two things: * How to decide whether 'foo.o' is out of date: it is out of date if it does not exist, or if either 'foo.c' or 'defs.h' is more recent than it. * How to update the file 'foo.o': by running 'cc' as stated. The recipe does not explicitly mention 'defs.h', but we presume that 'foo.c' includes it, and that that is why 'defs.h' was added to the prerequisites.  File: make.info, Node: Rule Syntax, Next: Prerequisite Types, Prev: Rule Example, Up: Rules 4.2 Rule Syntax =============== In general, a rule looks like this: TARGETS : PREREQUISITES RECIPE ... or like this: TARGETS : PREREQUISITES ; RECIPE RECIPE ... The TARGETS are file names, separated by spaces. Wildcard characters may be used (*note Using Wildcard Characters in File Names: Wildcards.) and a name of the form 'A(M)' represents member M in archive file A (*note Archive Members as Targets: Archive Members.). Usually there is only one target per rule, but occasionally there is a reason to have more (*note Multiple Targets in a Rule: Multiple Targets.). The RECIPE lines start with a tab character (or the first character in the value of the '.RECIPEPREFIX' variable; *note Special Variables::). The first recipe line may appear on the line after the prerequisites, with a tab character, or may appear on the same line, with a semicolon. Either way, the effect is the same. There are other differences in the syntax of recipes. *Note Writing Recipes in Rules: Recipes. Because dollar signs are used to start 'make' variable references, if you really want a dollar sign in a target or prerequisite you must write two of them, '$$' (*note How to Use Variables: Using Variables.). If you have enabled secondary expansion (*note Secondary Expansion::) and you want a literal dollar sign in the prerequisites list, you must actually write _four_ dollar signs ('$$$$'). You may split a long line by inserting a backslash followed by a newline, but this is not required, as 'make' places no limit on the length of a line in a makefile. A rule tells 'make' two things: when the targets are out of date, and how to update them when necessary. The criterion for being out of date is specified in terms of the PREREQUISITES, which consist of file names separated by spaces. (Wildcards and archive members (*note Archives::) are allowed here too.) A target is out of date if it does not exist or if it is older than any of the prerequisites (by comparison of last-modification times). The idea is that the contents of the target file are computed based on information in the prerequisites, so if any of the prerequisites changes, the contents of the existing target file are no longer necessarily valid. How to update is specified by a RECIPE. This is one or more lines to be executed by the shell (normally 'sh'), but with some extra features (*note Writing Recipes in Rules: Recipes.).  File: make.info, Node: Prerequisite Types, Next: Wildcards, Prev: Rule Syntax, Up: Rules 4.3 Types of Prerequisites ========================== There are actually two different types of prerequisites understood by GNU 'make': normal prerequisites such as described in the previous section, and "order-only" prerequisites. A normal prerequisite makes two statements: first, it imposes an order in which recipes will be invoked: the recipes for all prerequisites of a target will be completed before the recipe for the target is run. Second, it imposes a dependency relationship: if any prerequisite is newer than the target, then the target is considered out-of-date and must be rebuilt. Normally, this is exactly what you want: if a target's prerequisite is updated, then the target should also be updated. Occasionally, however, you have a situation where you want to impose a specific ordering on the rules to be invoked _without_ forcing the target to be updated if one of those rules is executed. In that case, you want to define "order-only" prerequisites. Order-only prerequisites can be specified by placing a pipe symbol ('|') in the prerequisites list: any prerequisites to the left of the pipe symbol are normal; any prerequisites to the right are order-only: TARGETS : NORMAL-PREREQUISITES | ORDER-ONLY-PREREQUISITES The normal prerequisites section may of course be empty. Also, you may still declare multiple lines of prerequisites for the same target: they are appended appropriately (normal prerequisites are appended to the list of normal prerequisites; order-only prerequisites are appended to the list of order-only prerequisites). Note that if you declare the same file to be both a normal and an order-only prerequisite, the normal prerequisite takes precedence (since they have a strict superset of the behavior of an order-only prerequisite). Consider an example where your targets are to be placed in a separate directory, and that directory might not exist before 'make' is run. In this situation, you want the directory to be created before any targets are placed into it but, because the timestamps on directories change whenever a file is added, removed, or renamed, we certainly don't want to rebuild all the targets whenever the directory's timestamp changes. One way to manage this is with order-only prerequisites: make the directory an order-only prerequisite on all the targets: OBJDIR := objdir OBJS := $(addprefix $(OBJDIR)/,foo.o bar.o baz.o) $(OBJDIR)/%.o : %.c $(COMPILE.c) $(OUTPUT_OPTION) $< all: $(OBJS) $(OBJS): | $(OBJDIR) $(OBJDIR): mkdir $(OBJDIR) Now the rule to create the 'objdir' directory will be run, if needed, before any '.o' is built, but no '.o' will be built because the 'objdir' directory timestamp changed.  File: make.info, Node: Wildcards, Next: Directory Search, Prev: Prerequisite Types, Up: Rules 4.4 Using Wildcard Characters in File Names =========================================== A single file name can specify many files using "wildcard characters". The wildcard characters in 'make' are '*', '?' and '[...]', the same as in the Bourne shell. For example, '*.c' specifies a list of all the files (in the working directory) whose names end in '.c'. The character '~' at the beginning of a file name also has special significance. If alone, or followed by a slash, it represents your home directory. For example '~/bin' expands to '/home/you/bin'. If the '~' is followed by a word, the string represents the home directory of the user named by that word. For example '~john/bin' expands to '/home/john/bin'. On systems which don't have a home directory for each user (such as MS-DOS or MS-Windows), this functionality can be simulated by setting the environment variable HOME. Wildcard expansion is performed by 'make' automatically in targets and in prerequisites. In recipes, the shell is responsible for wildcard expansion. In other contexts, wildcard expansion happens only if you request it explicitly with the 'wildcard' function. The special significance of a wildcard character can be turned off by preceding it with a backslash. Thus, 'foo\*bar' would refer to a specific file whose name consists of 'foo', an asterisk, and 'bar'. * Menu: * Wildcard Examples:: Several examples. * Wildcard Pitfall:: Problems to avoid. * Wildcard Function:: How to cause wildcard expansion where it does not normally take place.  File: make.info, Node: Wildcard Examples, Next: Wildcard Pitfall, Prev: Wildcards, Up: Wildcards 4.4.1 Wildcard Examples ----------------------- Wildcards can be used in the recipe of a rule, where they are expanded by the shell. For example, here is a rule to delete all the object files: clean: rm -f *.o Wildcards are also useful in the prerequisites of a rule. With the following rule in the makefile, 'make print' will print all the '.c' files that have changed since the last time you printed them: print: *.c lpr -p $? touch print This rule uses 'print' as an empty target file; see *note Empty Target Files to Record Events: Empty Targets. (The automatic variable '$?' is used to print only those files that have changed; see *note Automatic Variables::.) Wildcard expansion does not happen when you define a variable. Thus, if you write this: objects = *.o then the value of the variable 'objects' is the actual string '*.o'. However, if you use the value of 'objects' in a target or prerequisite, wildcard expansion will take place there. If you use the value of 'objects' in a recipe, the shell may perform wildcard expansion when the recipe runs. To set 'objects' to the expansion, instead use: objects := $(wildcard *.o) *Note Wildcard Function::.  File: make.info, Node: Wildcard Pitfall, Next: Wildcard Function, Prev: Wildcard Examples, Up: Wildcards 4.4.2 Pitfalls of Using Wildcards --------------------------------- Now here is an example of a naive way of using wildcard expansion, that does not do what you would intend. Suppose you would like to say that the executable file 'foo' is made from all the object files in the directory, and you write this: objects = *.o foo : $(objects) cc -o foo $(CFLAGS) $(objects) The value of 'objects' is the actual string '*.o'. Wildcard expansion happens in the rule for 'foo', so that each _existing_ '.o' file becomes a prerequisite of 'foo' and will be recompiled if necessary. But what if you delete all the '.o' files? When a wildcard matches no files, it is left as it is, so then 'foo' will depend on the oddly-named file '*.o'. Since no such file is likely to exist, 'make' will give you an error saying it cannot figure out how to make '*.o'. This is not what you want! Actually it is possible to obtain the desired result with wildcard expansion, but you need more sophisticated techniques, including the 'wildcard' function and string substitution. *Note The Function 'wildcard': Wildcard Function. Microsoft operating systems (MS-DOS and MS-Windows) use backslashes to separate directories in pathnames, like so: c:\foo\bar\baz.c This is equivalent to the Unix-style 'c:/foo/bar/baz.c' (the 'c:' part is the so-called drive letter). When 'make' runs on these systems, it supports backslashes as well as the Unix-style forward slashes in pathnames. However, this support does _not_ include the wildcard expansion, where backslash is a quote character. Therefore, you _must_ use Unix-style slashes in these cases.  File: make.info, Node: Wildcard Function, Prev: Wildcard Pitfall, Up: Wildcards 4.4.3 The Function 'wildcard' ----------------------------- Wildcard expansion happens automatically in rules. But wildcard expansion does not normally take place when a variable is set, or inside the arguments of a function. If you want to do wildcard expansion in such places, you need to use the 'wildcard' function, like this: $(wildcard PATTERN...) This string, used anywhere in a makefile, is replaced by a space-separated list of names of existing files that match one of the given file name patterns. If no existing file name matches a pattern, then that pattern is omitted from the output of the 'wildcard' function. Note that this is different from how unmatched wildcards behave in rules, where they are used verbatim rather than ignored (*note Wildcard Pitfall::). One use of the 'wildcard' function is to get a list of all the C source files in a directory, like this: $(wildcard *.c) We can change the list of C source files into a list of object files by replacing the '.c' suffix with '.o' in the result, like this: $(patsubst %.c,%.o,$(wildcard *.c)) (Here we have used another function, 'patsubst'. *Note Functions for String Substitution and Analysis: Text Functions.) Thus, a makefile to compile all C source files in the directory and then link them together could be written as follows: objects := $(patsubst %.c,%.o,$(wildcard *.c)) foo : $(objects) cc -o foo $(objects) (This takes advantage of the implicit rule for compiling C programs, so there is no need to write explicit rules for compiling the files. *Note The Two Flavors of Variables: Flavors, for an explanation of ':=', which is a variant of '='.)  File: make.info, Node: Directory Search, Next: Phony Targets, Prev: Wildcards, Up: Rules 4.5 Searching Directories for Prerequisites =========================================== For large systems, it is often desirable to put sources in a separate directory from the binaries. The "directory search" features of 'make' facilitate this by searching several directories automatically to find a prerequisite. When you redistribute the files among directories, you do not need to change the individual rules, just the search paths. * Menu: * General Search:: Specifying a search path that applies to every prerequisite. * Selective Search:: Specifying a search path for a specified class of names. * Search Algorithm:: When and how search paths are applied. * Recipes/Search:: How to write recipes that work together with search paths. * Implicit/Search:: How search paths affect implicit rules. * Libraries/Search:: Directory search for link libraries.  File: make.info, Node: General Search, Next: Selective Search, Prev: Directory Search, Up: Directory Search 4.5.1 'VPATH': Search Path for All Prerequisites ------------------------------------------------ The value of the 'make' variable 'VPATH' specifies a list of directories that 'make' should search. Most often, the directories are expected to contain prerequisite files that are not in the current directory; however, 'make' uses 'VPATH' as a search list for both prerequisites and targets of rules. Thus, if a file that is listed as a target or prerequisite does not exist in the current directory, 'make' searches the directories listed in 'VPATH' for a file with that name. If a file is found in one of them, that file may become the prerequisite (see below). Rules may then specify the names of files in the prerequisite list as if they all existed in the current directory. *Note Writing Recipes with Directory Search: Recipes/Search. In the 'VPATH' variable, directory names are separated by colons or blanks. The order in which directories are listed is the order followed by 'make' in its search. (On MS-DOS and MS-Windows, semi-colons are used as separators of directory names in 'VPATH', since the colon can be used in the pathname itself, after the drive letter.) For example, VPATH = src:../headers specifies a path containing two directories, 'src' and '../headers', which 'make' searches in that order. With this value of 'VPATH', the following rule, foo.o : foo.c is interpreted as if it were written like this: foo.o : src/foo.c assuming the file 'foo.c' does not exist in the current directory but is found in the directory 'src'.  File: make.info, Node: Selective Search, Next: Search Algorithm, Prev: General Search, Up: Directory Search 4.5.2 The 'vpath' Directive --------------------------- Similar to the 'VPATH' variable, but more selective, is the 'vpath' directive (note lower case), which allows you to specify a search path for a particular class of file names: those that match a particular pattern. Thus you can supply certain search directories for one class of file names and other directories (or none) for other file names. There are three forms of the 'vpath' directive: 'vpath PATTERN DIRECTORIES' Specify the search path DIRECTORIES for file names that match PATTERN. The search path, DIRECTORIES, is a list of directories to be searched, separated by colons (semi-colons on MS-DOS and MS-Windows) or blanks, just like the search path used in the 'VPATH' variable. 'vpath PATTERN' Clear out the search path associated with PATTERN. 'vpath' Clear all search paths previously specified with 'vpath' directives. A 'vpath' pattern is a string containing a '%' character. The string must match the file name of a prerequisite that is being searched for, the '%' character matching any sequence of zero or more characters (as in pattern rules; *note Defining and Redefining Pattern Rules: Pattern Rules.). For example, '%.h' matches files that end in '.h'. (If there is no '%', the pattern must match the prerequisite exactly, which is not useful very often.) '%' characters in a 'vpath' directive's pattern can be quoted with preceding backslashes ('\'). Backslashes that would otherwise quote '%' characters can be quoted with more backslashes. Backslashes that quote '%' characters or other backslashes are removed from the pattern before it is compared to file names. Backslashes that are not in danger of quoting '%' characters go unmolested. When a prerequisite fails to exist in the current directory, if the PATTERN in a 'vpath' directive matches the name of the prerequisite file, then the DIRECTORIES in that directive are searched just like (and before) the directories in the 'VPATH' variable. For example, vpath %.h ../headers tells 'make' to look for any prerequisite whose name ends in '.h' in the directory '../headers' if the file is not found in the current directory. If several 'vpath' patterns match the prerequisite file's name, then 'make' processes each matching 'vpath' directive one by one, searching all the directories mentioned in each directive. 'make' handles multiple 'vpath' directives in the order in which they appear in the makefile; multiple directives with the same pattern are independent of each other. Thus, vpath %.c foo vpath % blish vpath %.c bar will look for a file ending in '.c' in 'foo', then 'blish', then 'bar', while vpath %.c foo:bar vpath % blish will look for a file ending in '.c' in 'foo', then 'bar', then 'blish'.  File: make.info, Node: Search Algorithm, Next: Recipes/Search, Prev: Selective Search, Up: Directory Search 4.5.3 How Directory Searches are Performed ------------------------------------------ When a prerequisite is found through directory search, regardless of type (general or selective), the pathname located may not be the one that 'make' actually provides you in the prerequisite list. Sometimes the path discovered through directory search is thrown away. The algorithm 'make' uses to decide whether to keep or abandon a path found via directory search is as follows: 1. If a target file does not exist at the path specified in the makefile, directory search is performed. 2. If the directory search is successful, that path is kept and this file is tentatively stored as the target. 3. All prerequisites of this target are examined using this same method. 4. After processing the prerequisites, the target may or may not need to be rebuilt: a. If the target does _not_ need to be rebuilt, the path to the file found during directory search is used for any prerequisite lists which contain this target. In short, if 'make' doesn't need to rebuild the target then you use the path found via directory search. b. If the target _does_ need to be rebuilt (is out-of-date), the pathname found during directory search is _thrown away_, and the target is rebuilt using the file name specified in the makefile. In short, if 'make' must rebuild, then the target is rebuilt locally, not in the directory found via directory search. This algorithm may seem complex, but in practice it is quite often exactly what you want. Other versions of 'make' use a simpler algorithm: if the file does not exist, and it is found via directory search, then that pathname is always used whether or not the target needs to be built. Thus, if the target is rebuilt it is created at the pathname discovered during directory search. If, in fact, this is the behavior you want for some or all of your directories, you can use the 'GPATH' variable to indicate this to 'make'. 'GPATH' has the same syntax and format as 'VPATH' (that is, a space- or colon-delimited list of pathnames). If an out-of-date target is found by directory search in a directory that also appears in 'GPATH', then that pathname is not thrown away. The target is rebuilt using the expanded path.  File: make.info, Node: Recipes/Search, Next: Implicit/Search, Prev: Search Algorithm, Up: Directory Search 4.5.4 Writing Recipes with Directory Search ------------------------------------------- When a prerequisite is found in another directory through directory search, this cannot change the recipe of the rule; they will execute as written. Therefore, you must write the recipe with care so that it will look for the prerequisite in the directory where 'make' finds it. This is done with the "automatic variables" such as '$^' (*note Automatic Variables::). For instance, the value of '$^' is a list of all the prerequisites of the rule, including the names of the directories in which they were found, and the value of '$@' is the target. Thus: foo.o : foo.c cc -c $(CFLAGS) $^ -o $@ (The variable 'CFLAGS' exists so you can specify flags for C compilation by implicit rules; we use it here for consistency so it will affect all C compilations uniformly; *note Variables Used by Implicit Rules: Implicit Variables.) Often the prerequisites include header files as well, which you do not want to mention in the recipe. The automatic variable '$<' is just the first prerequisite: VPATH = src:../headers foo.o : foo.c defs.h hack.h cc -c $(CFLAGS) $< -o $@  File: make.info, Node: Implicit/Search, Next: Libraries/Search, Prev: Recipes/Search, Up: Directory Search 4.5.5 Directory Search and Implicit Rules ----------------------------------------- The search through the directories specified in 'VPATH' or with 'vpath' also happens during consideration of implicit rules (*note Using Implicit Rules: Implicit Rules.). For example, when a file 'foo.o' has no explicit rule, 'make' considers implicit rules, such as the built-in rule to compile 'foo.c' if that file exists. If such a file is lacking in the current directory, the appropriate directories are searched for it. If 'foo.c' exists (or is mentioned in the makefile) in any of the directories, the implicit rule for C compilation is applied. The recipes of implicit rules normally use automatic variables as a matter of necessity; consequently they will use the file names found by directory search with no extra effort.  File: make.info, Node: Libraries/Search, Prev: Implicit/Search, Up: Directory Search 4.5.6 Directory Search for Link Libraries ----------------------------------------- Directory search applies in a special way to libraries used with the linker. This special feature comes into play when you write a prerequisite whose name is of the form '-lNAME'. (You can tell something strange is going on here because the prerequisite is normally the name of a file, and the _file name_ of a library generally looks like 'libNAME.a', not like '-lNAME'.) When a prerequisite's name has the form '-lNAME', 'make' handles it specially by searching for the file 'libNAME.so', and, if it is not found, for the file 'libNAME.a' in the current directory, in directories specified by matching 'vpath' search paths and the 'VPATH' search path, and then in the directories '/lib', '/usr/lib', and 'PREFIX/lib' (normally '/usr/local/lib', but MS-DOS/MS-Windows versions of 'make' behave as if PREFIX is defined to be the root of the DJGPP installation tree). For example, if there is a '/usr/lib/libcurses.a' library on your system (and no '/usr/lib/libcurses.so' file), then foo : foo.c -lcurses cc $^ -o $@ would cause the command 'cc foo.c /usr/lib/libcurses.a -o foo' to be executed when 'foo' is older than 'foo.c' or than '/usr/lib/libcurses.a'. Although the default set of files to be searched for is 'libNAME.so' and 'libNAME.a', this is customizable via the '.LIBPATTERNS' variable. Each word in the value of this variable is a pattern string. When a prerequisite like '-lNAME' is seen, 'make' will replace the percent in each pattern in the list with NAME and perform the above directory searches using each library file name. The default value for '.LIBPATTERNS' is 'lib%.so lib%.a', which provides the default behavior described above. You can turn off link library expansion completely by setting this variable to an empty value.  File: make.info, Node: Phony Targets, Next: Force Targets, Prev: Directory Search, Up: Rules 4.6 Phony Targets ================= A phony target is one that is not really the name of a file; rather it is just a name for a recipe to be executed when you make an explicit request. There are two reasons to use a phony target: to avoid a conflict with a file of the same name, and to improve performance. If you write a rule whose recipe will not create the target file, the recipe will be executed every time the target comes up for remaking. Here is an example: clean: rm *.o temp Because the 'rm' command does not create a file named 'clean', probably no such file will ever exist. Therefore, the 'rm' command will be executed every time you say 'make clean'. In this example, the 'clean' target will not work properly if a file named 'clean' is ever created in this directory. Since it has no prerequisites, 'clean' would always be considered up to date and its recipe would not be executed. To avoid this problem you can explicitly declare the target to be phony by making it a prerequisite of the special target '.PHONY' (*note Special Built-in Target Names: Special Targets.) as follows: .PHONY: clean clean: rm *.o temp Once this is done, 'make clean' will run the recipe regardless of whether there is a file named 'clean'. Phony targets are also useful in conjunction with recursive invocations of 'make' (*note Recursive Use of 'make': Recursion.). In this situation the makefile will often contain a variable which lists a number of sub-directories to be built. A simplistic way to handle this is to define one rule with a recipe that loops over the sub-directories, like this: SUBDIRS = foo bar baz subdirs: for dir in $(SUBDIRS); do \ $(MAKE) -C $$dir; \ done There are problems with this method, however. First, any error detected in a sub-make is ignored by this rule, so it will continue to build the rest of the directories even when one fails. This can be overcome by adding shell commands to note the error and exit, but then it will do so even if 'make' is invoked with the '-k' option, which is unfortunate. Second, and perhaps more importantly, you cannot take advantage of 'make''s ability to build targets in parallel (*note Parallel Execution: Parallel.), since there is only one rule. By declaring the sub-directories as '.PHONY' targets (you must do this as the sub-directory obviously always exists; otherwise it won't be built) you can remove these problems: SUBDIRS = foo bar baz .PHONY: subdirs $(SUBDIRS) subdirs: $(SUBDIRS) $(SUBDIRS): $(MAKE) -C $@ foo: baz Here we've also declared that the 'foo' sub-directory cannot be built until after the 'baz' sub-directory is complete; this kind of relationship declaration is particularly important when attempting parallel builds. The implicit rule search (*note Implicit Rules::) is skipped for '.PHONY' targets. This is why declaring a target as '.PHONY' is good for performance, even if you are not worried about the actual file existing. A phony target should not be a prerequisite of a real target file; if it is, its recipe will be run every time 'make' goes to update that file. As long as a phony target is never a prerequisite of a real target, the phony target recipe will be executed only when the phony target is a specified goal (*note Arguments to Specify the Goals: Goals.). Phony targets can have prerequisites. When one directory contains multiple programs, it is most convenient to describe all of the programs in one makefile './Makefile'. Since the target remade by default will be the first one in the makefile, it is common to make this a phony target named 'all' and give it, as prerequisites, all the individual programs. For example: all : prog1 prog2 prog3 .PHONY : all prog1 : prog1.o utils.o cc -o prog1 prog1.o utils.o prog2 : prog2.o cc -o prog2 prog2.o prog3 : prog3.o sort.o utils.o cc -o prog3 prog3.o sort.o utils.o Now you can say just 'make' to remake all three programs, or specify as arguments the ones to remake (as in 'make prog1 prog3'). Phoniness is not inherited: the prerequisites of a phony target are not themselves phony, unless explicitly declared to be so. When one phony target is a prerequisite of another, it serves as a subroutine of the other. For example, here 'make cleanall' will delete the object files, the difference files, and the file 'program': .PHONY: cleanall cleanobj cleandiff cleanall : cleanobj cleandiff rm program cleanobj : rm *.o cleandiff : rm *.diff  File: make.info, Node: Force Targets, Next: Empty Targets, Prev: Phony Targets, Up: Rules 4.7 Rules without Recipes or Prerequisites ========================================== If a rule has no prerequisites or recipe, and the target of the rule is a nonexistent file, then 'make' imagines this target to have been updated whenever its rule is run. This implies that all targets depending on this one will always have their recipe run. An example will illustrate this: clean: FORCE rm $(objects) FORCE: Here the target 'FORCE' satisfies the special conditions, so the target 'clean' that depends on it is forced to run its recipe. There is nothing special about the name 'FORCE', but that is one name commonly used this way. As you can see, using 'FORCE' this way has the same results as using '.PHONY: clean'. Using '.PHONY' is more explicit and more efficient. However, other versions of 'make' do not support '.PHONY'; thus 'FORCE' appears in many makefiles. *Note Phony Targets::.  File: make.info, Node: Empty Targets, Next: Special Targets, Prev: Force Targets, Up: Rules 4.8 Empty Target Files to Record Events ======================================= The "empty target" is a variant of the phony target; it is used to hold recipes for an action that you request explicitly from time to time. Unlike a phony target, this target file can really exist; but the file's contents do not matter, and usually are empty. The purpose of the empty target file is to record, with its last-modification time, when the rule's recipe was last executed. It does so because one of the commands in the recipe is a 'touch' command to update the target file. The empty target file should have some prerequisites (otherwise it doesn't make sense). When you ask to remake the empty target, the recipe is executed if any prerequisite is more recent than the target; in other words, if a prerequisite has changed since the last time you remade the target. Here is an example: print: foo.c bar.c lpr -p $? touch print With this rule, 'make print' will execute the 'lpr' command if either source file has changed since the last 'make print'. The automatic variable '$?' is used to print only those files that have changed (*note Automatic Variables::).  File: make.info, Node: Special Targets, Next: Multiple Targets, Prev: Empty Targets, Up: Rules 4.9 Special Built-in Target Names ================================= Certain names have special meanings if they appear as targets. '.PHONY' The prerequisites of the special target '.PHONY' are considered to be phony targets. When it is time to consider such a target, 'make' will run its recipe unconditionally, regardless of whether a file with that name exists or what its last-modification time is. *Note Phony Targets: Phony Targets. '.SUFFIXES' The prerequisites of the special target '.SUFFIXES' are the list of suffixes to be used in checking for suffix rules. *Note Old-Fashioned Suffix Rules: Suffix Rules. '.DEFAULT' The recipe specified for '.DEFAULT' is used for any target for which no rules are found (either explicit rules or implicit rules). *Note Last Resort::. If a '.DEFAULT' recipe is specified, every file mentioned as a prerequisite, but not as a target in a rule, will have that recipe executed on its behalf. *Note Implicit Rule Search Algorithm: Implicit Rule Search. '.PRECIOUS' The targets which '.PRECIOUS' depends on are given the following special treatment: if 'make' is killed or interrupted during the execution of their recipes, the target is not deleted. *Note Interrupting or Killing 'make': Interrupts. Also, if the target is an intermediate file, it will not be deleted after it is no longer needed, as is normally done. *Note Chains of Implicit Rules: Chained Rules. In this latter respect it overlaps with the '.SECONDARY' special target. You can also list the target pattern of an implicit rule (such as '%.o') as a prerequisite file of the special target '.PRECIOUS' to preserve intermediate files created by rules whose target patterns match that file's name. '.INTERMEDIATE' The targets which '.INTERMEDIATE' depends on are treated as intermediate files. *Note Chains of Implicit Rules: Chained Rules. '.INTERMEDIATE' with no prerequisites has no effect. '.SECONDARY' The targets which '.SECONDARY' depends on are treated as intermediate files, except that they are never automatically deleted. *Note Chains of Implicit Rules: Chained Rules. '.SECONDARY' with no prerequisites causes all targets to be treated as secondary (i.e., no target is removed because it is considered intermediate). '.SECONDEXPANSION' If '.SECONDEXPANSION' is mentioned as a target anywhere in the makefile, then all prerequisite lists defined _after_ it appears will be expanded a second time after all makefiles have been read in. *Note Secondary Expansion: Secondary Expansion. '.DELETE_ON_ERROR' If '.DELETE_ON_ERROR' is mentioned as a target anywhere in the makefile, then 'make' will delete the target of a rule if it has changed and its recipe exits with a nonzero exit status, just as it does when it receives a signal. *Note Errors in Recipes: Errors. '.IGNORE' If you specify prerequisites for '.IGNORE', then 'make' will ignore errors in execution of the recipe for those particular files. The recipe for '.IGNORE' (if any) is ignored. If mentioned as a target with no prerequisites, '.IGNORE' says to ignore errors in execution of recipes for all files. This usage of '.IGNORE' is supported only for historical compatibility. Since this affects every recipe in the makefile, it is not very useful; we recommend you use the more selective ways to ignore errors in specific recipes. *Note Errors in Recipes: Errors. '.LOW_RESOLUTION_TIME' If you specify prerequisites for '.LOW_RESOLUTION_TIME', 'make' assumes that these files are created by commands that generate low resolution time stamps. The recipe for the '.LOW_RESOLUTION_TIME' target are ignored. The high resolution file time stamps of many modern file systems lessen the chance of 'make' incorrectly concluding that a file is up to date. Unfortunately, some hosts do not provide a way to set a high resolution file time stamp, so commands like 'cp -p' that explicitly set a file's time stamp must discard its sub-second part. If a file is created by such a command, you should list it as a prerequisite of '.LOW_RESOLUTION_TIME' so that 'make' does not mistakenly conclude that the file is out of date. For example: .LOW_RESOLUTION_TIME: dst dst: src cp -p src dst Since 'cp -p' discards the sub-second part of 'src''s time stamp, 'dst' is typically slightly older than 'src' even when it is up to date. The '.LOW_RESOLUTION_TIME' line causes 'make' to consider 'dst' to be up to date if its time stamp is at the start of the same second that 'src''s time stamp is in. Due to a limitation of the archive format, archive member time stamps are always low resolution. You need not list archive members as prerequisites of '.LOW_RESOLUTION_TIME', as 'make' does this automatically. '.SILENT' If you specify prerequisites for '.SILENT', then 'make' will not print the recipe used to remake those particular files before executing them. The recipe for '.SILENT' is ignored. If mentioned as a target with no prerequisites, '.SILENT' says not to print any recipes before executing them. This usage of '.SILENT' is supported only for historical compatibility. We recommend you use the more selective ways to silence specific recipes. *Note Recipe Echoing: Echoing. If you want to silence all recipes for a particular run of 'make', use the '-s' or '--silent' option (*note Options Summary::). '.EXPORT_ALL_VARIABLES' Simply by being mentioned as a target, this tells 'make' to export all variables to child processes by default. *Note Communicating Variables to a Sub-'make': Variables/Recursion. '.NOTPARALLEL' If '.NOTPARALLEL' is mentioned as a target, then this invocation of 'make' will be run serially, even if the '-j' option is given. Any recursively invoked 'make' command will still run recipes in parallel (unless its makefile also contains this target). Any prerequisites on this target are ignored. '.ONESHELL' If '.ONESHELL' is mentioned as a target, then when a target is built all lines of the recipe will be given to a single invocation of the shell rather than each line being invoked separately (*note Recipe Execution: Execution.). '.POSIX' If '.POSIX' is mentioned as a target, then the makefile will be parsed and run in POSIX-conforming mode. This does _not_ mean that only POSIX-conforming makefiles will be accepted: all advanced GNU 'make' features are still available. Rather, this target causes 'make' to behave as required by POSIX in those areas where 'make''s default behavior differs. In particular, if this target is mentioned then recipes will be invoked as if the shell had been passed the '-e' flag: the first failing command in a recipe will cause the recipe to fail immediately. Any defined implicit rule suffix also counts as a special target if it appears as a target, and so does the concatenation of two suffixes, such as '.c.o'. These targets are suffix rules, an obsolete way of defining implicit rules (but a way still widely used). In principle, any target name could be special in this way if you break it in two and add both pieces to the suffix list. In practice, suffixes normally begin with '.', so these special target names also begin with '.'. *Note Old-Fashioned Suffix Rules: Suffix Rules.  File: make.info, Node: Multiple Targets, Next: Multiple Rules, Prev: Special Targets, Up: Rules 4.10 Multiple Targets in a Rule =============================== A rule with multiple targets is equivalent to writing many rules, each with one target, and all identical aside from that. The same recipe applies to all the targets, but its effect may vary because you can substitute the actual target name into the recipe using '$@'. The rule contributes the same prerequisites to all the targets also. This is useful in two cases. * You want just prerequisites, no recipe. For example: kbd.o command.o files.o: command.h gives an additional prerequisite to each of the three object files mentioned. * Similar recipes work for all the targets. The recipes do not need to be absolutely identical, since the automatic variable '$@' can be used to substitute the particular target to be remade into the commands (*note Automatic Variables::). For example: bigoutput littleoutput : text.g generate text.g -$(subst output,,$@) > $@ is equivalent to bigoutput : text.g generate text.g -big > bigoutput littleoutput : text.g generate text.g -little > littleoutput Here we assume the hypothetical program 'generate' makes two types of output, one if given '-big' and one if given '-little'. *Note Functions for String Substitution and Analysis: Text Functions, for an explanation of the 'subst' function. Suppose you would like to vary the prerequisites according to the target, much as the variable '$@' allows you to vary the recipe. You cannot do this with multiple targets in an ordinary rule, but you can do it with a "static pattern rule". *Note Static Pattern Rules: Static Pattern.  File: make.info, Node: Multiple Rules, Next: Static Pattern, Prev: Multiple Targets, Up: Rules 4.11 Multiple Rules for One Target ================================== One file can be the target of several rules. All the prerequisites mentioned in all the rules are merged into one list of prerequisites for the target. If the target is older than any prerequisite from any rule, the recipe is executed. There can only be one recipe to be executed for a file. If more than one rule gives a recipe for the same file, 'make' uses the last one given and prints an error message. (As a special case, if the file's name begins with a dot, no error message is printed. This odd behavior is only for compatibility with other implementations of 'make'... you should avoid using it). Occasionally it is useful to have the same target invoke multiple recipes which are defined in different parts of your makefile; you can use "double-colon rules" (*note Double-Colon::) for this. An extra rule with just prerequisites can be used to give a few extra prerequisites to many files at once. For example, makefiles often have a variable, such as 'objects', containing a list of all the compiler output files in the system being made. An easy way to say that all of them must be recompiled if 'config.h' changes is to write the following: objects = foo.o bar.o foo.o : defs.h bar.o : defs.h test.h $(objects) : config.h This could be inserted or taken out without changing the rules that really specify how to make the object files, making it a convenient form to use if you wish to add the additional prerequisite intermittently. Another wrinkle is that the additional prerequisites could be specified with a variable that you set with a command line argument to 'make' (*note Overriding Variables: Overriding.). For example, extradeps= $(objects) : $(extradeps) means that the command 'make extradeps=foo.h' will consider 'foo.h' as a prerequisite of each object file, but plain 'make' will not. If none of the explicit rules for a target has a recipe, then 'make' searches for an applicable implicit rule to find one *note Using Implicit Rules: Implicit Rules.).  File: make.info, Node: Static Pattern, Next: Double-Colon, Prev: Multiple Rules, Up: Rules 4.12 Static Pattern Rules ========================= "Static pattern rules" are rules which specify multiple targets and construct the prerequisite names for each target based on the target name. They are more general than ordinary rules with multiple targets because the targets do not have to have identical prerequisites. Their prerequisites must be _analogous_, but not necessarily _identical_. * Menu: * Static Usage:: The syntax of static pattern rules. * Static versus Implicit:: When are they better than implicit rules?  File: make.info, Node: Static Usage, Next: Static versus Implicit, Prev: Static Pattern, Up: Static Pattern 4.12.1 Syntax of Static Pattern Rules ------------------------------------- Here is the syntax of a static pattern rule: TARGETS ...: TARGET-PATTERN: PREREQ-PATTERNS ... RECIPE ... The TARGETS list specifies the targets that the rule applies to. The targets can contain wildcard characters, just like the targets of ordinary rules (*note Using Wildcard Characters in File Names: Wildcards.). The TARGET-PATTERN and PREREQ-PATTERNS say how to compute the prerequisites of each target. Each target is matched against the TARGET-PATTERN to extract a part of the target name, called the "stem". This stem is substituted into each of the PREREQ-PATTERNS to make the prerequisite names (one from each PREREQ-PATTERN). Each pattern normally contains the character '%' just once. When the TARGET-PATTERN matches a target, the '%' can match any part of the target name; this part is called the "stem". The rest of the pattern must match exactly. For example, the target 'foo.o' matches the pattern '%.o', with 'foo' as the stem. The targets 'foo.c' and 'foo.out' do not match that pattern. The prerequisite names for each target are made by substituting the stem for the '%' in each prerequisite pattern. For example, if one prerequisite pattern is '%.c', then substitution of the stem 'foo' gives the prerequisite name 'foo.c'. It is legitimate to write a prerequisite pattern that does not contain '%'; then this prerequisite is the same for all targets. '%' characters in pattern rules can be quoted with preceding backslashes ('\'). Backslashes that would otherwise quote '%' characters can be quoted with more backslashes. Backslashes that quote '%' characters or other backslashes are removed from the pattern before it is compared to file names or has a stem substituted into it. Backslashes that are not in danger of quoting '%' characters go unmolested. For example, the pattern 'the\%weird\\%pattern\\' has 'the%weird\' preceding the operative '%' character, and 'pattern\\' following it. The final two backslashes are left alone because they cannot affect any '%' character. Here is an example, which compiles each of 'foo.o' and 'bar.o' from the corresponding '.c' file: objects = foo.o bar.o all: $(objects) $(objects): %.o: %.c $(CC) -c $(CFLAGS) $< -o $@ Here '$<' is the automatic variable that holds the name of the prerequisite and '$@' is the automatic variable that holds the name of the target; see *note Automatic Variables::. Each target specified must match the target pattern; a warning is issued for each target that does not. If you have a list of files, only some of which will match the pattern, you can use the 'filter' function to remove non-matching file names (*note Functions for String Substitution and Analysis: Text Functions.): files = foo.elc bar.o lose.o $(filter %.o,$(files)): %.o: %.c $(CC) -c $(CFLAGS) $< -o $@ $(filter %.elc,$(files)): %.elc: %.el emacs -f batch-byte-compile $< In this example the result of '$(filter %.o,$(files))' is 'bar.o lose.o', and the first static pattern rule causes each of these object files to be updated by compiling the corresponding C source file. The result of '$(filter %.elc,$(files))' is 'foo.elc', so that file is made from 'foo.el'. Another example shows how to use '$*' in static pattern rules: bigoutput littleoutput : %output : text.g generate text.g -$* > $@ When the 'generate' command is run, '$*' will expand to the stem, either 'big' or 'little'.  File: make.info, Node: Static versus Implicit, Prev: Static Usage, Up: Static Pattern 4.12.2 Static Pattern Rules versus Implicit Rules ------------------------------------------------- A static pattern rule has much in common with an implicit rule defined as a pattern rule (*note Defining and Redefining Pattern Rules: Pattern Rules.). Both have a pattern for the target and patterns for constructing the names of prerequisites. The difference is in how 'make' decides _when_ the rule applies. An implicit rule _can_ apply to any target that matches its pattern, but it _does_ apply only when the target has no recipe otherwise specified, and only when the prerequisites can be found. If more than one implicit rule appears applicable, only one applies; the choice depends on the order of rules. By contrast, a static pattern rule applies to the precise list of targets that you specify in the rule. It cannot apply to any other target and it invariably does apply to each of the targets specified. If two conflicting rules apply, and both have recipes, that's an error. The static pattern rule can be better than an implicit rule for these reasons: * You may wish to override the usual implicit rule for a few files whose names cannot be categorized syntactically but can be given in an explicit list. * If you cannot be sure of the precise contents of the directories you are using, you may not be sure which other irrelevant files might lead 'make' to use the wrong implicit rule. The choice might depend on the order in which the implicit rule search is done. With static pattern rules, there is no uncertainty: each rule applies to precisely the targets specified.  File: make.info, Node: Double-Colon, Next: Automatic Prerequisites, Prev: Static Pattern, Up: Rules 4.13 Double-Colon Rules ======================= "Double-colon" rules are explicit rules written with '::' instead of ':' after the target names. They are handled differently from ordinary rules when the same target appears in more than one rule. Pattern rules with double-colons have an entirely different meaning (*note Match-Anything Rules::). When a target appears in multiple rules, all the rules must be the same type: all ordinary, or all double-colon. If they are double-colon, each of them is independent of the others. Each double-colon rule's recipe is executed if the target is older than any prerequisites of that rule. If there are no prerequisites for that rule, its recipe is always executed (even if the target already exists). This can result in executing none, any, or all of the double-colon rules. Double-colon rules with the same target are in fact completely separate from one another. Each double-colon rule is processed individually, just as rules with different targets are processed. The double-colon rules for a target are executed in the order they appear in the makefile. However, the cases where double-colon rules really make sense are those where the order of executing the recipes would not matter. Double-colon rules are somewhat obscure and not often very useful; they provide a mechanism for cases in which the method used to update a target differs depending on which prerequisite files caused the update, and such cases are rare. Each double-colon rule should specify a recipe; if it does not, an implicit rule will be used if one applies. *Note Using Implicit Rules: Implicit Rules.  File: make.info, Node: Automatic Prerequisites, Prev: Double-Colon, Up: Rules 4.14 Generating Prerequisites Automatically =========================================== In the makefile for a program, many of the rules you need to write often say only that some object file depends on some header file. For example, if 'main.c' uses 'defs.h' via an '#include', you would write: main.o: defs.h You need this rule so that 'make' knows that it must remake 'main.o' whenever 'defs.h' changes. You can see that for a large program you would have to write dozens of such rules in your makefile. And, you must always be very careful to update the makefile every time you add or remove an '#include'. To avoid this hassle, most modern C compilers can write these rules for you, by looking at the '#include' lines in the source files. Usually this is done with the '-M' option to the compiler. For example, the command: cc -M main.c generates the output: main.o : main.c defs.h Thus you no longer have to write all those rules yourself. The compiler will do it for you. Note that such a rule constitutes mentioning 'main.o' in a makefile, so it can never be considered an intermediate file by implicit rule search. This means that 'make' won't ever remove the file after using it; *note Chains of Implicit Rules: Chained Rules. With old 'make' programs, it was traditional practice to use this compiler feature to generate prerequisites on demand with a command like 'make depend'. That command would create a file 'depend' containing all the automatically-generated prerequisites; then the makefile could use 'include' to read them in (*note Include::). In GNU 'make', the feature of remaking makefiles makes this practice obsolete--you need never tell 'make' explicitly to regenerate the prerequisites, because it always regenerates any makefile that is out of date. *Note Remaking Makefiles::. The practice we recommend for automatic prerequisite generation is to have one makefile corresponding to each source file. For each source file 'NAME.c' there is a makefile 'NAME.d' which lists what files the object file 'NAME.o' depends on. That way only the source files that have changed need to be rescanned to produce the new prerequisites. Here is the pattern rule to generate a file of prerequisites (i.e., a makefile) called 'NAME.d' from a C source file called 'NAME.c': %.d: %.c @set -e; rm -f $@; \ $(CC) -M $(CPPFLAGS) $< > $@.$$$$; \ sed 's,\($*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@; \ rm -f $@.$$$$ *Note Pattern Rules::, for information on defining pattern rules. The '-e' flag to the shell causes it to exit immediately if the '$(CC)' command (or any other command) fails (exits with a nonzero status). With the GNU C compiler, you may wish to use the '-MM' flag instead of '-M'. This omits prerequisites on system header files. *Note Options Controlling the Preprocessor: (gcc)Preprocessor Options, for details. The purpose of the 'sed' command is to translate (for example): main.o : main.c defs.h into: main.o main.d : main.c defs.h This makes each '.d' file depend on all the source and header files that the corresponding '.o' file depends on. 'make' then knows it must regenerate the prerequisites whenever any of the source or header files changes. Once you've defined the rule to remake the '.d' files, you then use the 'include' directive to read them all in. *Note Include::. For example: sources = foo.c bar.c include $(sources:.c=.d) (This example uses a substitution variable reference to translate the list of source files 'foo.c bar.c' into a list of prerequisite makefiles, 'foo.d bar.d'. *Note Substitution Refs::, for full information on substitution references.) Since the '.d' files are makefiles like any others, 'make' will remake them as necessary with no further work from you. *Note Remaking Makefiles::. Note that the '.d' files contain target definitions; you should be sure to place the 'include' directive _after_ the first, default goal in your makefiles or run the risk of having a random object file become the default goal. *Note How Make Works::.  File: make.info, Node: Recipes, Next: Using Variables, Prev: Rules, Up: Top 5 Writing Recipes in Rules ************************** The recipe of a rule consists of one or more shell command lines to be executed, one at a time, in the order they appear. Typically, the result of executing these commands is that the target of the rule is brought up to date. Users use many different shell programs, but recipes in makefiles are always interpreted by '/bin/sh' unless the makefile specifies otherwise. *Note Recipe Execution: Execution. * Menu: * Recipe Syntax:: Recipe syntax features and pitfalls. * Echoing:: How to control when recipes are echoed. * Execution:: How recipes are executed. * Parallel:: How recipes can be executed in parallel. * Errors:: What happens after a recipe execution error. * Interrupts:: What happens when a recipe is interrupted. * Recursion:: Invoking 'make' from makefiles. * Canned Recipes:: Defining canned recipes. * Empty Recipes:: Defining useful, do-nothing recipes.  File: make.info, Node: Recipe Syntax, Next: Echoing, Prev: Recipes, Up: Recipes 5.1 Recipe Syntax ================= Makefiles have the unusual property that there are really two distinct syntaxes in one file. Most of the makefile uses 'make' syntax (*note Writing Makefiles: Makefiles.). However, recipes are meant to be interpreted by the shell and so they are written using shell syntax. The 'make' program does not try to understand shell syntax: it performs only a very few specific translations on the content of the recipe before handing it to the shell. Each line in the recipe must start with a tab (or the first character in the value of the '.RECIPEPREFIX' variable; *note Special Variables::), except that the first recipe line may be attached to the target-and-prerequisites line with a semicolon in between. _Any_ line in the makefile that begins with a tab and appears in a "rule context" (that is, after a rule has been started until another rule or variable definition) will be considered part of a recipe for that rule. Blank lines and lines of just comments may appear among the recipe lines; they are ignored. Some consequences of these rules include: * A blank line that begins with a tab is not blank: it's an empty recipe (*note Empty Recipes::). * A comment in a recipe is not a 'make' comment; it will be passed to the shell as-is. Whether the shell treats it as a comment or not depends on your shell. * A variable definition in a "rule context" which is indented by a tab as the first character on the line, will be considered part of a recipe, not a 'make' variable definition, and passed to the shell. * A conditional expression ('ifdef', 'ifeq', etc. *note Syntax of Conditionals: Conditional Syntax.) in a "rule context" which is indented by a tab as the first character on the line, will be considered part of a recipe and be passed to the shell. * Menu: * Splitting Recipe Lines:: Breaking long recipe lines for readability. * Variables in Recipes:: Using 'make' variables in recipes.  File: make.info, Node: Splitting Recipe Lines, Next: Variables in Recipes, Prev: Recipe Syntax, Up: Recipe Syntax 5.1.1 Splitting Recipe Lines ---------------------------- One of the few ways in which 'make' does interpret recipes is checking for a backslash just before the newline. As in normal makefile syntax, a single logical recipe line can be split into multiple physical lines in the makefile by placing a backslash before each newline. A sequence of lines like this is considered a single recipe line, and one instance of the shell will be invoked to run it. However, in contrast to how they are treated in other places in a makefile (*note Splitting Long Lines: Splitting Lines.), backslash/newline pairs are _not_ removed from the recipe. Both the backslash and the newline characters are preserved and passed to the shell. How the backslash/newline is interpreted depends on your shell. If the first character of the next line after the backslash/newline is the recipe prefix character (a tab by default; *note Special Variables::), then that character (and only that character) is removed. Whitespace is never added to the recipe. For example, the recipe for the all target in this makefile: all : @echo no\ space @echo no\ space @echo one \ space @echo one\ space consists of four separate shell commands where the output is: nospace nospace one space one space As a more complex example, this makefile: all : ; @echo 'hello \ world' ; echo "hello \ world" will invoke one shell with a command of: echo 'hello \ world' ; echo "hello \ world" which, according to shell quoting rules, will yield the following output: hello \ world hello world Notice how the backslash/newline pair was removed inside the string quoted with double quotes ('"..."'), but not from the string quoted with single quotes (''...''). This is the way the default shell ('/bin/sh') handles backslash/newline pairs. If you specify a different shell in your makefiles it may treat them differently. Sometimes you want to split a long line inside of single quotes, but you don't want the backslash/newline to appear in the quoted content. This is often the case when passing scripts to languages such as Perl, where extraneous backslashes inside the script can change its meaning or even be a syntax error. One simple way of handling this is to place the quoted string, or even the entire command, into a 'make' variable then use the variable in the recipe. In this situation the newline quoting rules for makefiles will be used, and the backslash/newline will be removed. If we rewrite our example above using this method: HELLO = 'hello \ world' all : ; @echo $(HELLO) we will get output like this: hello world If you like, you can also use target-specific variables (*note Target-specific Variable Values: Target-specific.) to obtain a tighter correspondence between the variable and the recipe that uses it.  File: make.info, Node: Variables in Recipes, Prev: Splitting Recipe Lines, Up: Recipe Syntax 5.1.2 Using Variables in Recipes -------------------------------- The other way in which 'make' processes recipes is by expanding any variable references in them (*note Basics of Variable References: Reference.). This occurs after make has finished reading all the makefiles and the target is determined to be out of date; so, the recipes for targets which are not rebuilt are never expanded. Variable and function references in recipes have identical syntax and semantics to references elsewhere in the makefile. They also have the same quoting rules: if you want a dollar sign to appear in your recipe, you must double it ('$$'). For shells like the default shell, that use dollar signs to introduce variables, it's important to keep clear in your mind whether the variable you want to reference is a 'make' variable (use a single dollar sign) or a shell variable (use two dollar signs). For example: LIST = one two three all: for i in $(LIST); do \ echo $$i; \ done results in the following command being passed to the shell: for i in one two three; do \ echo $i; \ done which generates the expected result: one two three  File: make.info, Node: Echoing, Next: Execution, Prev: Recipe Syntax, Up: Recipes 5.2 Recipe Echoing ================== Normally 'make' prints each line of the recipe before it is executed. We call this "echoing" because it gives the appearance that you are typing the lines yourself. When a line starts with '@', the echoing of that line is suppressed. The '@' is discarded before the line is passed to the shell. Typically you would use this for a command whose only effect is to print something, such as an 'echo' command to indicate progress through the makefile: @echo About to make distribution files When 'make' is given the flag '-n' or '--just-print' it only echoes most recipes, without executing them. *Note Summary of Options: Options Summary. In this case even the recipe lines starting with '@' are printed. This flag is useful for finding out which recipes 'make' thinks are necessary without actually doing them. The '-s' or '--silent' flag to 'make' prevents all echoing, as if all recipes started with '@'. A rule in the makefile for the special target '.SILENT' without prerequisites has the same effect (*note Special Built-in Target Names: Special Targets.). '.SILENT' is essentially obsolete since '@' is more flexible.  File: make.info, Node: Execution, Next: Parallel, Prev: Echoing, Up: Recipes 5.3 Recipe Execution ==================== When it is time to execute recipes to update a target, they are executed by invoking a new sub-shell for each line of the recipe, unless the '.ONESHELL' special target is in effect (*note Using One Shell: One Shell.) (In practice, 'make' may take shortcuts that do not affect the results.) *Please note:* this implies that setting shell variables and invoking shell commands such as 'cd' that set a context local to each process will not affect the following lines in the recipe.(1) If you want to use 'cd' to affect the next statement, put both statements in a single recipe line. Then 'make' will invoke one shell to run the entire line, and the shell will execute the statements in sequence. For example: foo : bar/lose cd $(@D) && gobble $(@F) > ../$@ Here we use the shell AND operator ('&&') so that if the 'cd' command fails, the script will fail without trying to invoke the 'gobble' command in the wrong directory, which could cause problems (in this case it would certainly cause '../foo' to be truncated, at least). * Menu: * One Shell:: One shell for all lines in a recipe. * Choosing the Shell:: How 'make' chooses the shell used to run recipes. ---------- Footnotes ---------- (1) On MS-DOS, the value of current working directory is *global*, so changing it _will_ affect the following recipe lines on those systems.  File: make.info, Node: One Shell, Next: Choosing the Shell, Prev: Execution, Up: Execution 5.3.1 Using One Shell --------------------- Sometimes you would prefer that all the lines in the recipe be passed to a single invocation of the shell. There are generally two situations where this is useful: first, it can improve performance in makefiles where recipes consist of many command lines, by avoiding extra processes. Second, you might want newlines to be included in your recipe command (for example perhaps you are using a very different interpreter as your 'SHELL'). If the '.ONESHELL' special target appears anywhere in the makefile then _all_ recipe lines for each target will be provided to a single invocation of the shell. Newlines between recipe lines will be preserved. For example: .ONESHELL: foo : bar/lose cd $(@D) gobble $(@F) > ../$@ would now work as expected even though the commands are on different recipe lines. If '.ONESHELL' is provided, then only the first line of the recipe will be checked for the special prefix characters ('@', '-', and '+'). Subsequent lines will include the special characters in the recipe line when the 'SHELL' is invoked. If you want your recipe to start with one of these special characters you'll need to arrange for them to not be the first characters on the first line, perhaps by adding a comment or similar. For example, this would be a syntax error in Perl because the first '@' is removed by make: .ONESHELL: SHELL = /usr/bin/perl .SHELLFLAGS = -e show : @f = qw(a b c); print "@f\n"; However, either of these alternatives would work properly: .ONESHELL: SHELL = /usr/bin/perl .SHELLFLAGS = -e show : # Make sure "@" is not the first character on the first line @f = qw(a b c); print "@f\n"; or .ONESHELL: SHELL = /usr/bin/perl .SHELLFLAGS = -e show : my @f = qw(a b c); print "@f\n"; As a special feature, if 'SHELL' is determined to be a POSIX-style shell, the special prefix characters in "internal" recipe lines will _removed_ before the recipe is processed. This feature is intended to allow existing makefiles to add the '.ONESHELL' special target and still run properly without extensive modifications. Since the special prefix characters are not legal at the beginning of a line in a POSIX shell script this is not a loss in functionality. For example, this works as expected: .ONESHELL: foo : bar/lose @cd $(@D) @gobble $(@F) > ../$@ Even with this special feature, however, makefiles with '.ONESHELL' will behave differently in ways that could be noticeable. For example, normally if any line in the recipe fails, that causes the rule to fail and no more recipe lines are processed. Under '.ONESHELL' a failure of any but the final recipe line will not be noticed by 'make'. You can modify '.SHELLFLAGS' to add the '-e' option to the shell which will cause any failure anywhere in the command line to cause the shell to fail, but this could itself cause your recipe to behave differently. Ultimately you may need to harden your recipe lines to allow them to work with '.ONESHELL'.  File: make.info, Node: Choosing the Shell, Prev: One Shell, Up: Execution 5.3.2 Choosing the Shell ------------------------ The program used as the shell is taken from the variable 'SHELL'. If this variable is not set in your makefile, the program '/bin/sh' is used as the shell. The argument(s) passed to the shell are taken from the variable '.SHELLFLAGS'. The default value of '.SHELLFLAGS' is '-c' normally, or '-ec' in POSIX-conforming mode. Unlike most variables, the variable 'SHELL' is never set from the environment. This is because the 'SHELL' environment variable is used to specify your personal choice of shell program for interactive use. It would be very bad for personal choices like this to affect the functioning of makefiles. *Note Variables from the Environment: Environment. Furthermore, when you do set 'SHELL' in your makefile that value is _not_ exported in the environment to recipe lines that 'make' invokes. Instead, the value inherited from the user's environment, if any, is exported. You can override this behavior by explicitly exporting 'SHELL' (*note Communicating Variables to a Sub-'make': Variables/Recursion.), forcing it to be passed in the environment to recipe lines. However, on MS-DOS and MS-Windows the value of 'SHELL' in the environment *is* used, since on those systems most users do not set this variable, and therefore it is most likely set specifically to be used by 'make'. On MS-DOS, if the setting of 'SHELL' is not suitable for 'make', you can set the variable 'MAKESHELL' to the shell that 'make' should use; if set it will be used as the shell instead of the value of 'SHELL'. Choosing a Shell in DOS and Windows ................................... Choosing a shell in MS-DOS and MS-Windows is much more complex than on other systems. On MS-DOS, if 'SHELL' is not set, the value of the variable 'COMSPEC' (which is always set) is used instead. The processing of lines that set the variable 'SHELL' in Makefiles is different on MS-DOS. The stock shell, 'command.com', is ridiculously limited in its functionality and many users of 'make' tend to install a replacement shell. Therefore, on MS-DOS, 'make' examines the value of 'SHELL', and changes its behavior based on whether it points to a Unix-style or DOS-style shell. This allows reasonable functionality even if 'SHELL' points to 'command.com'. If 'SHELL' points to a Unix-style shell, 'make' on MS-DOS additionally checks whether that shell can indeed be found; if not, it ignores the line that sets 'SHELL'. In MS-DOS, GNU 'make' searches for the shell in the following places: 1. In the precise place pointed to by the value of 'SHELL'. For example, if the makefile specifies 'SHELL = /bin/sh', 'make' will look in the directory '/bin' on the current drive. 2. In the current directory. 3. In each of the directories in the 'PATH' variable, in order. In every directory it examines, 'make' will first look for the specific file ('sh' in the example above). If this is not found, it will also look in that directory for that file with one of the known extensions which identify executable files. For example '.exe', '.com', '.bat', '.btm', '.sh', and some others. If any of these attempts is successful, the value of 'SHELL' will be set to the full pathname of the shell as found. However, if none of these is found, the value of 'SHELL' will not be changed, and thus the line that sets it will be effectively ignored. This is so 'make' will only support features specific to a Unix-style shell if such a shell is actually installed on the system where 'make' runs. Note that this extended search for the shell is limited to the cases where 'SHELL' is set from the Makefile; if it is set in the environment or command line, you are expected to set it to the full pathname of the shell, exactly as things are on Unix. The effect of the above DOS-specific processing is that a Makefile that contains 'SHELL = /bin/sh' (as many Unix makefiles do), will work on MS-DOS unaltered if you have e.g. 'sh.exe' installed in some directory along your 'PATH'.  File: make.info, Node: Parallel, Next: Errors, Prev: Execution, Up: Recipes 5.4 Parallel Execution ====================== GNU 'make' knows how to execute several recipes at once. Normally, 'make' will execute only one recipe at a time, waiting for it to finish before executing the next. However, the '-j' or '--jobs' option tells 'make' to execute many recipes simultaneously. You can inhibit parallelism in a particular makefile with the '.NOTPARALLEL' pseudo-target (*note Special Built-in Target Names: Special Targets.). On MS-DOS, the '-j' option has no effect, since that system doesn't support multi-processing. If the '-j' option is followed by an integer, this is the number of recipes to execute at once; this is called the number of "job slots". If there is nothing looking like an integer after the '-j' option, there is no limit on the number of job slots. The default number of job slots is one, which means serial execution (one thing at a time). Handling recursive 'make' invocations raises issues for parallel execution. For more information on this, see *note Communicating Options to a Sub-'make': Options/Recursion. If a recipe fails (is killed by a signal or exits with a nonzero status), and errors are not ignored for that recipe (*note Errors in Recipes: Errors.), the remaining recipe lines to remake the same target will not be run. If a recipe fails and the '-k' or '--keep-going' option was not given (*note Summary of Options: Options Summary.), 'make' aborts execution. If make terminates for any reason (including a signal) with child processes running, it waits for them to finish before actually exiting. When the system is heavily loaded, you will probably want to run fewer jobs than when it is lightly loaded. You can use the '-l' option to tell 'make' to limit the number of jobs to run at once, based on the load average. The '-l' or '--max-load' option is followed by a floating-point number. For example, -l 2.5 will not let 'make' start more than one job if the load average is above 2.5. The '-l' option with no following number removes the load limit, if one was given with a previous '-l' option. More precisely, when 'make' goes to start up a job, and it already has at least one job running, it checks the current load average; if it is not lower than the limit given with '-l', 'make' waits until the load average goes below that limit, or until all the other jobs finish. By default, there is no load limit. * Menu: * Parallel Output:: Handling output during parallel execution * Parallel Input:: Handling input during parallel execution  File: make.info, Node: Parallel Output, Next: Parallel Input, Prev: Parallel, Up: Parallel 5.4.1 Output During Parallel Execution -------------------------------------- When running several recipes in parallel the output from each recipe appears as soon as it is generated, with the result that messages from different recipes may be interspersed, sometimes even appearing on the same line. This can make reading the output very difficult. To avoid this you can use the '--output-sync' ('-O') option. This option instructs 'make' to save the output from the commands it invokes and print it all once the commands are completed. Additionally, if there are multiple recursive 'make' invocations running in parallel, they will communicate so that only one of them is generating output at a time. If working directory printing is enabled (*note The '--print-directory' Option: -w Option.), the enter/leave messages are printed around each output grouping. If you prefer not to see these messages add the '--no-print-directory' option to 'MAKEFLAGS'. There are four levels of granularity when synchronizing output, specified by giving an argument to the option (e.g., '-Oline' or '--output-sync=recurse'). 'none' This is the default: all output is sent directly as it is generated and no synchronization is performed. 'line' Output from each individual line of the recipe is grouped and printed as soon as that line is complete. If a recipe consists of multiple lines, they may be interspersed with lines from other recipes. 'target' Output from the entire recipe for each target is grouped and printed once the target is complete. This is the default if the '--output-sync' or '-O' option is given with no argument. 'recurse' Output from each recursive invocation of 'make' is grouped and printed once the recursive invocation is complete. Regardless of the mode chosen, the total build time will be the same. The only difference is in how the output appears. The 'target' and 'recurse' modes both collect the output of the entire recipe of a target and display it uninterrupted when the recipe completes. The difference between them is in how recipes that contain recursive invocations of 'make' are treated (*note Recursive Use of 'make': Recursion.). For all recipes which have no recursive lines, the 'target' and 'recurse' modes behave identically. If the 'recurse' mode is chosen, recipes that contain recursive 'make' invocations are treated the same as other targets: the output from the recipe, including the output from the recursive 'make', is saved and printed after the entire recipe is complete. This ensures output from all the targets built by a given recursive 'make' instance are grouped together, which may make the output easier to understand. However it also leads to long periods of time during the build where no output is seen, followed by large bursts of output. If you are not watching the build as it proceeds, but instead viewing a log of the build after the fact, this may be the best option for you. If you are watching the output, the long gaps of quiet during the build can be frustrating. The 'target' output synchronization mode detects when 'make' is going to be invoked recursively, using the standard methods, and it will not synchronize the output of those lines. The recursive 'make' will perform the synchronization for its targets and the output from each will be displayed immediately when it completes. Be aware that output from recursive lines of the recipe are not synchronized (for example if the recursive line prints a message before running 'make', that message will not be synchronized). The 'line' mode can be useful for front-ends that are watching the output of 'make' to track when recipes are started and completed. Some programs invoked by 'make' may behave differently if they determine they're writing output to a terminal versus a file (often described as "interactive" vs. "non-interactive" modes). For example, many programs that can display colorized output will not do so if they determine they are not writing to a terminal. If your makefile invokes a program like this then using the output synchronization options will cause the program to believe it's running in "non-interactive" mode even though the output will ultimately go to the terminal.  File: make.info, Node: Parallel Input, Prev: Parallel Output, Up: Parallel 5.4.2 Input During Parallel Execution ------------------------------------- Two processes cannot both take input from the same device at the same time. To make sure that only one recipe tries to take input from the terminal at once, 'make' will invalidate the standard input streams of all but one running recipe. If another recipe attempts to read from standard input it will usually incur a fatal error (a 'Broken pipe' signal). It is unpredictable which recipe will have a valid standard input stream (which will come from the terminal, or wherever you redirect the standard input of 'make'). The first recipe run will always get it first, and the first recipe started after that one finishes will get it next, and so on. We will change how this aspect of 'make' works if we find a better alternative. In the mean time, you should not rely on any recipe using standard input at all if you are using the parallel execution feature; but if you are not using this feature, then standard input works normally in all recipes.  File: make.info, Node: Errors, Next: Interrupts, Prev: Parallel, Up: Recipes 5.5 Errors in Recipes ===================== After each shell invocation returns, 'make' looks at its exit status. If the shell completed successfully (the exit status is zero), the next line in the recipe is executed in a new shell; after the last line is finished, the rule is finished. If there is an error (the exit status is nonzero), 'make' gives up on the current rule, and perhaps on all rules. Sometimes the failure of a certain recipe line does not indicate a problem. For example, you may use the 'mkdir' command to ensure that a directory exists. If the directory already exists, 'mkdir' will report an error, but you probably want 'make' to continue regardless. To ignore errors in a recipe line, write a '-' at the beginning of the line's text (after the initial tab). The '-' is discarded before the line is passed to the shell for execution. For example, clean: -rm -f *.o This causes 'make' to continue even if 'rm' is unable to remove a file. When you run 'make' with the '-i' or '--ignore-errors' flag, errors are ignored in all recipes of all rules. A rule in the makefile for the special target '.IGNORE' has the same effect, if there are no prerequisites. These ways of ignoring errors are obsolete because '-' is more flexible. When errors are to be ignored, because of either a '-' or the '-i' flag, 'make' treats an error return just like success, except that it prints out a message that tells you the status code the shell exited with, and says that the error has been ignored. When an error happens that 'make' has not been told to ignore, it implies that the current target cannot be correctly remade, and neither can any other that depends on it either directly or indirectly. No further recipes will be executed for these targets, since their preconditions have not been achieved. Normally 'make' gives up immediately in this circumstance, returning a nonzero status. However, if the '-k' or '--keep-going' flag is specified, 'make' continues to consider the other prerequisites of the pending targets, remaking them if necessary, before it gives up and returns nonzero status. For example, after an error in compiling one object file, 'make -k' will continue compiling other object files even though it already knows that linking them will be impossible. *Note Summary of Options: Options Summary. The usual behavior assumes that your purpose is to get the specified targets up to date; once 'make' learns that this is impossible, it might as well report the failure immediately. The '-k' option says that the real purpose is to test as many of the changes made in the program as possible, perhaps to find several independent problems so that you can correct them all before the next attempt to compile. This is why Emacs' 'compile' command passes the '-k' flag by default. Usually when a recipe line fails, if it has changed the target file at all, the file is corrupted and cannot be used--or at least it is not completely updated. Yet the file's time stamp says that it is now up to date, so the next time 'make' runs, it will not try to update that file. The situation is just the same as when the shell is killed by a signal; *note Interrupts::. So generally the right thing to do is to delete the target file if the recipe fails after beginning to change the file. 'make' will do this if '.DELETE_ON_ERROR' appears as a target. This is almost always what you want 'make' to do, but it is not historical practice; so for compatibility, you must explicitly request it.  File: make.info, Node: Interrupts, Next: Recursion, Prev: Errors, Up: Recipes 5.6 Interrupting or Killing 'make' ================================== If 'make' gets a fatal signal while a shell is executing, it may delete the target file that the recipe was supposed to update. This is done if the target file's last-modification time has changed since 'make' first checked it. The purpose of deleting the target is to make sure that it is remade from scratch when 'make' is next run. Why is this? Suppose you type 'Ctrl-c' while a compiler is running, and it has begun to write an object file 'foo.o'. The 'Ctrl-c' kills the compiler, resulting in an incomplete file whose last-modification time is newer than the source file 'foo.c'. But 'make' also receives the 'Ctrl-c' signal and deletes this incomplete file. If 'make' did not do this, the next invocation of 'make' would think that 'foo.o' did not require updating--resulting in a strange error message from the linker when it tries to link an object file half of which is missing. You can prevent the deletion of a target file in this way by making the special target '.PRECIOUS' depend on it. Before remaking a target, 'make' checks to see whether it appears on the prerequisites of '.PRECIOUS', and thereby decides whether the target should be deleted if a signal happens. Some reasons why you might do this are that the target is updated in some atomic fashion, or exists only to record a modification-time (its contents do not matter), or must exist at all times to prevent other sorts of trouble.  File: make.info, Node: Recursion, Next: Canned Recipes, Prev: Interrupts, Up: Recipes 5.7 Recursive Use of 'make' =========================== Recursive use of 'make' means using 'make' as a command in a makefile. This technique is useful when you want separate makefiles for various subsystems that compose a larger system. For example, suppose you have a sub-directory 'subdir' which has its own makefile, and you would like the containing directory's makefile to run 'make' on the sub-directory. You can do it by writing this: subsystem: cd subdir && $(MAKE) or, equivalently, this (*note Summary of Options: Options Summary.): subsystem: $(MAKE) -C subdir You can write recursive 'make' commands just by copying this example, but there are many things to know about how they work and why, and about how the sub-'make' relates to the top-level 'make'. You may also find it useful to declare targets that invoke recursive 'make' commands as '.PHONY' (for more discussion on when this is useful, see *note Phony Targets::). For your convenience, when GNU 'make' starts (after it has processed any '-C' options) it sets the variable 'CURDIR' to the pathname of the current working directory. This value is never touched by 'make' again: in particular note that if you include files from other directories the value of 'CURDIR' does not change. The value has the same precedence it would have if it were set in the makefile (by default, an environment variable 'CURDIR' will not override this value). Note that setting this variable has no impact on the operation of 'make' (it does not cause 'make' to change its working directory, for example). * Menu: * MAKE Variable:: The special effects of using '$(MAKE)'. * Variables/Recursion:: How to communicate variables to a sub-'make'. * Options/Recursion:: How to communicate options to a sub-'make'. * -w Option:: How the '-w' or '--print-directory' option helps debug use of recursive 'make' commands.  File: make.info, Node: MAKE Variable, Next: Variables/Recursion, Prev: Recursion, Up: Recursion 5.7.1 How the 'MAKE' Variable Works ----------------------------------- Recursive 'make' commands should always use the variable 'MAKE', not the explicit command name 'make', as shown here: subsystem: cd subdir && $(MAKE) The value of this variable is the file name with which 'make' was invoked. If this file name was '/bin/make', then the recipe executed is 'cd subdir && /bin/make'. If you use a special version of 'make' to run the top-level makefile, the same special version will be executed for recursive invocations. As a special feature, using the variable 'MAKE' in the recipe of a rule alters the effects of the '-t' ('--touch'), '-n' ('--just-print'), or '-q' ('--question') option. Using the 'MAKE' variable has the same effect as using a '+' character at the beginning of the recipe line. *Note Instead of Executing the Recipes: Instead of Execution. This special feature is only enabled if the 'MAKE' variable appears directly in the recipe: it does not apply if the 'MAKE' variable is referenced through expansion of another variable. In the latter case you must use the '+' token to get these special effects. Consider the command 'make -t' in the above example. (The '-t' option marks targets as up to date without actually running any recipes; see *note Instead of Execution::.) Following the usual definition of '-t', a 'make -t' command in the example would create a file named 'subsystem' and do nothing else. What you really want it to do is run 'cd subdir && make -t'; but that would require executing the recipe, and '-t' says not to execute recipes. The special feature makes this do what you want: whenever a recipe line of a rule contains the variable 'MAKE', the flags '-t', '-n' and '-q' do not apply to that line. Recipe lines containing 'MAKE' are executed normally despite the presence of a flag that causes most recipes not to be run. The usual 'MAKEFLAGS' mechanism passes the flags to the sub-'make' (*note Communicating Options to a Sub-'make': Options/Recursion.), so your request to touch the files, or print the recipes, is propagated to the subsystem.  File: make.info, Node: Variables/Recursion, Next: Options/Recursion, Prev: MAKE Variable, Up: Recursion 5.7.2 Communicating Variables to a Sub-'make' --------------------------------------------- Variable values of the top-level 'make' can be passed to the sub-'make' through the environment by explicit request. These variables are defined in the sub-'make' as defaults, but they do not override variables defined in the makefile used by the sub-'make' unless you use the '-e' switch (*note Summary of Options: Options Summary.). To pass down, or "export", a variable, 'make' adds the variable and its value to the environment for running each line of the recipe. The sub-'make', in turn, uses the environment to initialize its table of variable values. *Note Variables from the Environment: Environment. Except by explicit request, 'make' exports a variable only if it is either defined in the environment initially or set on the command line, and if its name consists only of letters, numbers, and underscores. Some shells cannot cope with environment variable names consisting of characters other than letters, numbers, and underscores. The value of the 'make' variable 'SHELL' is not exported. Instead, the value of the 'SHELL' variable from the invoking environment is passed to the sub-'make'. You can force 'make' to export its value for 'SHELL' by using the 'export' directive, described below. *Note Choosing the Shell::. The special variable 'MAKEFLAGS' is always exported (unless you unexport it). 'MAKEFILES' is exported if you set it to anything. 'make' automatically passes down variable values that were defined on the command line, by putting them in the 'MAKEFLAGS' variable. *Note Options/Recursion::. Variables are _not_ normally passed down if they were created by default by 'make' (*note Variables Used by Implicit Rules: Implicit Variables.). The sub-'make' will define these for itself. If you want to export specific variables to a sub-'make', use the 'export' directive, like this: export VARIABLE ... If you want to _prevent_ a variable from being exported, use the 'unexport' directive, like this: unexport VARIABLE ... In both of these forms, the arguments to 'export' and 'unexport' are expanded, and so could be variables or functions which expand to a (list of) variable names to be (un)exported. As a convenience, you can define a variable and export it at the same time by doing: export VARIABLE = value has the same result as: VARIABLE = value export VARIABLE and export VARIABLE := value has the same result as: VARIABLE := value export VARIABLE Likewise, export VARIABLE += value is just like: VARIABLE += value export VARIABLE *Note Appending More Text to Variables: Appending. You may notice that the 'export' and 'unexport' directives work in 'make' in the same way they work in the shell, 'sh'. If you want all variables to be exported by default, you can use 'export' by itself: export This tells 'make' that variables which are not explicitly mentioned in an 'export' or 'unexport' directive should be exported. Any variable given in an 'unexport' directive will still _not_ be exported. If you use 'export' by itself to export variables by default, variables whose names contain characters other than alphanumerics and underscores will not be exported unless specifically mentioned in an 'export' directive. The behavior elicited by an 'export' directive by itself was the default in older versions of GNU 'make'. If your makefiles depend on this behavior and you want to be compatible with old versions of 'make', you can write a rule for the special target '.EXPORT_ALL_VARIABLES' instead of using the 'export' directive. This will be ignored by old 'make's, while the 'export' directive will cause a syntax error. Likewise, you can use 'unexport' by itself to tell 'make' _not_ to export variables by default. Since this is the default behavior, you would only need to do this if 'export' had been used by itself earlier (in an included makefile, perhaps). You *cannot* use 'export' and 'unexport' by themselves to have variables exported for some recipes and not for others. The last 'export' or 'unexport' directive that appears by itself determines the behavior for the entire run of 'make'. As a special feature, the variable 'MAKELEVEL' is changed when it is passed down from level to level. This variable's value is a string which is the depth of the level as a decimal number. The value is '0' for the top-level 'make'; '1' for a sub-'make', '2' for a sub-sub-'make', and so on. The incrementation happens when 'make' sets up the environment for a recipe. The main use of 'MAKELEVEL' is to test it in a conditional directive (*note Conditional Parts of Makefiles: Conditionals.); this way you can write a makefile that behaves one way if run recursively and another way if run directly by you. You can use the variable 'MAKEFILES' to cause all sub-'make' commands to use additional makefiles. The value of 'MAKEFILES' is a whitespace-separated list of file names. This variable, if defined in the outer-level makefile, is passed down through the environment; then it serves as a list of extra makefiles for the sub-'make' to read before the usual or specified ones. *Note The Variable 'MAKEFILES': MAKEFILES Variable.  File: make.info, Node: Options/Recursion, Next: -w Option, Prev: Variables/Recursion, Up: Recursion 5.7.3 Communicating Options to a Sub-'make' ------------------------------------------- Flags such as '-s' and '-k' are passed automatically to the sub-'make' through the variable 'MAKEFLAGS'. This variable is set up automatically by 'make' to contain the flag letters that 'make' received. Thus, if you do 'make -ks' then 'MAKEFLAGS' gets the value 'ks'. As a consequence, every sub-'make' gets a value for 'MAKEFLAGS' in its environment. In response, it takes the flags from that value and processes them as if they had been given as arguments. *Note Summary of Options: Options Summary. Likewise variables defined on the command line are passed to the sub-'make' through 'MAKEFLAGS'. Words in the value of 'MAKEFLAGS' that contain '=', 'make' treats as variable definitions just as if they appeared on the command line. *Note Overriding Variables: Overriding. The options '-C', '-f', '-o', and '-W' are not put into 'MAKEFLAGS'; these options are not passed down. The '-j' option is a special case (*note Parallel Execution: Parallel.). If you set it to some numeric value 'N' and your operating system supports it (most any UNIX system will; others typically won't), the parent 'make' and all the sub-'make's will communicate to ensure that there are only 'N' jobs running at the same time between them all. Note that any job that is marked recursive (*note Instead of Executing Recipes: Instead of Execution.) doesn't count against the total jobs (otherwise we could get 'N' sub-'make's running and have no slots left over for any real work!) If your operating system doesn't support the above communication, then '-j 1' is always put into 'MAKEFLAGS' instead of the value you specified. This is because if the '-j' option were passed down to sub-'make's, you would get many more jobs running in parallel than you asked for. If you give '-j' with no numeric argument, meaning to run as many jobs as possible in parallel, this is passed down, since multiple infinities are no more than one. If you do not want to pass the other flags down, you must change the value of 'MAKEFLAGS', like this: subsystem: cd subdir && $(MAKE) MAKEFLAGS= The command line variable definitions really appear in the variable 'MAKEOVERRIDES', and 'MAKEFLAGS' contains a reference to this variable. If you do want to pass flags down normally, but don't want to pass down the command line variable definitions, you can reset 'MAKEOVERRIDES' to empty, like this: MAKEOVERRIDES = This is not usually useful to do. However, some systems have a small fixed limit on the size of the environment, and putting so much information into the value of 'MAKEFLAGS' can exceed it. If you see the error message 'Arg list too long', this may be the problem. (For strict compliance with POSIX.2, changing 'MAKEOVERRIDES' does not affect 'MAKEFLAGS' if the special target '.POSIX' appears in the makefile. You probably do not care about this.) A similar variable 'MFLAGS' exists also, for historical compatibility. It has the same value as 'MAKEFLAGS' except that it does not contain the command line variable definitions, and it always begins with a hyphen unless it is empty ('MAKEFLAGS' begins with a hyphen only when it begins with an option that has no single-letter version, such as '--warn-undefined-variables'). 'MFLAGS' was traditionally used explicitly in the recursive 'make' command, like this: subsystem: cd subdir && $(MAKE) $(MFLAGS) but now 'MAKEFLAGS' makes this usage redundant. If you want your makefiles to be compatible with old 'make' programs, use this technique; it will work fine with more modern 'make' versions too. The 'MAKEFLAGS' variable can also be useful if you want to have certain options, such as '-k' (*note Summary of Options: Options Summary.), set each time you run 'make'. You simply put a value for 'MAKEFLAGS' in your environment. You can also set 'MAKEFLAGS' in a makefile, to specify additional flags that should also be in effect for that makefile. (Note that you cannot use 'MFLAGS' this way. That variable is set only for compatibility; 'make' does not interpret a value you set for it in any way.) When 'make' interprets the value of 'MAKEFLAGS' (either from the environment or from a makefile), it first prepends a hyphen if the value does not already begin with one. Then it chops the value into words separated by blanks, and parses these words as if they were options given on the command line (except that '-C', '-f', '-h', '-o', '-W', and their long-named versions are ignored; and there is no error for an invalid option). If you do put 'MAKEFLAGS' in your environment, you should be sure not to include any options that will drastically affect the actions of 'make' and undermine the purpose of makefiles and of 'make' itself. For instance, the '-t', '-n', and '-q' options, if put in one of these variables, could have disastrous consequences and would certainly have at least surprising and probably annoying effects. If you'd like to run other implementations of 'make' in addition to GNU 'make', and hence do not want to add GNU 'make'-specific flags to the 'MAKEFLAGS' variable, you can add them to the 'GNUMAKEFLAGS' variable instead. This variable is parsed just before 'MAKEFLAGS', in the same way as 'MAKEFLAGS'. When 'make' constructs 'MAKEFLAGS' to pass to a recursive 'make' it will include all flags, even those taken from 'GNUMAKEFLAGS'. As a result, after parsing 'GNUMAKEFLAGS' GNU 'make' sets this variable to the empty string to avoid duplicating flags during recursion. It's best to use 'GNUMAKEFLAGS' only with flags which won't materially change the behavior of your makefiles. If your makefiles require GNU make anyway then simply use 'MAKEFLAGS'. Flags such as '--no-print-directory' or '--output-sync' may be appropriate for 'GNUMAKEFLAGS'.  File: make.info, Node: -w Option, Prev: Options/Recursion, Up: Recursion 5.7.4 The '--print-directory' Option ------------------------------------ If you use several levels of recursive 'make' invocations, the '-w' or '--print-directory' option can make the output a lot easier to understand by showing each directory as 'make' starts processing it and as 'make' finishes processing it. For example, if 'make -w' is run in the directory '/u/gnu/make', 'make' will print a line of the form: make: Entering directory `/u/gnu/make'. before doing anything else, and a line of the form: make: Leaving directory `/u/gnu/make'. when processing is completed. Normally, you do not need to specify this option because 'make' does it for you: '-w' is turned on automatically when you use the '-C' option, and in sub-'make's. 'make' will not automatically turn on '-w' if you also use '-s', which says to be silent, or if you use '--no-print-directory' to explicitly disable it.  File: make.info, Node: Canned Recipes, Next: Empty Recipes, Prev: Recursion, Up: Recipes 5.8 Defining Canned Recipes =========================== When the same sequence of commands is useful in making various targets, you can define it as a canned sequence with the 'define' directive, and refer to the canned sequence from the recipes for those targets. The canned sequence is actually a variable, so the name must not conflict with other variable names. Here is an example of defining a canned recipe: define run-yacc = yacc $(firstword $^) mv y.tab.c $@ endef Here 'run-yacc' is the name of the variable being defined; 'endef' marks the end of the definition; the lines in between are the commands. The 'define' directive does not expand variable references and function calls in the canned sequence; the '$' characters, parentheses, variable names, and so on, all become part of the value of the variable you are defining. *Note Defining Multi-Line Variables: Multi-Line, for a complete explanation of 'define'. The first command in this example runs Yacc on the first prerequisite of whichever rule uses the canned sequence. The output file from Yacc is always named 'y.tab.c'. The second command moves the output to the rule's target file name. To use the canned sequence, substitute the variable into the recipe of a rule. You can substitute it like any other variable (*note Basics of Variable References: Reference.). Because variables defined by 'define' are recursively expanded variables, all the variable references you wrote inside the 'define' are expanded now. For example: foo.c : foo.y $(run-yacc) 'foo.y' will be substituted for the variable '$^' when it occurs in 'run-yacc''s value, and 'foo.c' for '$@'. This is a realistic example, but this particular one is not needed in practice because 'make' has an implicit rule to figure out these commands based on the file names involved (*note Using Implicit Rules: Implicit Rules.). In recipe execution, each line of a canned sequence is treated just as if the line appeared on its own in the rule, preceded by a tab. In particular, 'make' invokes a separate sub-shell for each line. You can use the special prefix characters that affect command lines ('@', '-', and '+') on each line of a canned sequence. *Note Writing Recipes in Rules: Recipes. For example, using this canned sequence: define frobnicate = @echo "frobnicating target $@" frob-step-1 $< -o $@-step-1 frob-step-2 $@-step-1 -o $@ endef 'make' will not echo the first line, the 'echo' command. But it _will_ echo the following two recipe lines. On the other hand, prefix characters on the recipe line that refers to a canned sequence apply to every line in the sequence. So the rule: frob.out: frob.in @$(frobnicate) does not echo _any_ recipe lines. (*Note Recipe Echoing: Echoing, for a full explanation of '@'.)  File: make.info, Node: Empty Recipes, Prev: Canned Recipes, Up: Recipes 5.9 Using Empty Recipes ======================= It is sometimes useful to define recipes which do nothing. This is done simply by giving a recipe that consists of nothing but whitespace. For example: target: ; defines an empty recipe for 'target'. You could also use a line beginning with a recipe prefix character to define an empty recipe, but this would be confusing because such a line looks empty. You may be wondering why you would want to define a recipe that does nothing. The only reason this is useful is to prevent a target from getting implicit recipes (from implicit rules or the '.DEFAULT' special target; *note Implicit Rules:: and *note Defining Last-Resort Default Rules: Last Resort.). You may be inclined to define empty recipes for targets that are not actual files, but only exist so that their prerequisites can be remade. However, this is not the best way to do that, because the prerequisites may not be remade properly if the target file actually does exist. *Note Phony Targets: Phony Targets, for a better way to do this.  File: make.info, Node: Using Variables, Next: Conditionals, Prev: Recipes, Up: Top 6 How to Use Variables ********************** A "variable" is a name defined in a makefile to represent a string of text, called the variable's "value". These values are substituted by explicit request into targets, prerequisites, recipes, and other parts of the makefile. (In some other versions of 'make', variables are called "macros".) Variables and functions in all parts of a makefile are expanded when read, except for in recipes, the right-hand sides of variable definitions using '=', and the bodies of variable definitions using the 'define' directive. Variables can represent lists of file names, options to pass to compilers, programs to run, directories to look in for source files, directories to write output in, or anything else you can imagine. A variable name may be any sequence of characters not containing ':', '#', '=', or whitespace. However, variable names containing characters other than letters, numbers, and underscores should be considered carefully, as in some shells they cannot be passed through the environment to a sub-'make' (*note Communicating Variables to a Sub-'make': Variables/Recursion.). Variable names beginning with '.' and an uppercase letter may be given special meaning in future versions of 'make'. Variable names are case-sensitive. The names 'foo', 'FOO', and 'Foo' all refer to different variables. It is traditional to use upper case letters in variable names, but we recommend using lower case letters for variable names that serve internal purposes in the makefile, and reserving upper case for parameters that control implicit rules or for parameters that the user should override with command options (*note Overriding Variables: Overriding.). A few variables have names that are a single punctuation character or just a few characters. These are the "automatic variables", and they have particular specialized uses. *Note Automatic Variables::. * Menu: * Reference:: How to use the value of a variable. * Flavors:: Variables come in two flavors. * Advanced:: Advanced features for referencing a variable. * Values:: All the ways variables get their values. * Setting:: How to set a variable in the makefile. * Appending:: How to append more text to the old value of a variable. * Override Directive:: How to set a variable in the makefile even if the user has set it with a command argument. * Multi-Line:: An alternate way to set a variable to a multi-line string. * Undefine Directive:: How to undefine a variable so that it appears as if it was never set. * Environment:: Variable values can come from the environment. * Target-specific:: Variable values can be defined on a per-target basis. * Pattern-specific:: Target-specific variable values can be applied to a group of targets that match a pattern. * Suppressing Inheritance:: Suppress inheritance of variables. * Special Variables:: Variables with special meaning or behavior.  File: make.info, Node: Reference, Next: Flavors, Prev: Using Variables, Up: Using Variables 6.1 Basics of Variable References ================================= To substitute a variable's value, write a dollar sign followed by the name of the variable in parentheses or braces: either '$(foo)' or '${foo}' is a valid reference to the variable 'foo'. This special significance of '$' is why you must write '$$' to have the effect of a single dollar sign in a file name or recipe. Variable references can be used in any context: targets, prerequisites, recipes, most directives, and new variable values. Here is an example of a common case, where a variable holds the names of all the object files in a program: objects = program.o foo.o utils.o program : $(objects) cc -o program $(objects) $(objects) : defs.h Variable references work by strict textual substitution. Thus, the rule foo = c prog.o : prog.$(foo) $(foo)$(foo) -$(foo) prog.$(foo) could be used to compile a C program 'prog.c'. Since spaces before the variable value are ignored in variable assignments, the value of 'foo' is precisely 'c'. (Don't actually write your makefiles this way!) A dollar sign followed by a character other than a dollar sign, open-parenthesis or open-brace treats that single character as the variable name. Thus, you could reference the variable 'x' with '$x'. However, this practice is strongly discouraged, except in the case of the automatic variables (*note Automatic Variables::).  File: make.info, Node: Flavors, Next: Advanced, Prev: Reference, Up: Using Variables 6.2 The Two Flavors of Variables ================================ There are two ways that a variable in GNU 'make' can have a value; we call them the two "flavors" of variables. The two flavors are distinguished in how they are defined and in what they do when expanded. The first flavor of variable is a "recursively expanded" variable. Variables of this sort are defined by lines using '=' (*note Setting Variables: Setting.) or by the 'define' directive (*note Defining Multi-Line Variables: Multi-Line.). The value you specify is installed verbatim; if it contains references to other variables, these references are expanded whenever this variable is substituted (in the course of expanding some other string). When this happens, it is called "recursive expansion". For example, foo = $(bar) bar = $(ugh) ugh = Huh? all:;echo $(foo) will echo 'Huh?': '$(foo)' expands to '$(bar)' which expands to '$(ugh)' which finally expands to 'Huh?'. This flavor of variable is the only sort supported by most other versions of 'make'. It has its advantages and its disadvantages. An advantage (most would say) is that: CFLAGS = $(include_dirs) -O include_dirs = -Ifoo -Ibar will do what was intended: when 'CFLAGS' is expanded in a recipe, it will expand to '-Ifoo -Ibar -O'. A major disadvantage is that you cannot append something on the end of a variable, as in CFLAGS = $(CFLAGS) -O because it will cause an infinite loop in the variable expansion. (Actually 'make' detects the infinite loop and reports an error.) Another disadvantage is that any functions (*note Functions for Transforming Text: Functions.) referenced in the definition will be executed every time the variable is expanded. This makes 'make' run slower; worse, it causes the 'wildcard' and 'shell' functions to give unpredictable results because you cannot easily control when they are called, or even how many times. To avoid all the problems and inconveniences of recursively expanded variables, there is another flavor: simply expanded variables. "Simply expanded variables" are defined by lines using ':=' or '::=' (*note Setting Variables: Setting.). Both forms are equivalent in GNU 'make'; however only the '::=' form is described by the POSIX standard (support for '::=' was added to the POSIX standard in 2012, so older versions of 'make' won't accept this form either). The value of a simply expanded variable is scanned once and for all, expanding any references to other variables and functions, when the variable is defined. The actual value of the simply expanded variable is the result of expanding the text that you write. It does not contain any references to other variables; it contains their values _as of the time this variable was defined_. Therefore, x := foo y := $(x) bar x := later is equivalent to y := foo bar x := later When a simply expanded variable is referenced, its value is substituted verbatim. Here is a somewhat more complicated example, illustrating the use of ':=' in conjunction with the 'shell' function. (*Note The 'shell' Function: Shell Function.) This example also shows use of the variable 'MAKELEVEL', which is changed when it is passed down from level to level. (*Note Communicating Variables to a Sub-'make': Variables/Recursion, for information about 'MAKELEVEL'.) ifeq (0,${MAKELEVEL}) whoami := $(shell whoami) host-type := $(shell arch) MAKE := ${MAKE} host-type=${host-type} whoami=${whoami} endif An advantage of this use of ':=' is that a typical 'descend into a directory' recipe then looks like this: ${subdirs}: ${MAKE} -C $@ all Simply expanded variables generally make complicated makefile programming more predictable because they work like variables in most programming languages. They allow you to redefine a variable using its own value (or its value processed in some way by one of the expansion functions) and to use the expansion functions much more efficiently (*note Functions for Transforming Text: Functions.). You can also use them to introduce controlled leading whitespace into variable values. Leading whitespace characters are discarded from your input before substitution of variable references and function calls; this means you can include leading spaces in a variable value by protecting them with variable references, like this: nullstring := space := $(nullstring) # end of the line Here the value of the variable 'space' is precisely one space. The comment '# end of the line' is included here just for clarity. Since trailing space characters are _not_ stripped from variable values, just a space at the end of the line would have the same effect (but be rather hard to read). If you put whitespace at the end of a variable value, it is a good idea to put a comment like that at the end of the line to make your intent clear. Conversely, if you do _not_ want any whitespace characters at the end of your variable value, you must remember not to put a random comment on the end of the line after some whitespace, such as this: dir := /foo/bar # directory to put the frobs in Here the value of the variable 'dir' is '/foo/bar ' (with four trailing spaces), which was probably not the intention. (Imagine something like '$(dir)/file' with this definition!) There is another assignment operator for variables, '?='. This is called a conditional variable assignment operator, because it only has an effect if the variable is not yet defined. This statement: FOO ?= bar is exactly equivalent to this (*note The 'origin' Function: Origin Function.): ifeq ($(origin FOO), undefined) FOO = bar endif Note that a variable set to an empty value is still defined, so '?=' will not set that variable.  File: make.info, Node: Advanced, Next: Values, Prev: Flavors, Up: Using Variables 6.3 Advanced Features for Reference to Variables ================================================ This section describes some advanced features you can use to reference variables in more flexible ways. * Menu: * Substitution Refs:: Referencing a variable with substitutions on the value. * Computed Names:: Computing the name of the variable to refer to.  File: make.info, Node: Substitution Refs, Next: Computed Names, Prev: Advanced, Up: Advanced 6.3.1 Substitution References ----------------------------- A "substitution reference" substitutes the value of a variable with alterations that you specify. It has the form '$(VAR:A=B)' (or '${VAR:A=B}') and its meaning is to take the value of the variable VAR, replace every A at the end of a word with B in that value, and substitute the resulting string. When we say "at the end of a word", we mean that A must appear either followed by whitespace or at the end of the value in order to be replaced; other occurrences of A in the value are unaltered. For example: foo := a.o b.o c.o bar := $(foo:.o=.c) sets 'bar' to 'a.c b.c c.c'. *Note Setting Variables: Setting. A substitution reference is actually an abbreviation for use of the 'patsubst' expansion function (*note Functions for String Substitution and Analysis: Text Functions.). We provide substitution references as well as 'patsubst' for compatibility with other implementations of 'make'. Another type of substitution reference lets you use the full power of the 'patsubst' function. It has the same form '$(VAR:A=B)' described above, except that now A must contain a single '%' character. This case is equivalent to '$(patsubst A,B,$(VAR))'. *Note Functions for String Substitution and Analysis: Text Functions, for a description of the 'patsubst' function. For example: foo := a.o b.o c.o bar := $(foo:%.o=%.c) sets 'bar' to 'a.c b.c c.c'.  File: make.info, Node: Computed Names, Prev: Substitution Refs, Up: Advanced 6.3.2 Computed Variable Names ----------------------------- Computed variable names are a complicated concept needed only for sophisticated makefile programming. For most purposes you need not consider them, except to know that making a variable with a dollar sign in its name might have strange results. However, if you are the type that wants to understand everything, or you are actually interested in what they do, read on. Variables may be referenced inside the name of a variable. This is called a "computed variable name" or a "nested variable reference". For example, x = y y = z a := $($(x)) defines 'a' as 'z': the '$(x)' inside '$($(x))' expands to 'y', so '$($(x))' expands to '$(y)' which in turn expands to 'z'. Here the name of the variable to reference is not stated explicitly; it is computed by expansion of '$(x)'. The reference '$(x)' here is nested within the outer variable reference. The previous example shows two levels of nesting, but any number of levels is possible. For example, here are three levels: x = y y = z z = u a := $($($(x))) Here the innermost '$(x)' expands to 'y', so '$($(x))' expands to '$(y)' which in turn expands to 'z'; now we have '$(z)', which becomes 'u'. References to recursively-expanded variables within a variable name are re-expanded in the usual fashion. For example: x = $(y) y = z z = Hello a := $($(x)) defines 'a' as 'Hello': '$($(x))' becomes '$($(y))' which becomes '$(z)' which becomes 'Hello'. Nested variable references can also contain modified references and function invocations (*note Functions for Transforming Text: Functions.), just like any other reference. For example, using the 'subst' function (*note Functions for String Substitution and Analysis: Text Functions.): x = variable1 variable2 := Hello y = $(subst 1,2,$(x)) z = y a := $($($(z))) eventually defines 'a' as 'Hello'. It is doubtful that anyone would ever want to write a nested reference as convoluted as this one, but it works: '$($($(z)))' expands to '$($(y))' which becomes '$($(subst 1,2,$(x)))'. This gets the value 'variable1' from 'x' and changes it by substitution to 'variable2', so that the entire string becomes '$(variable2)', a simple variable reference whose value is 'Hello'. A computed variable name need not consist entirely of a single variable reference. It can contain several variable references, as well as some invariant text. For example, a_dirs := dira dirb 1_dirs := dir1 dir2 a_files := filea fileb 1_files := file1 file2 ifeq "$(use_a)" "yes" a1 := a else a1 := 1 endif ifeq "$(use_dirs)" "yes" df := dirs else df := files endif dirs := $($(a1)_$(df)) will give 'dirs' the same value as 'a_dirs', '1_dirs', 'a_files' or '1_files' depending on the settings of 'use_a' and 'use_dirs'. Computed variable names can also be used in substitution references: a_objects := a.o b.o c.o 1_objects := 1.o 2.o 3.o sources := $($(a1)_objects:.o=.c) defines 'sources' as either 'a.c b.c c.c' or '1.c 2.c 3.c', depending on the value of 'a1'. The only restriction on this sort of use of nested variable references is that they cannot specify part of the name of a function to be called. This is because the test for a recognized function name is done before the expansion of nested references. For example, ifdef do_sort func := sort else func := strip endif bar := a d b g q c foo := $($(func) $(bar)) attempts to give 'foo' the value of the variable 'sort a d b g q c' or 'strip a d b g q c', rather than giving 'a d b g q c' as the argument to either the 'sort' or the 'strip' function. This restriction could be removed in the future if that change is shown to be a good idea. You can also use computed variable names in the left-hand side of a variable assignment, or in a 'define' directive, as in: dir = foo $(dir)_sources := $(wildcard $(dir)/*.c) define $(dir)_print = lpr $($(dir)_sources) endef This example defines the variables 'dir', 'foo_sources', and 'foo_print'. Note that "nested variable references" are quite different from "recursively expanded variables" (*note The Two Flavors of Variables: Flavors.), though both are used together in complex ways when doing makefile programming.  File: make.info, Node: Values, Next: Setting, Prev: Advanced, Up: Using Variables 6.4 How Variables Get Their Values ================================== Variables can get values in several different ways: * You can specify an overriding value when you run 'make'. *Note Overriding Variables: Overriding. * You can specify a value in the makefile, either with an assignment (*note Setting Variables: Setting.) or with a verbatim definition (*note Defining Multi-Line Variables: Multi-Line.). * Variables in the environment become 'make' variables. *Note Variables from the Environment: Environment. * Several "automatic" variables are given new values for each rule. Each of these has a single conventional use. *Note Automatic Variables::. * Several variables have constant initial values. *Note Variables Used by Implicit Rules: Implicit Variables.  File: make.info, Node: Setting, Next: Appending, Prev: Values, Up: Using Variables 6.5 Setting Variables ===================== To set a variable from the makefile, write a line starting with the variable name followed by '=' ':=', or '::='. Whatever follows the '=', ':=', or '::=' on the line becomes the value. For example, objects = main.o foo.o bar.o utils.o defines a variable named 'objects'. Whitespace around the variable name and immediately after the '=' is ignored. Variables defined with '=' are "recursively expanded" variables. Variables defined with ':=' or '::=' are "simply expanded" variables; these definitions can contain variable references which will be expanded before the definition is made. *Note The Two Flavors of Variables: Flavors. The variable name may contain function and variable references, which are expanded when the line is read to find the actual variable name to use. There is no limit on the length of the value of a variable except the amount of memory on the computer. You can split the value of a variable into multiple physical lines for readability (*note Splitting Long Lines: Splitting Lines.). Most variable names are considered to have the empty string as a value if you have never set them. Several variables have built-in initial values that are not empty, but you can set them in the usual ways (*note Variables Used by Implicit Rules: Implicit Variables.). Several special variables are set automatically to a new value for each rule; these are called the "automatic" variables (*note Automatic Variables::). If you'd like a variable to be set to a value only if it's not already set, then you can use the shorthand operator '?=' instead of '='. These two settings of the variable 'FOO' are identical (*note The 'origin' Function: Origin Function.): FOO ?= bar and ifeq ($(origin FOO), undefined) FOO = bar endif The shell assignment operator '!=' can be used to execute a program and set a variable to its output. This operator first evaluates the right-hand side, then passes that result to the shell for execution. If the result of the execution ends in a newline, that one newline is removed; all other newlines are replaced by spaces. The resulting string is then placed into the named recursively-expanded variable. For example: hash != printf '\043' file_list != find . -name '*.c' If the result of the execution could produce a '$', and you don't intend what follows that to be interpreted as a make variable or function reference, then you must replace every '$' with '$$' as part of the execution. Alternatively, you can set a simply expanded variable to the result of running a program using the 'shell' function call. *Note The 'shell' Function: Shell Function. For example: hash := $(shell printf '\043') var := $(shell find . -name "*.c")  File: make.info, Node: Appending, Next: Override Directive, Prev: Setting, Up: Using Variables 6.6 Appending More Text to Variables ==================================== Often it is useful to add more text to the value of a variable already defined. You do this with a line containing '+=', like this: objects += another.o This takes the value of the variable 'objects', and adds the text 'another.o' to it (preceded by a single space). Thus: objects = main.o foo.o bar.o utils.o objects += another.o sets 'objects' to 'main.o foo.o bar.o utils.o another.o'. Using '+=' is similar to: objects = main.o foo.o bar.o utils.o objects := $(objects) another.o but differs in ways that become important when you use more complex values. When the variable in question has not been defined before, '+=' acts just like normal '=': it defines a recursively-expanded variable. However, when there _is_ a previous definition, exactly what '+=' does depends on what flavor of variable you defined originally. *Note The Two Flavors of Variables: Flavors, for an explanation of the two flavors of variables. When you add to a variable's value with '+=', 'make' acts essentially as if you had included the extra text in the initial definition of the variable. If you defined it first with ':=' or '::=', making it a simply-expanded variable, '+=' adds to that simply-expanded definition, and expands the new text before appending it to the old value just as ':=' does (see *note Setting Variables: Setting, for a full explanation of ':=' or '::='). In fact, variable := value variable += more is exactly equivalent to: variable := value variable := $(variable) more On the other hand, when you use '+=' with a variable that you defined first to be recursively-expanded using plain '=', 'make' does something a bit different. Recall that when you define a recursively-expanded variable, 'make' does not expand the value you set for variable and function references immediately. Instead it stores the text verbatim, and saves these variable and function references to be expanded later, when you refer to the new variable (*note The Two Flavors of Variables: Flavors.). When you use '+=' on a recursively-expanded variable, it is this unexpanded text to which 'make' appends the new text you specify. variable = value variable += more is roughly equivalent to: temp = value variable = $(temp) more except that of course it never defines a variable called 'temp'. The importance of this comes when the variable's old value contains variable references. Take this common example: CFLAGS = $(includes) -O ... CFLAGS += -pg # enable profiling The first line defines the 'CFLAGS' variable with a reference to another variable, 'includes'. ('CFLAGS' is used by the rules for C compilation; *note Catalogue of Built-In Rules: Catalogue of Rules.) Using '=' for the definition makes 'CFLAGS' a recursively-expanded variable, meaning '$(includes) -O' is _not_ expanded when 'make' processes the definition of 'CFLAGS'. Thus, 'includes' need not be defined yet for its value to take effect. It only has to be defined before any reference to 'CFLAGS'. If we tried to append to the value of 'CFLAGS' without using '+=', we might do it like this: CFLAGS := $(CFLAGS) -pg # enable profiling This is pretty close, but not quite what we want. Using ':=' redefines 'CFLAGS' as a simply-expanded variable; this means 'make' expands the text '$(CFLAGS) -pg' before setting the variable. If 'includes' is not yet defined, we get ' -O -pg', and a later definition of 'includes' will have no effect. Conversely, by using '+=' we set 'CFLAGS' to the _unexpanded_ value '$(includes) -O -pg'. Thus we preserve the reference to 'includes', so if that variable gets defined at any later point, a reference like '$(CFLAGS)' still uses its value.  File: make.info, Node: Override Directive, Next: Multi-Line, Prev: Appending, Up: Using Variables 6.7 The 'override' Directive ============================ If a variable has been set with a command argument (*note Overriding Variables: Overriding.), then ordinary assignments in the makefile are ignored. If you want to set the variable in the makefile even though it was set with a command argument, you can use an 'override' directive, which is a line that looks like this: override VARIABLE = VALUE or override VARIABLE := VALUE To append more text to a variable defined on the command line, use: override VARIABLE += MORE TEXT *Note Appending More Text to Variables: Appending. Variable assignments marked with the 'override' flag have a higher priority than all other assignments, except another 'override'. Subsequent assignments or appends to this variable which are not marked 'override' will be ignored. The 'override' directive was not invented for escalation in the war between makefiles and command arguments. It was invented so you can alter and add to values that the user specifies with command arguments. For example, suppose you always want the '-g' switch when you run the C compiler, but you would like to allow the user to specify the other switches with a command argument just as usual. You could use this 'override' directive: override CFLAGS += -g You can also use 'override' directives with 'define' directives. This is done as you might expect: override define foo = bar endef *Note Defining Multi-Line Variables: Multi-Line.  File: make.info, Node: Multi-Line, Next: Undefine Directive, Prev: Override Directive, Up: Using Variables 6.8 Defining Multi-Line Variables ================================= Another way to set the value of a variable is to use the 'define' directive. This directive has an unusual syntax which allows newline characters to be included in the value, which is convenient for defining both canned sequences of commands (*note Defining Canned Recipes: Canned Recipes.), and also sections of makefile syntax to use with 'eval' (*note Eval Function::). The 'define' directive is followed on the same line by the name of the variable being defined and an (optional) assignment operator, and nothing more. The value to give the variable appears on the following lines. The end of the value is marked by a line containing just the word 'endef'. Aside from this difference in syntax, 'define' works just like any other variable definition. The variable name may contain function and variable references, which are expanded when the directive is read to find the actual variable name to use. You may omit the variable assignment operator if you prefer. If omitted, 'make' assumes it to be '=' and creates a recursively-expanded variable (*note The Two Flavors of Variables: Flavors.). When using a '+=' operator, the value is appended to the previous value as with any other append operation: with a single space separating the old and new values. You may nest 'define' directives: 'make' will keep track of nested directives and report an error if they are not all properly closed with 'endef'. Note that lines beginning with the recipe prefix character are considered part of a recipe, so any 'define' or 'endef' strings appearing on such a line will not be considered 'make' directives. define two-lines = echo foo echo $(bar) endef The value in an ordinary assignment cannot contain a newline; but the newlines that separate the lines of the value in a 'define' become part of the variable's value (except for the final newline which precedes the 'endef' and is not considered part of the value). When used in a recipe, the previous example is functionally equivalent to this: two-lines = echo foo; echo $(bar) since two commands separated by semicolon behave much like two separate shell commands. However, note that using two separate lines means 'make' will invoke the shell twice, running an independent sub-shell for each line. *Note Recipe Execution: Execution. If you want variable definitions made with 'define' to take precedence over command-line variable definitions, you can use the 'override' directive together with 'define': override define two-lines = foo $(bar) endef *Note The 'override' Directive: Override Directive.  File: make.info, Node: Undefine Directive, Next: Environment, Prev: Multi-Line, Up: Using Variables 6.9 Undefining Variables ======================== If you want to clear a variable, setting its value to empty is usually sufficient. Expanding such a variable will yield the same result (empty string) regardless of whether it was set or not. However, if you are using the 'flavor' (*note Flavor Function::) and 'origin' (*note Origin Function::) functions, there is a difference between a variable that was never set and a variable with an empty value. In such situations you may want to use the 'undefine' directive to make a variable appear as if it was never set. For example: foo := foo bar = bar undefine foo undefine bar $(info $(origin foo)) $(info $(flavor bar)) This example will print "undefined" for both variables. If you want to undefine a command-line variable definition, you can use the 'override' directive together with 'undefine', similar to how this is done for variable definitions: override undefine CFLAGS  File: make.info, Node: Environment, Next: Target-specific, Prev: Undefine Directive, Up: Using Variables 6.10 Variables from the Environment =================================== Variables in 'make' can come from the environment in which 'make' is run. Every environment variable that 'make' sees when it starts up is transformed into a 'make' variable with the same name and value. However, an explicit assignment in the makefile, or with a command argument, overrides the environment. (If the '-e' flag is specified, then values from the environment override assignments in the makefile. *Note Summary of Options: Options Summary. But this is not recommended practice.) Thus, by setting the variable 'CFLAGS' in your environment, you can cause all C compilations in most makefiles to use the compiler switches you prefer. This is safe for variables with standard or conventional meanings because you know that no makefile will use them for other things. (Note this is not totally reliable; some makefiles set 'CFLAGS' explicitly and therefore are not affected by the value in the environment.) When 'make' runs a recipe, variables defined in the makefile are placed into the environment of each shell. This allows you to pass values to sub-'make' invocations (*note Recursive Use of 'make': Recursion.). By default, only variables that came from the environment or the command line are passed to recursive invocations. You can use the 'export' directive to pass other variables. *Note Communicating Variables to a Sub-'make': Variables/Recursion, for full details. Other use of variables from the environment is not recommended. It is not wise for makefiles to depend for their functioning on environment variables set up outside their control, since this would cause different users to get different results from the same makefile. This is against the whole purpose of most makefiles. Such problems would be especially likely with the variable 'SHELL', which is normally present in the environment to specify the user's choice of interactive shell. It would be very undesirable for this choice to affect 'make'; so, 'make' handles the 'SHELL' environment variable in a special way; see *note Choosing the Shell::.  File: make.info, Node: Target-specific, Next: Pattern-specific, Prev: Environment, Up: Using Variables 6.11 Target-specific Variable Values ==================================== Variable values in 'make' are usually global; that is, they are the same regardless of where they are evaluated (unless they're reset, of course). One exception to that is automatic variables (*note Automatic Variables::). The other exception is "target-specific variable values". This feature allows you to define different values for the same variable, based on the target that 'make' is currently building. As with automatic variables, these values are only available within the context of a target's recipe (and in other target-specific assignments). Set a target-specific variable value like this: TARGET ... : VARIABLE-ASSIGNMENT Target-specific variable assignments can be prefixed with any or all of the special keywords 'export', 'override', or 'private'; these apply their normal behavior to this instance of the variable only. Multiple TARGET values create a target-specific variable value for each member of the target list individually. The VARIABLE-ASSIGNMENT can be any valid form of assignment; recursive ('='), simple (':=' or '::='), appending ('+='), or conditional ('?='). All variables that appear within the VARIABLE-ASSIGNMENT are evaluated within the context of the target: thus, any previously-defined target-specific variable values will be in effect. Note that this variable is actually distinct from any "global" value: the two variables do not have to have the same flavor (recursive vs. simple). Target-specific variables have the same priority as any other makefile variable. Variables provided on the command line (and in the environment if the '-e' option is in force) will take precedence. Specifying the 'override' directive will allow the target-specific variable value to be preferred. There is one more special feature of target-specific variables: when you define a target-specific variable that variable value is also in effect for all prerequisites of this target, and all their prerequisites, etc. (unless those prerequisites override that variable with their own target-specific variable value). So, for example, a statement like this: prog : CFLAGS = -g prog : prog.o foo.o bar.o will set 'CFLAGS' to '-g' in the recipe for 'prog', but it will also set 'CFLAGS' to '-g' in the recipes that create 'prog.o', 'foo.o', and 'bar.o', and any recipes which create their prerequisites. Be aware that a given prerequisite will only be built once per invocation of make, at most. If the same file is a prerequisite of multiple targets, and each of those targets has a different value for the same target-specific variable, then the first target to be built will cause that prerequisite to be built and the prerequisite will inherit the target-specific value from the first target. It will ignore the target-specific values from any other targets.  File: make.info, Node: Pattern-specific, Next: Suppressing Inheritance, Prev: Target-specific, Up: Using Variables 6.12 Pattern-specific Variable Values ===================================== In addition to target-specific variable values (*note Target-specific Variable Values: Target-specific.), GNU 'make' supports pattern-specific variable values. In this form, the variable is defined for any target that matches the pattern specified. Set a pattern-specific variable value like this: PATTERN ... : VARIABLE-ASSIGNMENT where PATTERN is a %-pattern. As with target-specific variable values, multiple PATTERN values create a pattern-specific variable value for each pattern individually. The VARIABLE-ASSIGNMENT can be any valid form of assignment. Any command line variable setting will take precedence, unless 'override' is specified. For example: %.o : CFLAGS = -O will assign 'CFLAGS' the value of '-O' for all targets matching the pattern '%.o'. If a target matches more than one pattern, the matching pattern-specific variables with longer stems are interpreted first. This results in more specific variables taking precedence over the more generic ones, for example: %.o: %.c $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@ lib/%.o: CFLAGS := -fPIC -g %.o: CFLAGS := -g all: foo.o lib/bar.o In this example the first definition of the 'CFLAGS' variable will be used to update 'lib/bar.o' even though the second one also applies to this target. Pattern-specific variables which result in the same stem length are considered in the order in which they were defined in the makefile. Pattern-specific variables are searched after any target-specific variables defined explicitly for that target, and before target-specific variables defined for the parent target.  File: make.info, Node: Suppressing Inheritance, Next: Special Variables, Prev: Pattern-specific, Up: Using Variables 6.13 Suppressing Inheritance ============================ As described in previous sections, 'make' variables are inherited by prerequisites. This capability allows you to modify the behavior of a prerequisite based on which targets caused it to be rebuilt. For example, you might set a target-specific variable on a 'debug' target, then running 'make debug' will cause that variable to be inherited by all prerequisites of 'debug', while just running 'make all' (for example) would not have that assignment. Sometimes, however, you may not want a variable to be inherited. For these situations, 'make' provides the 'private' modifier. Although this modifier can be used with any variable assignment, it makes the most sense with target- and pattern-specific variables. Any variable marked 'private' will be visible to its local target but will not be inherited by prerequisites of that target. A global variable marked 'private' will be visible in the global scope but will not be inherited by any target, and hence will not be visible in any recipe. As an example, consider this makefile: EXTRA_CFLAGS = prog: private EXTRA_CFLAGS = -L/usr/local/lib prog: a.o b.o Due to the 'private' modifier, 'a.o' and 'b.o' will not inherit the 'EXTRA_CFLAGS' variable assignment from the 'prog' target.  File: make.info, Node: Special Variables, Prev: Suppressing Inheritance, Up: Using Variables 6.14 Other Special Variables ============================ GNU 'make' supports some variables that have special properties. 'MAKEFILE_LIST' Contains the name of each makefile that is parsed by 'make', in the order in which it was parsed. The name is appended just before 'make' begins to parse the makefile. Thus, if the first thing a makefile does is examine the last word in this variable, it will be the name of the current makefile. Once the current makefile has used 'include', however, the last word will be the just-included makefile. If a makefile named 'Makefile' has this content: name1 := $(lastword $(MAKEFILE_LIST)) include inc.mk name2 := $(lastword $(MAKEFILE_LIST)) all: @echo name1 = $(name1) @echo name2 = $(name2) then you would expect to see this output: name1 = Makefile name2 = inc.mk '.DEFAULT_GOAL' Sets the default goal to be used if no targets were specified on the command line (*note Arguments to Specify the Goals: Goals.). The '.DEFAULT_GOAL' variable allows you to discover the current default goal, restart the default goal selection algorithm by clearing its value, or to explicitly set the default goal. The following example illustrates these cases: # Query the default goal. ifeq ($(.DEFAULT_GOAL),) $(warning no default goal is set) endif .PHONY: foo foo: ; @echo $@ $(warning default goal is $(.DEFAULT_GOAL)) # Reset the default goal. .DEFAULT_GOAL := .PHONY: bar bar: ; @echo $@ $(warning default goal is $(.DEFAULT_GOAL)) # Set our own. .DEFAULT_GOAL := foo This makefile prints: no default goal is set default goal is foo default goal is bar foo Note that assigning more than one target name to '.DEFAULT_GOAL' is invalid and will result in an error. 'MAKE_RESTARTS' This variable is set only if this instance of 'make' has restarted (*note How Makefiles Are Remade: Remaking Makefiles.): it will contain the number of times this instance has restarted. Note this is not the same as recursion (counted by the 'MAKELEVEL' variable). You should not set, modify, or export this variable. 'MAKE_TERMOUT' 'MAKE_TERMERR' When 'make' starts it will check whether stdout and stderr will show their output on a terminal. If so, it will set 'MAKE_TERMOUT' and 'MAKE_TERMERR', respectively, to the name of the terminal device (or 'true' if this cannot be determined). If set these variables will be marked for export. These variables will not be changed by 'make' and they will not be modified if already set. These values can be used (particularly in combination with output synchronization (*note Output During Parallel Execution: Parallel Output.) to determine whether 'make' itself is writing to a terminal; they can be tested to decide whether to force recipe commands to generate colorized output for example. If you invoke a sub-'make' and redirect its stdout or stderr it is your responsibility to reset or unexport these variables as well, if your makefiles rely on them. '.RECIPEPREFIX' The first character of the value of this variable is used as the character make assumes is introducing a recipe line. If the variable is empty (as it is by default) that character is the standard tab character. For example, this is a valid makefile: .RECIPEPREFIX = > all: > @echo Hello, world The value of '.RECIPEPREFIX' can be changed multiple times; once set it stays in effect for all rules parsed until it is modified. '.VARIABLES' Expands to a list of the _names_ of all global variables defined so far. This includes variables which have empty values, as well as built-in variables (*note Variables Used by Implicit Rules: Implicit Variables.), but does not include any variables which are only defined in a target-specific context. Note that any value you assign to this variable will be ignored; it will always return its special value. '.FEATURES' Expands to a list of special features supported by this version of 'make'. Possible values include, but are not limited to: 'archives' Supports 'ar' (archive) files using special file name syntax. *Note Using 'make' to Update Archive Files: Archives. 'check-symlink' Supports the '-L' ('--check-symlink-times') flag. *Note Summary of Options: Options Summary. 'else-if' Supports "else if" non-nested conditionals. *Note Syntax of Conditionals: Conditional Syntax. 'jobserver' Supports "job server" enhanced parallel builds. *Note Parallel Execution: Parallel. 'oneshell' Supports the '.ONESHELL' special target. *Note Using One Shell: One Shell. 'order-only' Supports order-only prerequisites. *Note Types of Prerequisites: Prerequisite Types. 'second-expansion' Supports secondary expansion of prerequisite lists. 'shortest-stem' Uses the "shortest stem" method of choosing which pattern, of multiple applicable options, will be used. *Note How Patterns Match: Pattern Match. 'target-specific' Supports target-specific and pattern-specific variable assignments. *Note Target-specific Variable Values: Target-specific. 'undefine' Supports the 'undefine' directive. *Note Undefine Directive::. 'guile' Has GNU Guile available as an embedded extension language. *Note GNU Guile Integration: Guile Integration. 'load' Supports dynamically loadable objects for creating custom extensions. *Note Loading Dynamic Objects: Loading Objects. '.INCLUDE_DIRS' Expands to a list of directories that 'make' searches for included makefiles (*note Including Other Makefiles: Include.).  File: make.info, Node: Conditionals, Next: Functions, Prev: Using Variables, Up: Top 7 Conditional Parts of Makefiles ******************************** A "conditional" directive causes part of a makefile to be obeyed or ignored depending on the values of variables. Conditionals can compare the value of one variable to another, or the value of a variable to a constant string. Conditionals control what 'make' actually "sees" in the makefile, so they _cannot_ be used to control recipes at the time of execution. * Menu: * Conditional Example:: Example of a conditional * Conditional Syntax:: The syntax of conditionals. * Testing Flags:: Conditionals that test flags.  File: make.info, Node: Conditional Example, Next: Conditional Syntax, Prev: Conditionals, Up: Conditionals 7.1 Example of a Conditional ============================ The following example of a conditional tells 'make' to use one set of libraries if the 'CC' variable is 'gcc', and a different set of libraries otherwise. It works by controlling which of two recipe lines will be used for the rule. The result is that 'CC=gcc' as an argument to 'make' changes not only which compiler is used but also which libraries are linked. libs_for_gcc = -lgnu normal_libs = foo: $(objects) ifeq ($(CC),gcc) $(CC) -o foo $(objects) $(libs_for_gcc) else $(CC) -o foo $(objects) $(normal_libs) endif This conditional uses three directives: one 'ifeq', one 'else' and one 'endif'. The 'ifeq' directive begins the conditional, and specifies the condition. It contains two arguments, separated by a comma and surrounded by parentheses. Variable substitution is performed on both arguments and then they are compared. The lines of the makefile following the 'ifeq' are obeyed if the two arguments match; otherwise they are ignored. The 'else' directive causes the following lines to be obeyed if the previous conditional failed. In the example above, this means that the second alternative linking command is used whenever the first alternative is not used. It is optional to have an 'else' in a conditional. The 'endif' directive ends the conditional. Every conditional must end with an 'endif'. Unconditional makefile text follows. As this example illustrates, conditionals work at the textual level: the lines of the conditional are treated as part of the makefile, or ignored, according to the condition. This is why the larger syntactic units of the makefile, such as rules, may cross the beginning or the end of the conditional. When the variable 'CC' has the value 'gcc', the above example has this effect: foo: $(objects) $(CC) -o foo $(objects) $(libs_for_gcc) When the variable 'CC' has any other value, the effect is this: foo: $(objects) $(CC) -o foo $(objects) $(normal_libs) Equivalent results can be obtained in another way by conditionalizing a variable assignment and then using the variable unconditionally: libs_for_gcc = -lgnu normal_libs = ifeq ($(CC),gcc) libs=$(libs_for_gcc) else libs=$(normal_libs) endif foo: $(objects) $(CC) -o foo $(objects) $(libs)  File: make.info, Node: Conditional Syntax, Next: Testing Flags, Prev: Conditional Example, Up: Conditionals 7.2 Syntax of Conditionals ========================== The syntax of a simple conditional with no 'else' is as follows: CONDITIONAL-DIRECTIVE TEXT-IF-TRUE endif The TEXT-IF-TRUE may be any lines of text, to be considered as part of the makefile if the condition is true. If the condition is false, no text is used instead. The syntax of a complex conditional is as follows: CONDITIONAL-DIRECTIVE TEXT-IF-TRUE else TEXT-IF-FALSE endif or: CONDITIONAL-DIRECTIVE-ONE TEXT-IF-ONE-IS-TRUE else CONDITIONAL-DIRECTIVE-TWO TEXT-IF-TWO-IS-TRUE else TEXT-IF-ONE-AND-TWO-ARE-FALSE endif There can be as many "'else' CONDITIONAL-DIRECTIVE" clauses as necessary. Once a given condition is true, TEXT-IF-TRUE is used and no other clause is used; if no condition is true then TEXT-IF-FALSE is used. The TEXT-IF-TRUE and TEXT-IF-FALSE can be any number of lines of text. The syntax of the CONDITIONAL-DIRECTIVE is the same whether the conditional is simple or complex; after an 'else' or not. There are four different directives that test different conditions. Here is a table of them: 'ifeq (ARG1, ARG2)' 'ifeq 'ARG1' 'ARG2'' 'ifeq "ARG1" "ARG2"' 'ifeq "ARG1" 'ARG2'' 'ifeq 'ARG1' "ARG2"' Expand all variable references in ARG1 and ARG2 and compare them. If they are identical, the TEXT-IF-TRUE is effective; otherwise, the TEXT-IF-FALSE, if any, is effective. Often you want to test if a variable has a non-empty value. When the value results from complex expansions of variables and functions, expansions you would consider empty may actually contain whitespace characters and thus are not seen as empty. However, you can use the 'strip' function (*note Text Functions::) to avoid interpreting whitespace as a non-empty value. For example: ifeq ($(strip $(foo)),) TEXT-IF-EMPTY endif will evaluate TEXT-IF-EMPTY even if the expansion of '$(foo)' contains whitespace characters. 'ifneq (ARG1, ARG2)' 'ifneq 'ARG1' 'ARG2'' 'ifneq "ARG1" "ARG2"' 'ifneq "ARG1" 'ARG2'' 'ifneq 'ARG1' "ARG2"' Expand all variable references in ARG1 and ARG2 and compare them. If they are different, the TEXT-IF-TRUE is effective; otherwise, the TEXT-IF-FALSE, if any, is effective. 'ifdef VARIABLE-NAME' The 'ifdef' form takes the _name_ of a variable as its argument, not a reference to a variable. The value of that variable has a non-empty value, the TEXT-IF-TRUE is effective; otherwise, the TEXT-IF-FALSE, if any, is effective. Variables that have never been defined have an empty value. The text VARIABLE-NAME is expanded, so it could be a variable or function that expands to the name of a variable. For example: bar = true foo = bar ifdef $(foo) frobozz = yes endif The variable reference '$(foo)' is expanded, yielding 'bar', which is considered to be the name of a variable. The variable 'bar' is not expanded, but its value is examined to determine if it is non-empty. Note that 'ifdef' only tests whether a variable has a value. It does not expand the variable to see if that value is nonempty. Consequently, tests using 'ifdef' return true for all definitions except those like 'foo ='. To test for an empty value, use 'ifeq ($(foo),)'. For example, bar = foo = $(bar) ifdef foo frobozz = yes else frobozz = no endif sets 'frobozz' to 'yes', while: foo = ifdef foo frobozz = yes else frobozz = no endif sets 'frobozz' to 'no'. 'ifndef VARIABLE-NAME' If the variable VARIABLE-NAME has an empty value, the TEXT-IF-TRUE is effective; otherwise, the TEXT-IF-FALSE, if any, is effective. The rules for expansion and testing of VARIABLE-NAME are identical to the 'ifdef' directive. Extra spaces are allowed and ignored at the beginning of the conditional directive line, but a tab is not allowed. (If the line begins with a tab, it will be considered part of a recipe for a rule.) Aside from this, extra spaces or tabs may be inserted with no effect anywhere except within the directive name or within an argument. A comment starting with '#' may appear at the end of the line. The other two directives that play a part in a conditional are 'else' and 'endif'. Each of these directives is written as one word, with no arguments. Extra spaces are allowed and ignored at the beginning of the line, and spaces or tabs at the end. A comment starting with '#' may appear at the end of the line. Conditionals affect which lines of the makefile 'make' uses. If the condition is true, 'make' reads the lines of the TEXT-IF-TRUE as part of the makefile; if the condition is false, 'make' ignores those lines completely. It follows that syntactic units of the makefile, such as rules, may safely be split across the beginning or the end of the conditional. 'make' evaluates conditionals when it reads a makefile. Consequently, you cannot use automatic variables in the tests of conditionals because they are not defined until recipes are run (*note Automatic Variables::). To prevent intolerable confusion, it is not permitted to start a conditional in one makefile and end it in another. However, you may write an 'include' directive within a conditional, provided you do not attempt to terminate the conditional inside the included file.  File: make.info, Node: Testing Flags, Prev: Conditional Syntax, Up: Conditionals 7.3 Conditionals that Test Flags ================================ You can write a conditional that tests 'make' command flags such as '-t' by using the variable 'MAKEFLAGS' together with the 'findstring' function (*note Functions for String Substitution and Analysis: Text Functions.). This is useful when 'touch' is not enough to make a file appear up to date. The 'findstring' function determines whether one string appears as a substring of another. If you want to test for the '-t' flag, use 't' as the first string and the value of 'MAKEFLAGS' as the other. For example, here is how to arrange to use 'ranlib -t' to finish marking an archive file up to date: archive.a: ... ifneq (,$(findstring t,$(MAKEFLAGS))) +touch archive.a +ranlib -t archive.a else ranlib archive.a endif The '+' prefix marks those recipe lines as "recursive" so that they will be executed despite use of the '-t' flag. *Note Recursive Use of 'make': Recursion.  File: make.info, Node: Functions, Next: Running, Prev: Conditionals, Up: Top 8 Functions for Transforming Text ********************************* "Functions" allow you to do text processing in the makefile to compute the files to operate on or the commands to use in recipes. You use a function in a "function call", where you give the name of the function and some text (the "arguments") for the function to operate on. The result of the function's processing is substituted into the makefile at the point of the call, just as a variable might be substituted. * Menu: * Syntax of Functions:: How to write a function call. * Text Functions:: General-purpose text manipulation functions. * File Name Functions:: Functions for manipulating file names. * Conditional Functions:: Functions that implement conditions. * Foreach Function:: Repeat some text with controlled variation. * File Function:: Write text to a file. * Call Function:: Expand a user-defined function. * Value Function:: Return the un-expanded value of a variable. * Eval Function:: Evaluate the arguments as makefile syntax. * Origin Function:: Find where a variable got its value. * Flavor Function:: Find out the flavor of a variable. * Make Control Functions:: Functions that control how make runs. * Shell Function:: Substitute the output of a shell command. * Guile Function:: Use GNU Guile embedded scripting language.  File: make.info, Node: Syntax of Functions, Next: Text Functions, Prev: Functions, Up: Functions 8.1 Function Call Syntax ======================== A function call resembles a variable reference. It can appear anywhere a variable reference can appear, and it is expanded using the same rules as variable references. A function call looks like this: $(FUNCTION ARGUMENTS) or like this: ${FUNCTION ARGUMENTS} Here FUNCTION is a function name; one of a short list of names that are part of 'make'. You can also essentially create your own functions by using the 'call' built-in function. The ARGUMENTS are the arguments of the function. They are separated from the function name by one or more spaces or tabs, and if there is more than one argument, then they are separated by commas. Such whitespace and commas are not part of an argument's value. The delimiters which you use to surround the function call, whether parentheses or braces, can appear in an argument only in matching pairs; the other kind of delimiters may appear singly. If the arguments themselves contain other function calls or variable references, it is wisest to use the same kind of delimiters for all the references; write '$(subst a,b,$(x))', not '$(subst a,b,${x})'. This is because it is clearer, and because only one type of delimiter is matched to find the end of the reference. The text written for each argument is processed by substitution of variables and function calls to produce the argument value, which is the text on which the function acts. The substitution is done in the order in which the arguments appear. Commas and unmatched parentheses or braces cannot appear in the text of an argument as written; leading spaces cannot appear in the text of the first argument as written. These characters can be put into the argument value by variable substitution. First define variables 'comma' and 'space' whose values are isolated comma and space characters, then substitute these variables where such characters are wanted, like this: comma:= , empty:= space:= $(empty) $(empty) foo:= a b c bar:= $(subst $(space),$(comma),$(foo)) # bar is now 'a,b,c'. Here the 'subst' function replaces each space with a comma, through the value of 'foo', and substitutes the result.  File: make.info, Node: Text Functions, Next: File Name Functions, Prev: Syntax of Functions, Up: Functions 8.2 Functions for String Substitution and Analysis ================================================== Here are some functions that operate on strings: '$(subst FROM,TO,TEXT)' Performs a textual replacement on the text TEXT: each occurrence of FROM is replaced by TO. The result is substituted for the function call. For example, $(subst ee,EE,feet on the street) substitutes the string 'fEEt on the strEEt'. '$(patsubst PATTERN,REPLACEMENT,TEXT)' Finds whitespace-separated words in TEXT that match PATTERN and replaces them with REPLACEMENT. Here PATTERN may contain a '%' which acts as a wildcard, matching any number of any characters within a word. If REPLACEMENT also contains a '%', the '%' is replaced by the text that matched the '%' in PATTERN. Only the first '%' in the PATTERN and REPLACEMENT is treated this way; any subsequent '%' is unchanged. '%' characters in 'patsubst' function invocations can be quoted with preceding backslashes ('\'). Backslashes that would otherwise quote '%' characters can be quoted with more backslashes. Backslashes that quote '%' characters or other backslashes are removed from the pattern before it is compared file names or has a stem substituted into it. Backslashes that are not in danger of quoting '%' characters go unmolested. For example, the pattern 'the\%weird\\%pattern\\' has 'the%weird\' preceding the operative '%' character, and 'pattern\\' following it. The final two backslashes are left alone because they cannot affect any '%' character. Whitespace between words is folded into single space characters; leading and trailing whitespace is discarded. For example, $(patsubst %.c,%.o,x.c.c bar.c) produces the value 'x.c.o bar.o'. Substitution references (*note Substitution References: Substitution Refs.) are a simpler way to get the effect of the 'patsubst' function: $(VAR:PATTERN=REPLACEMENT) is equivalent to $(patsubst PATTERN,REPLACEMENT,$(VAR)) The second shorthand simplifies one of the most common uses of 'patsubst': replacing the suffix at the end of file names. $(VAR:SUFFIX=REPLACEMENT) is equivalent to $(patsubst %SUFFIX,%REPLACEMENT,$(VAR)) For example, you might have a list of object files: objects = foo.o bar.o baz.o To get the list of corresponding source files, you could simply write: $(objects:.o=.c) instead of using the general form: $(patsubst %.o,%.c,$(objects)) '$(strip STRING)' Removes leading and trailing whitespace from STRING and replaces each internal sequence of one or more whitespace characters with a single space. Thus, '$(strip a b c )' results in 'a b c'. The function 'strip' can be very useful when used in conjunction with conditionals. When comparing something with the empty string '' using 'ifeq' or 'ifneq', you usually want a string of just whitespace to match the empty string (*note Conditionals::). Thus, the following may fail to have the desired results: .PHONY: all ifneq "$(needs_made)" "" all: $(needs_made) else all:;@echo 'Nothing to make!' endif Replacing the variable reference '$(needs_made)' with the function call '$(strip $(needs_made))' in the 'ifneq' directive would make it more robust. '$(findstring FIND,IN)' Searches IN for an occurrence of FIND. If it occurs, the value is FIND; otherwise, the value is empty. You can use this function in a conditional to test for the presence of a specific substring in a given string. Thus, the two examples, $(findstring a,a b c) $(findstring a,b c) produce the values 'a' and '' (the empty string), respectively. *Note Testing Flags::, for a practical application of 'findstring'. '$(filter PATTERN...,TEXT)' Returns all whitespace-separated words in TEXT that _do_ match any of the PATTERN words, removing any words that _do not_ match. The patterns are written using '%', just like the patterns used in the 'patsubst' function above. The 'filter' function can be used to separate out different types of strings (such as file names) in a variable. For example: sources := foo.c bar.c baz.s ugh.h foo: $(sources) cc $(filter %.c %.s,$(sources)) -o foo says that 'foo' depends of 'foo.c', 'bar.c', 'baz.s' and 'ugh.h' but only 'foo.c', 'bar.c' and 'baz.s' should be specified in the command to the compiler. '$(filter-out PATTERN...,TEXT)' Returns all whitespace-separated words in TEXT that _do not_ match any of the PATTERN words, removing the words that _do_ match one or more. This is the exact opposite of the 'filter' function. For example, given: objects=main1.o foo.o main2.o bar.o mains=main1.o main2.o the following generates a list which contains all the object files not in 'mains': $(filter-out $(mains),$(objects)) '$(sort LIST)' Sorts the words of LIST in lexical order, removing duplicate words. The output is a list of words separated by single spaces. Thus, $(sort foo bar lose) returns the value 'bar foo lose'. Incidentally, since 'sort' removes duplicate words, you can use it for this purpose even if you don't care about the sort order. '$(word N,TEXT)' Returns the Nth word of TEXT. The legitimate values of N start from 1. If N is bigger than the number of words in TEXT, the value is empty. For example, $(word 2, foo bar baz) returns 'bar'. '$(wordlist S,E,TEXT)' Returns the list of words in TEXT starting with word S and ending with word E (inclusive). The legitimate values of S start from 1; E may start from 0. If S is bigger than the number of words in TEXT, the value is empty. If E is bigger than the number of words in TEXT, words up to the end of TEXT are returned. If S is greater than E, nothing is returned. For example, $(wordlist 2, 3, foo bar baz) returns 'bar baz'. '$(words TEXT)' Returns the number of words in TEXT. Thus, the last word of TEXT is '$(word $(words TEXT),TEXT)'. '$(firstword NAMES...)' The argument NAMES is regarded as a series of names, separated by whitespace. The value is the first name in the series. The rest of the names are ignored. For example, $(firstword foo bar) produces the result 'foo'. Although '$(firstword TEXT)' is the same as '$(word 1,TEXT)', the 'firstword' function is retained for its simplicity. '$(lastword NAMES...)' The argument NAMES is regarded as a series of names, separated by whitespace. The value is the last name in the series. For example, $(lastword foo bar) produces the result 'bar'. Although '$(lastword TEXT)' is the same as '$(word $(words TEXT),TEXT)', the 'lastword' function was added for its simplicity and better performance. Here is a realistic example of the use of 'subst' and 'patsubst'. Suppose that a makefile uses the 'VPATH' variable to specify a list of directories that 'make' should search for prerequisite files (*note 'VPATH' Search Path for All Prerequisites: General Search.). This example shows how to tell the C compiler to search for header files in the same list of directories. The value of 'VPATH' is a list of directories separated by colons, such as 'src:../headers'. First, the 'subst' function is used to change the colons to spaces: $(subst :, ,$(VPATH)) This produces 'src ../headers'. Then 'patsubst' is used to turn each directory name into a '-I' flag. These can be added to the value of the variable 'CFLAGS', which is passed automatically to the C compiler, like this: override CFLAGS += $(patsubst %,-I%,$(subst :, ,$(VPATH))) The effect is to append the text '-Isrc -I../headers' to the previously given value of 'CFLAGS'. The 'override' directive is used so that the new value is assigned even if the previous value of 'CFLAGS' was specified with a command argument (*note The 'override' Directive: Override Directive.).  File: make.info, Node: File Name Functions, Next: Conditional Functions, Prev: Text Functions, Up: Functions 8.3 Functions for File Names ============================ Several of the built-in expansion functions relate specifically to taking apart file names or lists of file names. Each of the following functions performs a specific transformation on a file name. The argument of the function is regarded as a series of file names, separated by whitespace. (Leading and trailing whitespace is ignored.) Each file name in the series is transformed in the same way and the results are concatenated with single spaces between them. '$(dir NAMES...)' Extracts the directory-part of each file name in NAMES. The directory-part of the file name is everything up through (and including) the last slash in it. If the file name contains no slash, the directory part is the string './'. For example, $(dir src/foo.c hacks) produces the result 'src/ ./'. '$(notdir NAMES...)' Extracts all but the directory-part of each file name in NAMES. If the file name contains no slash, it is left unchanged. Otherwise, everything through the last slash is removed from it. A file name that ends with a slash becomes an empty string. This is unfortunate, because it means that the result does not always have the same number of whitespace-separated file names as the argument had; but we do not see any other valid alternative. For example, $(notdir src/foo.c hacks) produces the result 'foo.c hacks'. '$(suffix NAMES...)' Extracts the suffix of each file name in NAMES. If the file name contains a period, the suffix is everything starting with the last period. Otherwise, the suffix is the empty string. This frequently means that the result will be empty when NAMES is not, and if NAMES contains multiple file names, the result may contain fewer file names. For example, $(suffix src/foo.c src-1.0/bar.c hacks) produces the result '.c .c'. '$(basename NAMES...)' Extracts all but the suffix of each file name in NAMES. If the file name contains a period, the basename is everything starting up to (and not including) the last period. Periods in the directory part are ignored. If there is no period, the basename is the entire file name. For example, $(basename src/foo.c src-1.0/bar hacks) produces the result 'src/foo src-1.0/bar hacks'. '$(addsuffix SUFFIX,NAMES...)' The argument NAMES is regarded as a series of names, separated by whitespace; SUFFIX is used as a unit. The value of SUFFIX is appended to the end of each individual name and the resulting larger names are concatenated with single spaces between them. For example, $(addsuffix .c,foo bar) produces the result 'foo.c bar.c'. '$(addprefix PREFIX,NAMES...)' The argument NAMES is regarded as a series of names, separated by whitespace; PREFIX is used as a unit. The value of PREFIX is prepended to the front of each individual name and the resulting larger names are concatenated with single spaces between them. For example, $(addprefix src/,foo bar) produces the result 'src/foo src/bar'. '$(join LIST1,LIST2)' Concatenates the two arguments word by word: the two first words (one from each argument) concatenated form the first word of the result, the two second words form the second word of the result, and so on. So the Nth word of the result comes from the Nth word of each argument. If one argument has more words that the other, the extra words are copied unchanged into the result. For example, '$(join a b,.c .o)' produces 'a.c b.o'. Whitespace between the words in the lists is not preserved; it is replaced with a single space. This function can merge the results of the 'dir' and 'notdir' functions, to produce the original list of files which was given to those two functions. '$(wildcard PATTERN)' The argument PATTERN is a file name pattern, typically containing wildcard characters (as in shell file name patterns). The result of 'wildcard' is a space-separated list of the names of existing files that match the pattern. *Note Using Wildcard Characters in File Names: Wildcards. '$(realpath NAMES...)' For each file name in NAMES return the canonical absolute name. A canonical name does not contain any '.' or '..' components, nor any repeated path separators ('/') or symlinks. In case of a failure the empty string is returned. Consult the 'realpath(3)' documentation for a list of possible failure causes. '$(abspath NAMES...)' For each file name in NAMES return an absolute name that does not contain any '.' or '..' components, nor any repeated path separators ('/'). Note that, in contrast to 'realpath' function, 'abspath' does not resolve symlinks and does not require the file names to refer to an existing file or directory. Use the 'wildcard' function to test for existence.  File: make.info, Node: Conditional Functions, Next: Foreach Function, Prev: File Name Functions, Up: Functions 8.4 Functions for Conditionals ============================== There are three functions that provide conditional expansion. A key aspect of these functions is that not all of the arguments are expanded initially. Only those arguments which need to be expanded, will be expanded. '$(if CONDITION,THEN-PART[,ELSE-PART])' The 'if' function provides support for conditional expansion in a functional context (as opposed to the GNU 'make' makefile conditionals such as 'ifeq' (*note Syntax of Conditionals: Conditional Syntax.). The first argument, CONDITION, first has all preceding and trailing whitespace stripped, then is expanded. If it expands to any non-empty string, then the condition is considered to be true. If it expands to an empty string, the condition is considered to be false. If the condition is true then the second argument, THEN-PART, is evaluated and this is used as the result of the evaluation of the entire 'if' function. If the condition is false then the third argument, ELSE-PART, is evaluated and this is the result of the 'if' function. If there is no third argument, the 'if' function evaluates to nothing (the empty string). Note that only one of the THEN-PART or the ELSE-PART will be evaluated, never both. Thus, either can contain side-effects (such as 'shell' function calls, etc.) '$(or CONDITION1[,CONDITION2[,CONDITION3...]])' The 'or' function provides a "short-circuiting" OR operation. Each argument is expanded, in order. If an argument expands to a non-empty string the processing stops and the result of the expansion is that string. If, after all arguments are expanded, all of them are false (empty), then the result of the expansion is the empty string. '$(and CONDITION1[,CONDITION2[,CONDITION3...]])' The 'and' function provides a "short-circuiting" AND operation. Each argument is expanded, in order. If an argument expands to an empty string the processing stops and the result of the expansion is the empty string. If all arguments expand to a non-empty string then the result of the expansion is the expansion of the last argument.  File: make.info, Node: Foreach Function, Next: File Function, Prev: Conditional Functions, Up: Functions 8.5 The 'foreach' Function ========================== The 'foreach' function is very different from other functions. It causes one piece of text to be used repeatedly, each time with a different substitution performed on it. It resembles the 'for' command in the shell 'sh' and the 'foreach' command in the C-shell 'csh'. The syntax of the 'foreach' function is: $(foreach VAR,LIST,TEXT) The first two arguments, VAR and LIST, are expanded before anything else is done; note that the last argument, TEXT, is *not* expanded at the same time. Then for each word of the expanded value of LIST, the variable named by the expanded value of VAR is set to that word, and TEXT is expanded. Presumably TEXT contains references to that variable, so its expansion will be different each time. The result is that TEXT is expanded as many times as there are whitespace-separated words in LIST. The multiple expansions of TEXT are concatenated, with spaces between them, to make the result of 'foreach'. This simple example sets the variable 'files' to the list of all files in the directories in the list 'dirs': dirs := a b c d files := $(foreach dir,$(dirs),$(wildcard $(dir)/*)) Here TEXT is '$(wildcard $(dir)/*)'. The first repetition finds the value 'a' for 'dir', so it produces the same result as '$(wildcard a/*)'; the second repetition produces the result of '$(wildcard b/*)'; and the third, that of '$(wildcard c/*)'. This example has the same result (except for setting 'dirs') as the following example: files := $(wildcard a/* b/* c/* d/*) When TEXT is complicated, you can improve readability by giving it a name, with an additional variable: find_files = $(wildcard $(dir)/*) dirs := a b c d files := $(foreach dir,$(dirs),$(find_files)) Here we use the variable 'find_files' this way. We use plain '=' to define a recursively-expanding variable, so that its value contains an actual function call to be re-expanded under the control of 'foreach'; a simply-expanded variable would not do, since 'wildcard' would be called only once at the time of defining 'find_files'. The 'foreach' function has no permanent effect on the variable VAR; its value and flavor after the 'foreach' function call are the same as they were beforehand. The other values which are taken from LIST are in effect only temporarily, during the execution of 'foreach'. The variable VAR is a simply-expanded variable during the execution of 'foreach'. If VAR was undefined before the 'foreach' function call, it is undefined after the call. *Note The Two Flavors of Variables: Flavors. You must take care when using complex variable expressions that result in variable names because many strange things are valid variable names, but are probably not what you intended. For example, files := $(foreach Esta-escrito-en-espanol!,b c ch,$(find_files)) might be useful if the value of 'find_files' references the variable whose name is 'Esta-escrito-en-espanol!' (es un nombre bastante largo, no?), but it is more likely to be a mistake.  File: make.info, Node: File Function, Next: Call Function, Prev: Foreach Function, Up: Functions 8.6 The 'file' Function ======================= The 'file' function allows the makefile to write to a file. Two modes of writing are supported: overwrite, where the text is written to the beginning of the file and any existing content is lost, and append, where the text is written to the end of the file, preserving the existing content. In all cases the file is created if it does not exist. The syntax of the 'file' function is: $(file OP FILENAME[,TEXT]) The operator OP can be either '>' which indicates overwrite mode, or '>>' which indicates append mode. The FILENAME indicates the file to be written to. There may optionally be whitespace between the operator and the file name. When the 'file' function is expanded all its arguments are expanded first, then the file indicated by FILENAME will be opened in the mode described by OP. Finally TEXT will be written to the file. If TEXT does not already end in a newline, even if empty, a final newline will be written. If the TEXT argument is not given, nothing will be written. The result of evaluating the 'file' function is always the empty string. It is a fatal error if the file cannot be opened for writing, or if the write operation fails. For example, the 'file' function can be useful if your build system has a limited command line size and your recipe runs a command that can accept arguments from a file as well. Many commands use the convention that an argument prefixed with an '@' specifies a file containing more arguments. Then you might write your recipe in this way: program: $(OBJECTS) $(file >$@.in,$^) $(CMD) $(CMDFLAGS) @$@.in @rm $@.in If the command required each argument to be on a separate line of the input file, you might write your recipe like this: program: $(OBJECTS) $(file >$@.in) $(foreach O,$^,$(file >>$@.in,$O)) $(CMD) $(CMDFLAGS) @$@.in @rm $@.in  File: make.info, Node: Call Function, Next: Value Function, Prev: File Function, Up: Functions 8.7 The 'call' Function ======================= The 'call' function is unique in that it can be used to create new parameterized functions. You can write a complex expression as the value of a variable, then use 'call' to expand it with different values. The syntax of the 'call' function is: $(call VARIABLE,PARAM,PARAM,...) When 'make' expands this function, it assigns each PARAM to temporary variables '$(1)', '$(2)', etc. The variable '$(0)' will contain VARIABLE. There is no maximum number of parameter arguments. There is no minimum, either, but it doesn't make sense to use 'call' with no parameters. Then VARIABLE is expanded as a 'make' variable in the context of these temporary assignments. Thus, any reference to '$(1)' in the value of VARIABLE will resolve to the first PARAM in the invocation of 'call'. Note that VARIABLE is the _name_ of a variable, not a _reference_ to that variable. Therefore you would not normally use a '$' or parentheses when writing it. (You can, however, use a variable reference in the name if you want the name not to be a constant.) If VARIABLE is the name of a built-in function, the built-in function is always invoked (even if a 'make' variable by that name also exists). The 'call' function expands the PARAM arguments before assigning them to temporary variables. This means that VARIABLE values containing references to built-in functions that have special expansion rules, like 'foreach' or 'if', may not work as you expect. Some examples may make this clearer. This macro simply reverses its arguments: reverse = $(2) $(1) foo = $(call reverse,a,b) Here FOO will contain 'b a'. This one is slightly more interesting: it defines a macro to search for the first instance of a program in 'PATH': pathsearch = $(firstword $(wildcard $(addsuffix /$(1),$(subst :, ,$(PATH))))) LS := $(call pathsearch,ls) Now the variable LS contains '/bin/ls' or similar. The 'call' function can be nested. Each recursive invocation gets its own local values for '$(1)', etc. that mask the values of higher-level 'call'. For example, here is an implementation of a "map" function: map = $(foreach a,$(2),$(call $(1),$(a))) Now you can MAP a function that normally takes only one argument, such as 'origin', to multiple values in one step: o = $(call map,origin,o map MAKE) and end up with O containing something like 'file file default'. A final caution: be careful when adding whitespace to the arguments to 'call'. As with other functions, any whitespace contained in the second and subsequent arguments is kept; this can cause strange effects. It's generally safest to remove all extraneous whitespace when providing parameters to 'call'.  File: make.info, Node: Value Function, Next: Eval Function, Prev: Call Function, Up: Functions 8.8 The 'value' Function ======================== The 'value' function provides a way for you to use the value of a variable _without_ having it expanded. Please note that this does not undo expansions which have already occurred; for example if you create a simply expanded variable its value is expanded during the definition; in that case the 'value' function will return the same result as using the variable directly. The syntax of the 'value' function is: $(value VARIABLE) Note that VARIABLE is the _name_ of a variable, not a _reference_ to that variable. Therefore you would not normally use a '$' or parentheses when writing it. (You can, however, use a variable reference in the name if you want the name not to be a constant.) The result of this function is a string containing the value of VARIABLE, without any expansion occurring. For example, in this makefile: FOO = $PATH all: @echo $(FOO) @echo $(value FOO) The first output line would be 'ATH', since the "$P" would be expanded as a 'make' variable, while the second output line would be the current value of your '$PATH' environment variable, since the 'value' function avoided the expansion. The 'value' function is most often used in conjunction with the 'eval' function (*note Eval Function::).  File: make.info, Node: Eval Function, Next: Origin Function, Prev: Value Function, Up: Functions 8.9 The 'eval' Function ======================= The 'eval' function is very special: it allows you to define new makefile constructs that are not constant; which are the result of evaluating other variables and functions. The argument to the 'eval' function is expanded, then the results of that expansion are parsed as makefile syntax. The expanded results can define new 'make' variables, targets, implicit or explicit rules, etc. The result of the 'eval' function is always the empty string; thus, it can be placed virtually anywhere in a makefile without causing syntax errors. It's important to realize that the 'eval' argument is expanded _twice_; first by the 'eval' function, then the results of that expansion are expanded again when they are parsed as makefile syntax. This means you may need to provide extra levels of escaping for "$" characters when using 'eval'. The 'value' function (*note Value Function::) can sometimes be useful in these situations, to circumvent unwanted expansions. Here is an example of how 'eval' can be used; this example combines a number of concepts and other functions. Although it might seem overly complex to use 'eval' in this example, rather than just writing out the rules, consider two things: first, the template definition (in 'PROGRAM_template') could need to be much more complex than it is here; and second, you might put the complex, "generic" part of this example into another makefile, then include it in all the individual makefiles. Now your individual makefiles are quite straightforward. PROGRAMS = server client server_OBJS = server.o server_priv.o server_access.o server_LIBS = priv protocol client_OBJS = client.o client_api.o client_mem.o client_LIBS = protocol # Everything after this is generic .PHONY: all all: $(PROGRAMS) define PROGRAM_template = $(1): $$($(1)_OBJS) $$($(1)_LIBS:%=-l%) ALL_OBJS += $$($(1)_OBJS) endef $(foreach prog,$(PROGRAMS),$(eval $(call PROGRAM_template,$(prog)))) $(PROGRAMS): $(LINK.o) $^ $(LDLIBS) -o $@ clean: rm -f $(ALL_OBJS) $(PROGRAMS)  File: make.info, Node: Origin Function, Next: Flavor Function, Prev: Eval Function, Up: Functions 8.10 The 'origin' Function ========================== The 'origin' function is unlike most other functions in that it does not operate on the values of variables; it tells you something _about_ a variable. Specifically, it tells you where it came from. The syntax of the 'origin' function is: $(origin VARIABLE) Note that VARIABLE is the _name_ of a variable to inquire about, not a _reference_ to that variable. Therefore you would not normally use a '$' or parentheses when writing it. (You can, however, use a variable reference in the name if you want the name not to be a constant.) The result of this function is a string telling you how the variable VARIABLE was defined: 'undefined' if VARIABLE was never defined. 'default' if VARIABLE has a default definition, as is usual with 'CC' and so on. *Note Variables Used by Implicit Rules: Implicit Variables. Note that if you have redefined a default variable, the 'origin' function will return the origin of the later definition. 'environment' if VARIABLE was inherited from the environment provided to 'make'. 'environment override' if VARIABLE was inherited from the environment provided to 'make', and is overriding a setting for VARIABLE in the makefile as a result of the '-e' option (*note Summary of Options: Options Summary.). 'file' if VARIABLE was defined in a makefile. 'command line' if VARIABLE was defined on the command line. 'override' if VARIABLE was defined with an 'override' directive in a makefile (*note The 'override' Directive: Override Directive.). 'automatic' if VARIABLE is an automatic variable defined for the execution of the recipe for each rule (*note Automatic Variables::). This information is primarily useful (other than for your curiosity) to determine if you want to believe the value of a variable. For example, suppose you have a makefile 'foo' that includes another makefile 'bar'. You want a variable 'bletch' to be defined in 'bar' if you run the command 'make -f bar', even if the environment contains a definition of 'bletch'. However, if 'foo' defined 'bletch' before including 'bar', you do not want to override that definition. This could be done by using an 'override' directive in 'foo', giving that definition precedence over the later definition in 'bar'; unfortunately, the 'override' directive would also override any command line definitions. So, 'bar' could include: ifdef bletch ifeq "$(origin bletch)" "environment" bletch = barf, gag, etc. endif endif If 'bletch' has been defined from the environment, this will redefine it. If you want to override a previous definition of 'bletch' if it came from the environment, even under '-e', you could instead write: ifneq "$(findstring environment,$(origin bletch))" "" bletch = barf, gag, etc. endif Here the redefinition takes place if '$(origin bletch)' returns either 'environment' or 'environment override'. *Note Functions for String Substitution and Analysis: Text Functions.  File: make.info, Node: Flavor Function, Next: Make Control Functions, Prev: Origin Function, Up: Functions 8.11 The 'flavor' Function ========================== The 'flavor' function, like the 'origin' function, does not operate on the values of variables but rather it tells you something _about_ a variable. Specifically, it tells you the flavor of a variable (*note The Two Flavors of Variables: Flavors.). The syntax of the 'flavor' function is: $(flavor VARIABLE) Note that VARIABLE is the _name_ of a variable to inquire about, not a _reference_ to that variable. Therefore you would not normally use a '$' or parentheses when writing it. (You can, however, use a variable reference in the name if you want the name not to be a constant.) The result of this function is a string that identifies the flavor of the variable VARIABLE: 'undefined' if VARIABLE was never defined. 'recursive' if VARIABLE is a recursively expanded variable. 'simple' if VARIABLE is a simply expanded variable.  File: make.info, Node: Make Control Functions, Next: Shell Function, Prev: Flavor Function, Up: Functions 8.12 Functions That Control Make ================================ These functions control the way make runs. Generally, they are used to provide information to the user of the makefile or to cause make to stop if some sort of environmental error is detected. '$(error TEXT...)' Generates a fatal error where the message is TEXT. Note that the error is generated whenever this function is evaluated. So, if you put it inside a recipe or on the right side of a recursive variable assignment, it won't be evaluated until later. The TEXT will be expanded before the error is generated. For example, ifdef ERROR1 $(error error is $(ERROR1)) endif will generate a fatal error during the read of the makefile if the 'make' variable 'ERROR1' is defined. Or, ERR = $(error found an error!) .PHONY: err err: ; $(ERR) will generate a fatal error while 'make' is running, if the 'err' target is invoked. '$(warning TEXT...)' This function works similarly to the 'error' function, above, except that 'make' doesn't exit. Instead, TEXT is expanded and the resulting message is displayed, but processing of the makefile continues. The result of the expansion of this function is the empty string. '$(info TEXT...)' This function does nothing more than print its (expanded) argument(s) to standard output. No makefile name or line number is added. The result of the expansion of this function is the empty string.  File: make.info, Node: Shell Function, Next: Guile Function, Prev: Make Control Functions, Up: Functions 8.13 The 'shell' Function ========================= The 'shell' function is unlike any other function other than the 'wildcard' function (*note The Function 'wildcard': Wildcard Function.) in that it communicates with the world outside of 'make'. The 'shell' function performs the same function that backquotes ('`') perform in most shells: it does "command expansion". This means that it takes as an argument a shell command and evaluates to the output of the command. The only processing 'make' does on the result is to convert each newline (or carriage-return / newline pair) to a single space. If there is a trailing (carriage-return and) newline it will simply be removed. The commands run by calls to the 'shell' function are run when the function calls are expanded (*note How 'make' Reads a Makefile: Reading Makefiles.). Because this function involves spawning a new shell, you should carefully consider the performance implications of using the 'shell' function within recursively expanded variables vs. simply expanded variables (*note The Two Flavors of Variables: Flavors.). Here are some examples of the use of the 'shell' function: contents := $(shell cat foo) sets 'contents' to the contents of the file 'foo', with a space (rather than a newline) separating each line. files := $(shell echo *.c) sets 'files' to the expansion of '*.c'. Unless 'make' is using a very strange shell, this has the same result as '$(wildcard *.c)' (as long as at least one '.c' file exists).  File: make.info, Node: Guile Function, Prev: Shell Function, Up: Functions 8.14 The 'guile' Function ========================= If GNU 'make' is built with support for GNU Guile as an embedded extension language then the 'guile' function will be available. The 'guile' function takes one argument which is first expanded by 'make' in the normal fashion, then passed to the GNU Guile evaluator. The result of the evaluator is converted into a string and used as the expansion of the 'guile' function in the makefile. See *note GNU Guile Integration: Guile Integration. for details on writing extensions to 'make' in Guile. You can determine whether GNU Guile support is available by checking the '.FEATURES' variable for the word GUILE.  File: make.info, Node: Running, Next: Implicit Rules, Prev: Functions, Up: Top 9 How to Run 'make' ******************* A makefile that says how to recompile a program can be used in more than one way. The simplest use is to recompile every file that is out of date. Usually, makefiles are written so that if you run 'make' with no arguments, it does just that. But you might want to update only some of the files; you might want to use a different compiler or different compiler options; you might want just to find out which files are out of date without changing them. By giving arguments when you run 'make', you can do any of these things and many others. The exit status of 'make' is always one of three values: '0' The exit status is zero if 'make' is successful. '2' The exit status is two if 'make' encounters any errors. It will print messages describing the particular errors. '1' The exit status is one if you use the '-q' flag and 'make' determines that some target is not already up to date. *Note Instead of Executing Recipes: Instead of Execution. * Menu: * Makefile Arguments:: How to specify which makefile to use. * Goals:: How to use goal arguments to specify which parts of the makefile to use. * Instead of Execution:: How to use mode flags to specify what kind of thing to do with the recipes in the makefile other than simply execute them. * Avoiding Compilation:: How to avoid recompiling certain files. * Overriding:: How to override a variable to specify an alternate compiler and other things. * Testing:: How to proceed past some errors, to test compilation. * Options Summary:: Summary of Options  File: make.info, Node: Makefile Arguments, Next: Goals, Prev: Running, Up: Running 9.1 Arguments to Specify the Makefile ===================================== The way to specify the name of the makefile is with the '-f' or '--file' option ('--makefile' also works). For example, '-f altmake' says to use the file 'altmake' as the makefile. If you use the '-f' flag several times and follow each '-f' with an argument, all the specified files are used jointly as makefiles. If you do not use the '-f' or '--file' flag, the default is to try 'GNUmakefile', 'makefile', and 'Makefile', in that order, and use the first of these three which exists or can be made (*note Writing Makefiles: Makefiles.).  File: make.info, Node: Goals, Next: Instead of Execution, Prev: Makefile Arguments, Up: Running 9.2 Arguments to Specify the Goals ================================== The "goals" are the targets that 'make' should strive ultimately to update. Other targets are updated as well if they appear as prerequisites of goals, or prerequisites of prerequisites of goals, etc. By default, the goal is the first target in the makefile (not counting targets that start with a period). Therefore, makefiles are usually written so that the first target is for compiling the entire program or programs they describe. If the first rule in the makefile has several targets, only the first target in the rule becomes the default goal, not the whole list. You can manage the selection of the default goal from within your makefile using the '.DEFAULT_GOAL' variable (*note Other Special Variables: Special Variables.). You can also specify a different goal or goals with command line arguments to 'make'. Use the name of the goal as an argument. If you specify several goals, 'make' processes each of them in turn, in the order you name them. Any target in the makefile may be specified as a goal (unless it starts with '-' or contains an '=', in which case it will be parsed as a switch or variable definition, respectively). Even targets not in the makefile may be specified, if 'make' can find implicit rules that say how to make them. 'Make' will set the special variable 'MAKECMDGOALS' to the list of goals you specified on the command line. If no goals were given on the command line, this variable is empty. Note that this variable should be used only in special circumstances. An example of appropriate use is to avoid including '.d' files during 'clean' rules (*note Automatic Prerequisites::), so 'make' won't create them only to immediately remove them again: sources = foo.c bar.c ifneq ($(MAKECMDGOALS),clean) include $(sources:.c=.d) endif One use of specifying a goal is if you want to compile only a part of the program, or only one of several programs. Specify as a goal each file that you wish to remake. For example, consider a directory containing several programs, with a makefile that starts like this: .PHONY: all all: size nm ld ar as If you are working on the program 'size', you might want to say 'make size' so that only the files of that program are recompiled. Another use of specifying a goal is to make files that are not normally made. For example, there may be a file of debugging output, or a version of the program that is compiled specially for testing, which has a rule in the makefile but is not a prerequisite of the default goal. Another use of specifying a goal is to run the recipe associated with a phony target (*note Phony Targets::) or empty target (*note Empty Target Files to Record Events: Empty Targets.). Many makefiles contain a phony target named 'clean' which deletes everything except source files. Naturally, this is done only if you request it explicitly with 'make clean'. Following is a list of typical phony and empty target names. *Note Standard Targets::, for a detailed list of all the standard target names which GNU software packages use. 'all' Make all the top-level targets the makefile knows about. 'clean' Delete all files that are normally created by running 'make'. 'mostlyclean' Like 'clean', but may refrain from deleting a few files that people normally don't want to recompile. For example, the 'mostlyclean' target for GCC does not delete 'libgcc.a', because recompiling it is rarely necessary and takes a lot of time. 'distclean' 'realclean' 'clobber' Any of these targets might be defined to delete _more_ files than 'clean' does. For example, this would delete configuration files or links that you would normally create as preparation for compilation, even if the makefile itself cannot create these files. 'install' Copy the executable file into a directory that users typically search for commands; copy any auxiliary files that the executable uses into the directories where it will look for them. 'print' Print listings of the source files that have changed. 'tar' Create a tar file of the source files. 'shar' Create a shell archive (shar file) of the source files. 'dist' Create a distribution file of the source files. This might be a tar file, or a shar file, or a compressed version of one of the above, or even more than one of the above. 'TAGS' Update a tags table for this program. 'check' 'test' Perform self tests on the program this makefile builds.  File: make.info, Node: Instead of Execution, Next: Avoiding Compilation, Prev: Goals, Up: Running 9.3 Instead of Executing Recipes ================================ The makefile tells 'make' how to tell whether a target is up to date, and how to update each target. But updating the targets is not always what you want. Certain options specify other activities for 'make'. '-n' '--just-print' '--dry-run' '--recon' "No-op". Causes 'make' to print the recipes that are needed to make the targets up to date, but not actually execute them. Note that some recipes are still executed, even with this flag (*note How the 'MAKE' Variable Works: MAKE Variable.). Also any recipes needed to update included makefiles are still executed (*note How Makefiles Are Remade: Remaking Makefiles.). '-t' '--touch' "Touch". Marks targets as up to date without actually changing them. In other words, 'make' pretends to update the targets but does not really change their contents; instead only their modified times are updated. '-q' '--question' "Question". Silently check whether the targets are up to date, but do not execute recipes; the exit code shows whether any updates are needed. '-W FILE' '--what-if=FILE' '--assume-new=FILE' '--new-file=FILE' "What if". Each '-W' flag is followed by a file name. The given files' modification times are recorded by 'make' as being the present time, although the actual modification times remain the same. You can use the '-W' flag in conjunction with the '-n' flag to see what would happen if you were to modify specific files. With the '-n' flag, 'make' prints the recipe that it would normally execute but usually does not execute it. With the '-t' flag, 'make' ignores the recipes in the rules and uses (in effect) the command 'touch' for each target that needs to be remade. The 'touch' command is also printed, unless '-s' or '.SILENT' is used. For speed, 'make' does not actually invoke the program 'touch'. It does the work directly. With the '-q' flag, 'make' prints nothing and executes no recipes, but the exit status code it returns is zero if and only if the targets to be considered are already up to date. If the exit status is one, then some updating needs to be done. If 'make' encounters an error, the exit status is two, so you can distinguish an error from a target that is not up to date. It is an error to use more than one of these three flags in the same invocation of 'make'. The '-n', '-t', and '-q' options do not affect recipe lines that begin with '+' characters or contain the strings '$(MAKE)' or '${MAKE}'. Note that only the line containing the '+' character or the strings '$(MAKE)' or '${MAKE}' is run regardless of these options. Other lines in the same rule are not run unless they too begin with '+' or contain '$(MAKE)' or '${MAKE}' (*Note How the 'MAKE' Variable Works: MAKE Variable.) The '-t' flag prevents phony targets (*note Phony Targets::) from being updated, unless there are recipe lines beginning with '+' or containing '$(MAKE)' or '${MAKE}'. The '-W' flag provides two features: * If you also use the '-n' or '-q' flag, you can see what 'make' would do if you were to modify some files. * Without the '-n' or '-q' flag, when 'make' is actually executing recipes, the '-W' flag can direct 'make' to act as if some files had been modified, without actually running the recipes for those files. Note that the options '-p' and '-v' allow you to obtain other information about 'make' or about the makefiles in use (*note Summary of Options: Options Summary.).  File: make.info, Node: Avoiding Compilation, Next: Overriding, Prev: Instead of Execution, Up: Running 9.4 Avoiding Recompilation of Some Files ======================================== Sometimes you may have changed a source file but you do not want to recompile all the files that depend on it. For example, suppose you add a macro or a declaration to a header file that many other files depend on. Being conservative, 'make' assumes that any change in the header file requires recompilation of all dependent files, but you know that they do not need to be recompiled and you would rather not waste the time waiting for them to compile. If you anticipate the problem before changing the header file, you can use the '-t' flag. This flag tells 'make' not to run the recipes in the rules, but rather to mark the target up to date by changing its last-modification date. You would follow this procedure: 1. Use the command 'make' to recompile the source files that really need recompilation, ensuring that the object files are up-to-date before you begin. 2. Make the changes in the header files. 3. Use the command 'make -t' to mark all the object files as up to date. The next time you run 'make', the changes in the header files will not cause any recompilation. If you have already changed the header file at a time when some files do need recompilation, it is too late to do this. Instead, you can use the '-o FILE' flag, which marks a specified file as "old" (*note Summary of Options: Options Summary.). This means that the file itself will not be remade, and nothing else will be remade on its account. Follow this procedure: 1. Recompile the source files that need compilation for reasons independent of the particular header file, with 'make -o HEADERFILE'. If several header files are involved, use a separate '-o' option for each header file. 2. Touch all the object files with 'make -t'.  File: make.info, Node: Overriding, Next: Testing, Prev: Avoiding Compilation, Up: Running 9.5 Overriding Variables ======================== An argument that contains '=' specifies the value of a variable: 'V=X' sets the value of the variable V to X. If you specify a value in this way, all ordinary assignments of the same variable in the makefile are ignored; we say they have been "overridden" by the command line argument. The most common way to use this facility is to pass extra flags to compilers. For example, in a properly written makefile, the variable 'CFLAGS' is included in each recipe that runs the C compiler, so a file 'foo.c' would be compiled something like this: cc -c $(CFLAGS) foo.c Thus, whatever value you set for 'CFLAGS' affects each compilation that occurs. The makefile probably specifies the usual value for 'CFLAGS', like this: CFLAGS=-g Each time you run 'make', you can override this value if you wish. For example, if you say 'make CFLAGS='-g -O'', each C compilation will be done with 'cc -c -g -O'. (This also illustrates how you can use quoting in the shell to enclose spaces and other special characters in the value of a variable when you override it.) The variable 'CFLAGS' is only one of many standard variables that exist just so that you can change them this way. *Note Variables Used by Implicit Rules: Implicit Variables, for a complete list. You can also program the makefile to look at additional variables of your own, giving the user the ability to control other aspects of how the makefile works by changing the variables. When you override a variable with a command line argument, you can define either a recursively-expanded variable or a simply-expanded variable. The examples shown above make a recursively-expanded variable; to make a simply-expanded variable, write ':=' or '::=' instead of '='. But, unless you want to include a variable reference or function call in the _value_ that you specify, it makes no difference which kind of variable you create. There is one way that the makefile can change a variable that you have overridden. This is to use the 'override' directive, which is a line that looks like this: 'override VARIABLE = VALUE' (*note The 'override' Directive: Override Directive.).  File: make.info, Node: Testing, Next: Options Summary, Prev: Overriding, Up: Running 9.6 Testing the Compilation of a Program ======================================== Normally, when an error happens in executing a shell command, 'make' gives up immediately, returning a nonzero status. No further recipes are executed for any target. The error implies that the goal cannot be correctly remade, and 'make' reports this as soon as it knows. When you are compiling a program that you have just changed, this is not what you want. Instead, you would rather that 'make' try compiling every file that can be tried, to show you as many compilation errors as possible. On these occasions, you should use the '-k' or '--keep-going' flag. This tells 'make' to continue to consider the other prerequisites of the pending targets, remaking them if necessary, before it gives up and returns nonzero status. For example, after an error in compiling one object file, 'make -k' will continue compiling other object files even though it already knows that linking them will be impossible. In addition to continuing after failed shell commands, 'make -k' will continue as much as possible after discovering that it does not know how to make a target or prerequisite file. This will always cause an error message, but without '-k', it is a fatal error (*note Summary of Options: Options Summary.). The usual behavior of 'make' assumes that your purpose is to get the goals up to date; once 'make' learns that this is impossible, it might as well report the failure immediately. The '-k' flag says that the real purpose is to test as much as possible of the changes made in the program, perhaps to find several independent problems so that you can correct them all before the next attempt to compile. This is why Emacs' 'M-x compile' command passes the '-k' flag by default.  File: make.info, Node: Options Summary, Prev: Testing, Up: Running 9.7 Summary of Options ====================== Here is a table of all the options 'make' understands: '-b' '-m' These options are ignored for compatibility with other versions of 'make'. '-B' '--always-make' Consider all targets out-of-date. GNU 'make' proceeds to consider targets and their prerequisites using the normal algorithms; however, all targets so considered are always remade regardless of the status of their prerequisites. To avoid infinite recursion, if 'MAKE_RESTARTS' (*note Other Special Variables: Special Variables.) is set to a number greater than 0 this option is disabled when considering whether to remake makefiles (*note How Makefiles Are Remade: Remaking Makefiles.). '-C DIR' '--directory=DIR' Change to directory DIR before reading the makefiles. If multiple '-C' options are specified, each is interpreted relative to the previous one: '-C / -C etc' is equivalent to '-C /etc'. This is typically used with recursive invocations of 'make' (*note Recursive Use of 'make': Recursion.). '-d' Print debugging information in addition to normal processing. The debugging information says which files are being considered for remaking, which file-times are being compared and with what results, which files actually need to be remade, which implicit rules are considered and which are applied--everything interesting about how 'make' decides what to do. The '-d' option is equivalent to '--debug=a' (see below). '--debug[=OPTIONS]' Print debugging information in addition to normal processing. Various levels and types of output can be chosen. With no arguments, print the "basic" level of debugging. Possible arguments are below; only the first character is considered, and values must be comma- or space-separated. 'a (all)' All types of debugging output are enabled. This is equivalent to using '-d'. 'b (basic)' Basic debugging prints each target that was found to be out-of-date, and whether the build was successful or not. 'v (verbose)' A level above 'basic'; includes messages about which makefiles were parsed, prerequisites that did not need to be rebuilt, etc. This option also enables 'basic' messages. 'i (implicit)' Prints messages describing the implicit rule searches for each target. This option also enables 'basic' messages. 'j (jobs)' Prints messages giving details on the invocation of specific sub-commands. 'm (makefile)' By default, the above messages are not enabled while trying to remake the makefiles. This option enables messages while rebuilding makefiles, too. Note that the 'all' option does enable this option. This option also enables 'basic' messages. 'n (none)' Disable all debugging currently enabled. If additional debugging flags are encountered after this they will still take effect. '-e' '--environment-overrides' Give variables taken from the environment precedence over variables from makefiles. *Note Variables from the Environment: Environment. '--eval=STRING' Evaluate STRING as makefile syntax. This is a command-line version of the 'eval' function (*note Eval Function::). The evaluation is performed after the default rules and variables have been defined, but before any makefiles are read. '-f FILE' '--file=FILE' '--makefile=FILE' Read the file named FILE as a makefile. *Note Writing Makefiles: Makefiles. '-h' '--help' Remind you of the options that 'make' understands and then exit. '-i' '--ignore-errors' Ignore all errors in recipes executed to remake files. *Note Errors in Recipes: Errors. '-I DIR' '--include-dir=DIR' Specifies a directory DIR to search for included makefiles. *Note Including Other Makefiles: Include. If several '-I' options are used to specify several directories, the directories are searched in the order specified. '-j [JOBS]' '--jobs[=JOBS]' Specifies the number of recipes (jobs) to run simultaneously. With no argument, 'make' runs as many recipes simultaneously as possible. If there is more than one '-j' option, the last one is effective. *Note Parallel Execution: Parallel, for more information on how recipes are run. Note that this option is ignored on MS-DOS. '-k' '--keep-going' Continue as much as possible after an error. While the target that failed, and those that depend on it, cannot be remade, the other prerequisites of these targets can be processed all the same. *Note Testing the Compilation of a Program: Testing. '-l [LOAD]' '--load-average[=LOAD]' '--max-load[=LOAD]' Specifies that no new recipes should be started if there are other recipes running and the load average is at least LOAD (a floating-point number). With no argument, removes a previous load limit. *Note Parallel Execution: Parallel. '-L' '--check-symlink-times' On systems that support symbolic links, this option causes 'make' to consider the timestamps on any symbolic links in addition to the timestamp on the file referenced by those links. When this option is provided, the most recent timestamp among the file and the symbolic links is taken as the modification time for this target file. '-n' '--just-print' '--dry-run' '--recon' Print the recipe that would be executed, but do not execute it (except in certain circumstances). *Note Instead of Executing Recipes: Instead of Execution. '-o FILE' '--old-file=FILE' '--assume-old=FILE' Do not remake the file FILE even if it is older than its prerequisites, and do not remake anything on account of changes in FILE. Essentially the file is treated as very old and its rules are ignored. *Note Avoiding Recompilation of Some Files: Avoiding Compilation. '-O[TYPE]' '--output-sync[=TYPE]' Ensure that the complete output from each recipe is printed in one uninterrupted sequence. This option is only useful when using the '--jobs' option to run multiple recipes simultaneously (*note Parallel Execution: Parallel.) Without this option output will be displayed as it is generated by the recipes. With no type or the type 'target', output from the entire recipe of each target is grouped together. With the type 'line', output from each line in the recipe is grouped together. With the type 'recurse', the output from an entire recursive make is grouped together. With the type 'none', no output synchronization is performed. *Note Output During Parallel Execution: Parallel Output. '-p' '--print-data-base' Print the data base (rules and variable values) that results from reading the makefiles; then execute as usual or as otherwise specified. This also prints the version information given by the '-v' switch (see below). To print the data base without trying to remake any files, use 'make -qp'. To print the data base of predefined rules and variables, use 'make -p -f /dev/null'. The data base output contains file name and line number information for recipe and variable definitions, so it can be a useful debugging tool in complex environments. '-q' '--question' "Question mode". Do not run any recipes, or print anything; just return an exit status that is zero if the specified targets are already up to date, one if any remaking is required, or two if an error is encountered. *Note Instead of Executing Recipes: Instead of Execution. '-r' '--no-builtin-rules' Eliminate use of the built-in implicit rules (*note Using Implicit Rules: Implicit Rules.). You can still define your own by writing pattern rules (*note Defining and Redefining Pattern Rules: Pattern Rules.). The '-r' option also clears out the default list of suffixes for suffix rules (*note Old-Fashioned Suffix Rules: Suffix Rules.). But you can still define your own suffixes with a rule for '.SUFFIXES', and then define your own suffix rules. Note that only _rules_ are affected by the '-r' option; default variables remain in effect (*note Variables Used by Implicit Rules: Implicit Variables.); see the '-R' option below. '-R' '--no-builtin-variables' Eliminate use of the built-in rule-specific variables (*note Variables Used by Implicit Rules: Implicit Variables.). You can still define your own, of course. The '-R' option also automatically enables the '-r' option (see above), since it doesn't make sense to have implicit rules without any definitions for the variables that they use. '-s' '--silent' '--quiet' Silent operation; do not print the recipes as they are executed. *Note Recipe Echoing: Echoing. '-S' '--no-keep-going' '--stop' Cancel the effect of the '-k' option. This is never necessary except in a recursive 'make' where '-k' might be inherited from the top-level 'make' via 'MAKEFLAGS' (*note Recursive Use of 'make': Recursion.) or if you set '-k' in 'MAKEFLAGS' in your environment. '-t' '--touch' Touch files (mark them up to date without really changing them) instead of running their recipes. This is used to pretend that the recipes were done, in order to fool future invocations of 'make'. *Note Instead of Executing Recipes: Instead of Execution. '--trace' Show tracing information for 'make' execution. Prints the entire recipe to be executed, even for recipes that are normally silent (due to '.SILENT' or '@'). Also prints the makefile name and line number where the recipe was defined, and information on why the target is being rebuilt. '-v' '--version' Print the version of the 'make' program plus a copyright, a list of authors, and a notice that there is no warranty; then exit. '-w' '--print-directory' Print a message containing the working directory both before and after executing the makefile. This may be useful for tracking down errors from complicated nests of recursive 'make' commands. *Note Recursive Use of 'make': Recursion. (In practice, you rarely need to specify this option since 'make' does it for you; see *note The '--print-directory' Option: -w Option.) '--no-print-directory' Disable printing of the working directory under '-w'. This option is useful when '-w' is turned on automatically, but you do not want to see the extra messages. *Note The '--print-directory' Option: -w Option. '-W FILE' '--what-if=FILE' '--new-file=FILE' '--assume-new=FILE' Pretend that the target FILE has just been modified. When used with the '-n' flag, this shows you what would happen if you were to modify that file. Without '-n', it is almost the same as running a 'touch' command on the given file before running 'make', except that the modification time is changed only in the imagination of 'make'. *Note Instead of Executing Recipes: Instead of Execution. '--warn-undefined-variables' Issue a warning message whenever 'make' sees a reference to an undefined variable. This can be helpful when you are trying to debug makefiles which use variables in complex ways.