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 \input texinfo @c -*- Texinfo -*- @c %**start of header @setfilename make.info @include version.texi @set EDITION 0.75 @settitle GNU @code{make} @setchapternewpage odd @c Combine the variable and function indices: @syncodeindex vr fn @c Combine the program and concept indices: @syncodeindex pg cp @c FSF publishers: format makebook.texi instead of using this file directly. @c ISBN confirmed by Jasimin Huang on 25 Mar 2009 @set ISBN 1-882114-83-3 @c %**end of header @copying This file documents the GNU @code{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 @value{EDITION}, last updated @value{UPDATED}, of @cite{The GNU Make Manual}, for GNU @code{make} version @value{VERSION}. Copyright @copyright{} 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, 2015, 2016, 2017, 2018, 2019, 2020 Free Software Foundation, Inc. @quotation 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.'' @end quotation @end copying @c finalout @c ISPELL CHECK: done, 10 June 1993 --roland @c ISPELL CHECK: done, 2000-06-25 --Martin Buchholz @dircategory Software development @direntry * Make: (make). Remake files automatically. @end direntry @iftex @shorttitlepage GNU Make @end iftex @titlepage @title GNU Make @subtitle A Program for Directing Recompilation @subtitle GNU @code{make} Version @value{VERSION} @subtitle @value{UPDATED-MONTH} @author Richard M. Stallman, Roland McGrath, Paul D. Smith @page @vskip 0pt plus 1filll @insertcopying @sp 2 Published by the Free Software Foundation @* 51 Franklin St. -- Fifth Floor @* Boston, MA 02110-1301 USA @* ISBN @value{ISBN} @* @sp 2 Cover art by Etienne Suvasa. @end titlepage @summarycontents @contents @ifnottex @node Top, Overview, (dir), (dir) @top GNU @code{make} @insertcopying @end ifnottex @menu * Overview:: Overview of @code{make}. * Introduction:: An introduction to @code{make}. * Makefiles:: Makefiles tell @code{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 @code{make} on the command line. * Implicit Rules:: Use implicit rules to treat many files alike, based on their file names. * Archives:: How @code{make} can update library archives. * Extending make:: Using extensions to @code{make}. * Integrating make:: Integrating @code{make} with other tools. * Features:: Features GNU @code{make} has over other @code{make}s. * Missing:: What GNU @code{make} lacks from other @code{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 @code{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. @detailmenu --- The Detailed Node Listing --- Overview of @code{make} * Preparing:: Preparing and running @code{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 @code{make} processes this makefile. * Variables Simplify:: Variables make makefiles simpler. * make Deduces:: Letting @code{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 read in. * Parsing 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 @code{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 @code{make} variables in recipes. Recipe Execution * One Shell:: One shell for all lines in a recipe. * Choosing the Shell:: How @code{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 @code{make} * MAKE Variable:: The special effects of using @samp{$(MAKE)}. * Variables/Recursion:: How to communicate variables to a sub-@code{make}. * Options/Recursion:: How to communicate options to a sub-@code{make}. * -w Option:: How the @samp{-w} or @samp{--print-directory} option helps debug use of recursive @code{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 @code{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 @code{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 @code{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 @code{make} strings. * Guile Interface:: Invoking @code{make} functions from Guile. * Guile Example:: Example using Guile in @code{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 Integrating GNU @code{make} * Job Slots:: Share job slots with GNU @code{make}. * Terminal Output:: Control output to terminals. Sharing Job Slots with GNU @code{make} * POSIX Jobserver:: Using the jobserver on POSIX systems. * Windows Jobserver:: Using the jobserver on Windows systems. @end detailmenu @end menu @node Overview, Introduction, Top, Top @comment node-name, next, previous, up @chapter Overview of @code{make} The @code{make} utility automatically determines which pieces of a large program need to be recompiled, and issues commands to recompile them. This manual describes GNU @code{make}, which was implemented by Richard Stallman and Roland McGrath. Development since Version 3.76 has been handled by Paul D. Smith. GNU @code{make} conforms to section 6.2 of @cite{IEEE Standard 1003.2-1992} (POSIX.2). @cindex POSIX @cindex IEEE Standard 1003.2 @cindex standards conformance Our examples show C programs, since they are most common, but you can use @code{make} with any programming language whose compiler can be run with a shell command. Indeed, @code{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 @code{make}. * Reading:: On reading this text. * Bugs:: Problems and bugs. @end menu @node Preparing, Reading, Overview, Overview @ifnottex @heading Preparing and Running Make @end ifnottex To prepare to use @code{make}, you must write a file called the @dfn{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.@refill Once a suitable makefile exists, each time you change some source files, this simple shell command: @example make @end example @noindent suffices to perform all necessary recompilations. The @code{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 @code{make} to control which files should be recompiled, or how. @xref{Running, ,How to Run @code{make}}. @node Reading, Bugs, Preparing, Overview @section How to Read This Manual If you are new to @code{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. @ifnottex The exception is the second chapter, @ref{Introduction, ,An Introduction to Makefiles}, all of which is introductory. @end ifnottex @iftex The exception is @ref{Introduction, ,An Introduction to Makefiles}, all of which is introductory. @end iftex If you are familiar with other @code{make} programs, see @ref{Features, ,Features of GNU @code{make}}, which lists the enhancements GNU @code{make} has, and @ref{Missing, ,Incompatibilities and Missing Features}, which explains the few things GNU @code{make} lacks that others have. For a quick summary, see @ref{Options Summary}, @ref{Quick Reference}, and @ref{Special Targets}. @node Bugs, , Reading, Overview @section Problems and Bugs @cindex reporting bugs @cindex bugs, reporting @cindex problems and bugs, reporting If you have problems with GNU @code{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 @code{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: @example bug-make@@gnu.org @end example @noindent or use our Web-based project management tool, at: @example http://savannah.gnu.org/projects/make/ @end example @noindent In addition to the information above, please be careful to include the version number of @code{make} you are using. You can get this information with the command @samp{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 @samp{make --help}. @node Introduction, Makefiles, Overview, Top @comment node-name, next, previous, up @chapter An Introduction to Makefiles You need a file called a @dfn{makefile} to tell @code{make} what to do. Most often, the makefile tells @code{make} how to compile and link a program. @cindex makefile 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 @code{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 @ref{Complex Makefile}. When @code{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. @cindex recompilation @cindex editor @menu * Rule Introduction:: What a rule looks like. * Simple Makefile:: A simple makefile. * How Make Works:: How @code{make} processes this makefile. * Variables Simplify:: Variables make makefiles simpler. * make Deduces:: Letting @code{make} deduce the recipes. * Combine By Prerequisite:: Another style of makefile. * Cleanup:: Rules for cleaning the directory. @end menu @node Rule Introduction, Simple Makefile, Introduction, Introduction @comment node-name, next, previous, up @section What a Rule Looks Like @cindex rule, introduction to @cindex makefile rule parts @cindex parts of makefile rule A simple makefile consists of rules'' with the following shape: @cindex targets, introduction to @cindex prerequisites, introduction to @cindex recipes, introduction to @example @group @var{target} @dots{} : @var{prerequisites} @dots{} @var{recipe} @dots{} @dots{} @end group @end example A @dfn{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 @samp{clean} (@pxref{Phony Targets}). A @dfn{prerequisite} is a file that is used as input to create the target. A target often depends on several files. @cindex tabs in rules A @dfn{recipe} is an action that @code{make} carries out. A recipe may have more than one command, either on the same line or each on its own line. @strong{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 @code{.RECIPEPREFIX} variable to an alternate character (@pxref{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 @samp{clean} does not have prerequisites. A @dfn{rule}, then, explains how and when to remake certain files which are the targets of the particular rule. @code{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. @xref{Rules, , Writing 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. @node Simple Makefile, How Make Works, Rule Introduction, Introduction @section A Simple Makefile @cindex simple makefile @cindex makefile, simple Here is a straightforward makefile that describes the way an executable file called @code{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 @file{defs.h}, but only those defining editing commands include @file{command.h}, and only low level files that change the editor buffer include @file{buffer.h}. @example @group 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 @end group @end example @noindent We split each long line into two lines using backslash/newline; this is like using one long line, but is easier to read. @xref{Splitting Lines, , Splitting Long Lines}. @cindex continuation lines @cindex @code{\} (backslash), for continuation lines @cindex backslash (@code{\}), for continuation lines @cindex quoting newline, in makefile @cindex newline, quoting, in makefile To use this makefile to create the executable file called @file{edit}, type: @example make @end example To use this makefile to delete the executable file and all the object files from the directory, type: @example make clean @end example In the example makefile, the targets include the executable file @samp{edit}, and the object files @samp{main.o} and @samp{kbd.o}. The prerequisites are files such as @samp{main.c} and @samp{defs.h}. In fact, each @samp{.o} file is both a target and a prerequisite. Recipes include @w{@samp{cc -c main.c}} and @w{@samp{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, @file{edit} depends on each of the eight object files; the object file @file{main.o} depends on the source file @file{main.c} and on the header file @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 @code{.RECIPEPREFIX} variable; @pxref{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 @code{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 @code{make} does is execute the recipe you have specified when the target file needs to be updated.)@refill @cindex recipe The target @samp{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, @samp{clean} is not a prerequisite of any other rule. Consequently, @code{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 @dfn{phony targets}. @xref{Phony Targets}, for information about this kind of target. @xref{Errors, , Errors in Recipes}, to see how to cause @code{make} to ignore errors from @code{rm} or any other command. @cindex @code{clean} target @cindex @code{rm} (shell command) @node How Make Works, Variables Simplify, Simple Makefile, Introduction @comment node-name, next, previous, up @section How @code{make} Processes a Makefile @cindex processing a makefile @cindex makefile, how @code{make} processes By default, @code{make} starts with the first target (not targets whose names start with @samp{.}). This is called the @dfn{default goal}. (@dfn{Goals} are the targets that @code{make} strives ultimately to update. You can override this behavior using the command line (@pxref{Goals, , Arguments to Specify the Goals}) or with the @code{.DEFAULT_GOAL} special variable (@pxref{Special Variables, , Other Special Variables}). @cindex default goal @cindex goal, default @cindex goal In the simple example of the previous section, the default goal is to update the executable program @file{edit}; therefore, we put that rule first. Thus, when you give the command: @example make @end example @noindent @code{make} reads the makefile in the current directory and begins by processing the first rule. In the example, this rule is for relinking @file{edit}; but before @code{make} can fully process this rule, it must process the rules for the files that @file{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 @samp{.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 @code{make} to do so (with a command such as @w{@code{make clean}}). Before recompiling an object file, @code{make} considers updating its prerequisites, the source file and header files. This makefile does not specify anything to be done for them---the @samp{.c} and @samp{.h} files are not the targets of any rules---so @code{make} does nothing for these files. But @code{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, @code{make} decides whether to relink @file{edit}. This must be done if the file @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 @file{edit}, so @file{edit} is relinked. @cindex relinking Thus, if we change the file @file{insert.c} and run @code{make}, @code{make} will compile that file to update @file{insert.o}, and then link @file{edit}. If we change the file @file{command.h} and run @code{make}, @code{make} will recompile the object files @file{kbd.o}, @file{command.o} and @file{files.o} and then link the file @file{edit}. @node Variables Simplify, make Deduces, How Make Works, Introduction @section Variables Make Makefiles Simpler @cindex variables @cindex simplifying with variables In our example, we had to list all the object files twice in the rule for @file{edit} (repeated here): @example @group 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 @end group @end example @cindex @code{objects} 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. @dfn{Variables} allow a text string to be defined once and substituted in multiple places later (@pxref{Using Variables, ,How to Use Variables}). @cindex @code{OBJECTS} @cindex @code{objs} @cindex @code{OBJS} @cindex @code{obj} @cindex @code{OBJ} It is standard practice for every makefile to have a variable named @code{objects}, @code{OBJECTS}, @code{objs}, @code{OBJS}, @code{obj}, or @code{OBJ} which is a list of all object file names. We would define such a variable @code{objects} with a line like this in the makefile:@refill @example @group objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o @end group @end example @noindent Then, each place we want to put a list of the object file names, we can substitute the variable's value by writing @samp{$(objects)} (@pxref{Using Variables, ,How to Use Variables}). Here is how the complete simple makefile looks when you use a variable for the object files: @example @group 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 $(objects) @end group @end example @node make Deduces, Combine By Prerequisite, Variables Simplify, Introduction @section Letting @code{make} Deduce the Recipes @cindex deducing recipes (implicit rules) @cindex implicit rule, introduction to @cindex rule, implicit, introduction to It is not necessary to spell out the recipes for compiling the individual C source files, because @code{make} can figure them out: it has an @dfn{implicit rule} for updating a @samp{.o} file from a correspondingly named @samp{.c} file using a @samp{cc -c} command. For example, it will use the recipe @samp{cc -c main.c -o main.o} to compile @file{main.c} into @file{main.o}. We can therefore omit the recipes from the rules for the object files. @xref{Implicit Rules, ,Using Implicit Rules}.@refill When a @samp{.c} file is used automatically in this way, it is also automatically added to the list of prerequisites. We can therefore omit the @samp{.c} files from the prerequisites, provided we omit the recipe. Here is the entire example, with both of these changes, and a variable @code{objects} as suggested above: @example @group objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit :$(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) @end group @end example @noindent This is how we would write the makefile in actual practice. (The complications associated with @samp{clean} are described elsewhere. See @ref{Phony Targets}, and @ref{Errors, ,Errors in Recipes}.) Because implicit rules are so convenient, they are important. You will see them used frequently.@refill @node Combine By Prerequisite, Cleanup, make Deduces, Introduction @section Another Style of Makefile @cindex combining rules by prerequisite 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: @example @group objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit$(objects) $(objects) : defs.h kbd.o command.o files.o : command.h display.o insert.o search.o files.o : buffer.h @end group @end example @noindent Here @file{defs.h} is given as a prerequisite of all the object files; @file{command.h} and @file{buffer.h} are prerequisites of the specific object files listed for them. Whether this is better is a matter of taste: it is more compact, but some people dislike it because they find it clearer to put all the information about each target in one place. @node Cleanup, , Combine By Prerequisite, Introduction @section Rules for Cleaning the Directory @cindex cleaning up @cindex removing, to clean up Compiling a program is not the only thing you might want to write rules for. Makefiles commonly tell how to do a few other things besides compiling a program: for example, how to delete all the object files and executables so that the directory is @samp{clean}. @cindex @code{clean} target Here is how we could write a @code{make} rule for cleaning our example editor: @example @group clean: rm edit$(objects) @end group @end example In practice, we might want to write the rule in a somewhat more complicated manner to handle unanticipated situations. We would do this: @example @group .PHONY : clean clean : -rm edit $(objects) @end group @end example @noindent This prevents @code{make} from getting confused by an actual file called @file{clean} and causes it to continue in spite of errors from @code{rm}. (See @ref{Phony Targets}, and @ref{Errors, ,Errors in Recipes}.) @noindent 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 @code{edit}, which recompiles the editor, to remain the default goal. Since @code{clean} is not a prerequisite of @code{edit}, this rule will not run at all if we give the command @samp{make} with no arguments. In order to make the rule run, we have to type @samp{make clean}. @xref{Running, ,How to Run @code{make}}. @node Makefiles, Rules, Introduction, Top @chapter Writing Makefiles @cindex makefile, how to write The information that tells @code{make} how to recompile a system comes from reading a data base called the @dfn{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 read in. * Parsing Makefiles:: How makefiles are parsed. * Secondary Expansion:: How and when secondary expansion is performed. @end menu @node Makefile Contents, Makefile Names, Makefiles, Makefiles @section What Makefiles Contain Makefiles contain five kinds of things: @dfn{explicit rules}, @dfn{implicit rules}, @dfn{variable definitions}, @dfn{directives}, and @dfn{comments}. Rules, variables, and directives are described at length in later chapters.@refill @itemize @bullet @cindex rule, explicit, definition of @cindex explicit rule, definition of @item An @dfn{explicit rule} says when and how to remake one or more files, called the rule's @dfn{targets}. It lists the other files that the targets depend on, called the @dfn{prerequisites} of the target, and may also give a recipe to use to create or update the targets. @xref{Rules, ,Writing Rules}. @cindex rule, implicit, definition of @cindex implicit rule, definition of @item An @dfn{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. @xref{Implicit Rules, ,Using Implicit Rules}. @cindex variable definition @item A @dfn{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 @code{objects} as a list of all object files (@pxref{Variables Simplify, , Variables Make Makefiles Simpler}). @cindex directive @item A @dfn{directive} is an instruction for @code{make} to do something special while reading the makefile. These include: @itemize @bullet @item Reading another makefile (@pxref{Include, ,Including Other Makefiles}). @item Deciding (based on the values of variables) whether to use or ignore a part of the makefile (@pxref{Conditionals, ,Conditional Parts of Makefiles}). @item Defining a variable from a verbatim string containing multiple lines (@pxref{Multi-Line, ,Defining Multi-Line Variables}). @end itemize @cindex comments, in makefile @cindex @code{#} (comments), in makefile @item @samp{#} in a line of a makefile starts a @dfn{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 @code{#}, escape it with a backslash (e.g., @code{\#}). 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 @code{#} 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 @code{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 @code{make} comments or as recipe text, depending on the context in which the variable is evaluated. @end itemize @menu * Splitting Lines:: Splitting long lines in makefiles @end menu @node Splitting Lines, , Makefile Contents, Makefile Contents @subsection Splitting Long Lines @cindex splitting long lines @cindex long lines, splitting @cindex backslash (@code{\}), to quote newlines Makefiles use a line-based'' syntax in which the newline character is special and marks the end of a statement. GNU @code{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 (@code{\}) 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 (@pxref{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 @code{.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. @subsubheading Splitting Without Adding Whitespace @cindex whitespace, avoiding on line split @cindex removing whitespace from split lines If you need to split a line but do @emph{not} want any whitespace added, you can utilize a subtle trick: replace your backslash/newline pairs with the three characters dollar sign/backslash/newline: @example var := one$\ word @end example After @code{make} removes the backslash/newline and condenses the following line into a single space, this is equivalent to: @example var := one$word @end example Then @code{make} will perform variable expansion. The variable reference @samp{$ } refers to a variable with the one-character name  '' (space) which does not exist, and so expands to the empty string, giving a final assignment which is the equivalent of: @example var := oneword @end example @node Makefile Names, Include, Makefile Contents, Makefiles @section What Name to Give Your Makefile @cindex makefile name @cindex name of makefile @cindex default makefile name @cindex file name of makefile @c following paragraph rewritten to avoid overfull hbox By default, when @code{make} looks for the makefile, it tries the following names, in order: @file{GNUmakefile}, @file{makefile} and @file{Makefile}.@refill @findex Makefile @findex GNUmakefile @findex makefile @cindex @code{README} Normally you should call your makefile either @file{makefile} or @file{Makefile}. (We recommend @file{Makefile} because it appears prominently near the beginning of a directory listing, right near other important files such as @file{README}.) The first name checked, @file{GNUmakefile}, is not recommended for most makefiles. You should use this name if you have a makefile that is specific to GNU @code{make}, and will not be understood by other versions of @code{make}. Other @code{make} programs look for @file{makefile} and @file{Makefile}, but not @file{GNUmakefile}. If @code{make} finds none of these names, it does not use any makefile. Then you must specify a goal with a command argument, and @code{make} will attempt to figure out how to remake it using only its built-in implicit rules. @xref{Implicit Rules, ,Using Implicit Rules}. @cindex @code{-f} @cindex @code{--file} @cindex @code{--makefile} If you want to use a nonstandard name for your makefile, you can specify the makefile name with the @samp{-f} or @samp{--file} option. The arguments @w{@samp{-f @var{name}}} or @w{@samp{--file=@var{name}}} tell @code{make} to read the file @var{name} as the makefile. If you use more than one @samp{-f} or @samp{--file} option, you can specify several makefiles. All the makefiles are effectively concatenated in the order specified. The default makefile names @file{GNUmakefile}, @file{makefile} and @file{Makefile} are not checked automatically if you specify @samp{-f} or @samp{--file}.@refill @cindex specifying makefile name @cindex makefile name, how to specify @cindex name of makefile, how to specify @cindex file name of makefile, how to specify @node Include, MAKEFILES Variable, Makefile Names, Makefiles @section Including Other Makefiles @cindex including other makefiles @cindex makefile, including @findex include The @code{include} directive tells @code{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: @example include @var{filenames}@dots{} @end example @noindent @var{filenames} can contain shell file name patterns. If @var{filenames} is empty, nothing is included and no error is printed. @cindex shell file name pattern (in @code{include}) @cindex shell wildcards (in @code{include}) @cindex wildcard, in @code{include} 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 @code{.RECIPEPREFIX})---if the line begins with a tab, it will be considered a recipe line. Whitespace is required between @code{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 @samp{#} is allowed at the end of the line. If the file names contain any variable or function references, they are expanded. @xref{Using Variables, ,How to Use Variables}. For example, if you have three @file{.mk} files, @file{a.mk}, @file{b.mk}, and @file{c.mk}, and @code{$(bar)} expands to @code{bish bash}, then the following expression @example include foo *.mk$(bar) @end example is equivalent to @example include foo a.mk b.mk c.mk bish bash @end example When @code{make} processes an @code{include} directive, it suspends reading of the containing makefile and reads from each listed file in turn. When that is finished, @code{make} resumes reading the makefile in which the directive appears. One occasion for using @code{include} directives is when several programs, handled by individual makefiles in various directories, need to use a common set of variable definitions (@pxref{Setting, ,Setting Variables}) or pattern rules (@pxref{Pattern Rules, ,Defining and Redefining 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 @code{make}. @xref{Automatic Prerequisites}. @cindex prerequisites, automatic generation @cindex automatic generation of prerequisites @cindex generating prerequisites automatically @cindex @code{-I} @cindex @code{--include-dir} @cindex included makefiles, default directories @cindex default directories for included makefiles @findex /usr/gnu/include @findex /usr/local/include @findex /usr/include 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 @samp{-I} or @samp{--include-dir} option are searched (@pxref{Options Summary, ,Summary of Options}). Then the following directories (if they exist) are searched, in this order: @file{@var{prefix}/include} (normally @file{/usr/local/include} @footnote{GNU Make compiled for MS-DOS and MS-Windows behaves as if @var{prefix} has been defined to be the root of the DJGPP tree hierarchy.}) @file{/usr/gnu/include}, @file{/usr/local/include}, @file{/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 @code{include} continues. Once it has finished reading makefiles, @code{make} will try to remake any that are out of date or don't exist. @xref{Remaking Makefiles, ,How Makefiles Are Remade}. Only after it has tried to find a way to remake a makefile and failed, will @code{make} diagnose the missing makefile as a fatal error. If you want @code{make} to simply ignore a makefile which does not exist or cannot be remade, with no error message, use the @w{@code{-include}} directive instead of @code{include}, like this: @example -include @var{filenames}@dots{} @end example This acts like @code{include} in every way except that there is no error (not even a warning) if any of the @var{filenames} (or any prerequisites of any of the @var{filenames}) do not exist or cannot be remade. For compatibility with some other @code{make} implementations, @code{sinclude} is another name for @w{@code{-include}}. @node MAKEFILES Variable, Remaking Makefiles, Include, Makefiles @section The Variable @code{MAKEFILES} @cindex makefile, and @code{MAKEFILES} variable @cindex including (@code{MAKEFILES} variable) @vindex MAKEFILES If the environment variable @code{MAKEFILES} is defined, @code{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 @code{include} directive: various directories are searched for those files (@pxref{Include, ,Including Other Makefiles}). 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 @code{MAKEFILES} are not found.@refill @cindex recursion, and @code{MAKEFILES} variable The main use of @code{MAKEFILES} is in communication between recursive invocations of @code{make} (@pxref{Recursion, ,Recursive Use of @code{make}}). It usually is not desirable to set the environment variable before a top-level invocation of @code{make}, because it is usually better not to mess with a makefile from outside. However, if you are running @code{make} without a specific makefile, a makefile in @code{MAKEFILES} can do useful things to help the built-in implicit rules work better, such as defining search paths (@pxref{Directory Search}). Some users are tempted to set @code{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 @code{include} directives in the makefiles. @xref{Include, , Including Other Makefiles}. @node Remaking Makefiles, Overriding Makefiles, MAKEFILES Variable, Makefiles @section How Makefiles Are Remade @cindex updating makefiles @cindex remaking makefiles @cindex makefile, remaking of 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 @code{make} to get an up-to-date version of the makefile to read in. To this end, after reading in all makefiles @code{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 (@pxref{Implicit Rules, ,Using Implicit Rules}), it will be updated if necessary. After all makefiles have been checked, if any have actually been changed, @code{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.) Each restart will cause the special variable @code{MAKE_RESTARTS} to be updated (@pxref{Special Variables}).@refill If you know that one or more of your makefiles cannot be remade and you want to keep @code{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 (@pxref{Empty Recipes, ,Using 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 (@pxref{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 @code{make} is run, and then again after @code{make} starts over and reads the makefiles in again. This would cause an infinite loop: @code{make} would constantly remake the makefile, and never do anything else. So, to avoid this, @code{make} will @strong{not} attempt to remake makefiles which are specified as targets of a double-colon rule with a recipe but no prerequisites.@refill If you do not specify any makefiles to be read with @samp{-f} or @samp{--file} options, @code{make} will try the default makefile names; @pxref{Makefile Names, ,What Name to Give Your Makefile}. Unlike makefiles explicitly requested with @samp{-f} or @samp{--file} options, @code{make} is not certain that these makefiles should exist. However, if a default makefile does not exist but can be created by running @code{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, @code{make} will try to make each of them in the same order in which they are searched for (@pxref{Makefile Names, ,What Name to Give Your Makefile}) until it succeeds in making one, or it runs out of names to try. Note that it is not an error if @code{make} cannot find or make any makefile; a makefile is not always necessary.@refill When you use the @samp{-t} or @samp{--touch} option (@pxref{Instead of Execution, ,Instead of Executing Recipes}), you would not want to use an out-of-date makefile to decide which targets to touch. So the @samp{-t} option has no effect on updating makefiles; they are really updated even if @samp{-t} is specified. Likewise, @samp{-q} (or @samp{--question}) and @samp{-n} (or @samp{--just-print}) do not prevent updating of makefiles, because an out-of-date makefile would result in the wrong output for other targets. Thus, @samp{make -f mfile -n foo} will update @file{mfile}, read it in, and then print the recipe to update @file{foo} and its prerequisites without running it. The recipe printed for @file{foo} will be the one specified in the updated contents of @file{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 @samp{-t} and so on do apply to them. Thus, @samp{make -f mfile -n mfile foo} would read the makefile @file{mfile}, print the recipe needed to update it without actually running it, and then print the recipe needed to update @file{foo} without running that. The recipe for @file{foo} will be the one specified by the existing contents of @file{mfile}. @node Overriding Makefiles, Reading Makefiles, Remaking Makefiles, Makefiles @section Overriding Part of Another Makefile @cindex overriding makefiles @cindex makefile, overriding Sometimes it is useful to have a makefile that is mostly just like another makefile. You can often use the @samp{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. @cindex match-anything rule, used to override 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, @code{make} should look in another makefile. @xref{Pattern Rules}, for more information on pattern rules. For example, if you have a makefile called @file{Makefile} that says how to make the target @samp{foo} (and other targets), you can write a makefile called @file{GNUmakefile} that contains: @example foo: frobnicate > foo %: force @@$(MAKE) -f Makefile$@@ force: ; @end example If you say @samp{make foo}, @code{make} will find @file{GNUmakefile}, read it, and see that to make @file{foo}, it needs to run the recipe @samp{frobnicate > foo}. If you say @samp{make bar}, @code{make} will find no way to make @file{bar} in @file{GNUmakefile}, so it will use the recipe from the pattern rule: @samp{make -f Makefile bar}. If @file{Makefile} provides a rule for updating @file{bar}, @code{make} will apply the rule. And likewise for any other target that @file{GNUmakefile} does not say how to make. The way this works is that the pattern rule has a pattern of just @samp{%}, so it matches any target whatever. The rule specifies a prerequisite @file{force}, to guarantee that the recipe will be run even if the target file already exists. We give the @file{force} target an empty recipe to prevent @code{make} from searching for an implicit rule to build it---otherwise it would apply the same match-anything rule to @file{force} itself and create a prerequisite loop! @node Reading Makefiles, Parsing Makefiles, Overriding Makefiles, Makefiles @section How @code{make} Reads a Makefile @cindex reading makefiles @cindex makefile, reading GNU @code{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 and implicit and explicit rules, and builds a dependency graph of all the targets and their prerequisites. During the second phase, @code{make} uses this internalized data to determine which targets need to be updated and run the recipes necessary to update them. 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. Below is a summary of the different constructs that can be found in a makefile, and the phase in which expansion happens for each part of the construct. We say that expansion is @dfn{immediate} if it happens during the first phase: @code{make} will expand that part of the construct as the makefile is parsed. We say that expansion is @dfn{deferred} if it is not immediate. Expansion of a deferred construct part is delayed until the expansion is used: either when it is referenced in an immediate context, or when it is needed during 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. @subheading Variable Assignment @cindex +=, expansion @cindex =, expansion @cindex ?=, expansion @cindex +=, expansion @cindex !=, expansion @cindex define, expansion Variable definitions are parsed as follows: @example @var{immediate} = @var{deferred} @var{immediate} ?= @var{deferred} @var{immediate} := @var{immediate} @var{immediate} ::= @var{immediate} @var{immediate} += @var{deferred} or @var{immediate} @var{immediate} != @var{immediate} define @var{immediate} @var{deferred} endef define @var{immediate} = @var{deferred} endef define @var{immediate} ?= @var{deferred} endef define @var{immediate} := @var{immediate} endef define @var{immediate} ::= @var{immediate} endef define @var{immediate} += @var{deferred} or @var{immediate} endef define @var{immediate} != @var{immediate} endef @end example For the append operator @samp{+=}, the right-hand side is considered immediate if the variable was previously set as a simple variable (@samp{:=} or @samp{::=}), and deferred otherwise. For the shell assignment operator @samp{!=}, 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). @subheading Conditional Directives @cindex ifdef, expansion @cindex ifeq, expansion @cindex ifndef, expansion @cindex ifneq, expansion 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 @emph{must} move the condition into the recipe and use shell conditional syntax instead. @subheading Rule Definition @cindex target, expansion @cindex prerequisite, expansion @cindex implicit rule, expansion @cindex pattern rule, expansion @cindex explicit rule, expansion A rule is always expanded the same way, regardless of the form: @example @var{immediate} : @var{immediate} ; @var{deferred} @var{deferred} @end example That is, the target and prerequisite sections are expanded immediately, and the recipe used to build the target is always deferred. This is true for explicit rules, pattern rules, suffix rules, static pattern rules, and simple prerequisite definitions. @node Parsing Makefiles, Secondary Expansion, Reading Makefiles, Makefiles @section How Makefiles Are Parsed @cindex parsing makefiles @cindex makefiles, parsing GNU @code{make} parses makefiles line-by-line. Parsing proceeds using the following steps: @enumerate @item Read in a full logical line, including backslash-escaped lines (@pxref{Splitting Lines, , Splitting Long Lines}). @item Remove comments (@pxref{Makefile Contents, , What Makefiles Contain}). @item If the line begins with the recipe prefix character and we are in a rule context, add the line to the current recipe and read the next line (@pxref{Recipe Syntax}). @item Expand elements of the line which appear in an @emph{immediate} expansion context (@pxref{Reading Makefiles, , How @code{make} Reads a Makefile}).@refill @item Scan the line for a separator character, such as @samp{:} or @samp{=}, to determine whether the line is a macro assignment or a rule (@pxref{Recipe Syntax}). @item Internalize the resulting operation and read the next line. @end enumerate An important consequence of this is that a macro can expand to an entire rule, @emph{if it is one line long}. This will work: @example myrule = target : ; echo built $(myrule) @end example However, this will not work because @code{make} does not re-split lines after it has expanded them: @example define myrule target: echo built endef$(myrule) @end example The above makefile results in the definition of a target @samp{target} with prerequisites @samp{echo} and @samp{built}, as if the makefile contained @code{target: echo built}, rather than a rule with a recipe. Newlines still present in a line after expansion is complete are ignored as normal whitespace. In order to properly expand a multi-line macro you must use the @code{eval} function: this causes the @code{make} parser to be run on the results of the expanded macro (@pxref{Eval Function}). @node Secondary Expansion, , Parsing Makefiles, Makefiles @section Secondary Expansion @cindex secondary expansion @cindex expansion, secondary @findex .SECONDEXPANSION Previously we learned that GNU @code{make} works in two distinct phases: a read-in phase and a target-update phase (@pxref{Reading Makefiles, , How @code{make} Reads a Makefile}). GNU make also has the ability to enable a @emph{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 @code{.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 @emph{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 @emph{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: @example .SECONDEXPANSION: ONEVAR = onefile TWOVAR = twofile myfile: $(ONEVAR) $$(TWOVAR) @end example After the first expansion phase the prerequisites list of the @file{myfile} target will be @code{onefile} and @code{(TWOVAR)}; the first (unescaped) variable reference to @var{ONEVAR} is expanded, while the second (escaped) variable reference is simply unescaped, without being recognized as a variable reference. Now during the secondary expansion the first word is expanded again but since it contains no variable or function references it remains the value @file{onefile}, while the second word is now a normal reference to the variable @var{TWOVAR}, which is expanded to the value @file{twofile}. The final result is that there are two prerequisites, @file{onefile} and @file{twofile}. Obviously, this is not a very interesting case since the same result could more easily have been achieved simply by having both variables appear, unescaped, in the prerequisites list. One difference becomes apparent if the variables are reset; consider this example: @example .SECONDEXPANSION: AVAR = top onefile: (AVAR) twofile:$$(AVAR) AVAR = bottom @end example Here the prerequisite of @file{onefile} will be expanded immediately, and resolve to the value @file{top}, while the prerequisite of @file{twofile} will not be full expanded until the secondary expansion and yield a value of @file{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 @code{$@@}, @code{$*}, 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 @code{$}. Also, secondary expansion occurs for both explicit and implicit (pattern) rules. Knowing this, the possible uses for this feature increase dramatically. For example: @example .SECONDEXPANSION: main_OBJS := main.o try.o test.o lib_OBJS := lib.o api.o main lib: $$($$@@_OBJS) @end example Here, after the initial expansion the prerequisites of both the @file{main} and @file{lib} targets will be @code{$($@@_OBJS)}. During the secondary expansion, the @code{$@@} variable is set to the name of the target and so the expansion for the @file{main} target will yield @code{$(main_OBJS)}, or @code{main.o try.o test.o}, while the secondary expansion for the @file{lib} target will yield @code{$(lib_OBJS)}, or @code{lib.o api.o}. You can also mix in functions here, as long as they are properly escaped: @example main_SRCS := main.c try.c test.c lib_SRCS := lib.c api.c .SECONDEXPANSION: main lib: $$(patsubst %.c,%.o,$$($$@@_SRCS)) @end example This version allows users to specify source files rather than object files, but gives the same resulting prerequisites list as the previous example. Evaluation of automatic variables during the secondary expansion phase, especially of the target name variable @code{$$@@}, 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 @code{make} understands. The subtleties of using the different automatic variables are described below. @subheading Secondary Expansion of Explicit Rules @cindex secondary expansion and explicit rules @cindex explicit rules, secondary expansion of During the secondary expansion of explicit rules, @code{$$@@} and @code{$$%} evaluate, respectively, to the file name of the target and, when the target is an archive member, the target member name. The @code{$$<} variable evaluates to the first prerequisite in the first rule for this target. @code{$$^} and @code{$$+} evaluate to the list of all prerequisites of rules @emph{that have already appeared} for the same target (@code{$$+} with repetitions and @code{$$^} without). The following example will help illustrate these behaviors: @example .SECONDEXPANSION: foo: foo.1 bar.1$$< $$^$$+ # line #1 foo: foo.2 bar.2 $$<$$^ $$+ # line #2 foo: foo.3 bar.3$$< $$^$$+ # line #3 @end example In the first prerequisite list, all three variables (@code{$$<}, @code{$$^}, and @code{$$+}) expand to the empty string. In the second, they will have values @code{foo.1}, @code{foo.1 bar.1}, and @code{foo.1 bar.1} respectively. In the third they will have values @code{foo.1}, @code{foo.1 bar.1 foo.2 bar.2}, and @code{foo.1 bar.1 foo.2 bar.2 foo.1 foo.1 bar.1 foo.1 bar.1} respectively. Rules undergo secondary expansion in makefile order, except that the rule with the recipe is always evaluated last. The variables @code{$$?} and @code{$$*} are not available and expand to the empty string. @subheading Secondary Expansion of Static Pattern Rules @cindex secondary expansion and static pattern rules @cindex static pattern rules, secondary expansion of Rules for secondary expansion of static pattern rules are identical to those for explicit rules, above, with one exception: for static pattern rules the @code{$$*} variable is set to the pattern stem. As with explicit rules, @code{$$?} is not available and expands to the empty string. @subheading Secondary Expansion of Implicit Rules @cindex secondary expansion and implicit rules @cindex implicit rules, secondary expansion of As @code{make} searches for an implicit rule, it substitutes the stem and then performs secondary expansion for every rule with a matching target pattern. The value of the automatic variables is derived in the same fashion as for static pattern rules. As an example: @example .SECONDEXPANSION: foo: bar foo foz: fo%: bo% %oo:$$< $$^$$+ $$* @end example When the implicit rule is tried for target @file{foo}, @code{$$<} expands to @file{bar}, @code{$$^} expands to @file{bar boo}, @code{$$+} also expands to @file{bar boo}, and @code{$$*} expands to @file{f}. Note that the directory prefix (D), as described in @ref{Implicit Rule Search, ,Implicit Rule Search Algorithm}, is appended (after expansion) to all the patterns in the prerequisites list. As an example:@refill @example .SECONDEXPANSION: /tmp/foo.o: %.o:$$(addsuffix /%.c,foo bar) foo.h @@echo$^ @end example The prerequisite list printed, after the secondary expansion and directory prefix reconstruction, will be @file{/tmp/foo/foo.c /tmp/bar/foo.c foo.h}. If you are not interested in this reconstruction, you can use @code{$$*} instead of @code{%} in the prerequisites list. @node Rules, Recipes, Makefiles, Top @chapter Writing Rules @cindex writing rules @cindex rule, how to write @cindex target @cindex prerequisite A @dfn{rule} appears in the makefile and says when and how to remake certain files, called the rule's @dfn{targets} (most often only one per rule). It lists the other files that are the @dfn{prerequisites} of the target, and the @dfn{recipe} to use to create or update the target. @cindex default goal @cindex goal, default The order of rules is not significant, except for determining the @dfn{default goal}: the target for @code{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, @samp{/}, as well; and, a target that defines a pattern rule has no effect on the default goal. (@xref{Pattern Rules, ,Defining and Redefining 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 @samp{all}). @xref{Goals, ,Arguments to Specify the 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. @end menu @ifnottex @node Rule Example, Rule Syntax, Rules, Rules @section Rule Example Here is an example of a rule: @example foo.o : foo.c defs.h # module for twiddling the frobs cc -c -g foo.c @end example Its target is @file{foo.o} and its prerequisites are @file{foo.c} and @file{defs.h}. It has one command in the recipe: @samp{cc -c -g foo.c}. The recipe starts with a tab to identify it as a recipe. This rule says two things: @itemize @bullet @item How to decide whether @file{foo.o} is out of date: it is out of date if it does not exist, or if either @file{foo.c} or @file{defs.h} is more recent than it. @item How to update the file @file{foo.o}: by running @code{cc} as stated. The recipe does not explicitly mention @file{defs.h}, but we presume that @file{foo.c} includes it, and that is why @file{defs.h} was added to the prerequisites. @end itemize @end ifnottex @node Rule Syntax, Prerequisite Types, Rule Example, Rules @section Rule Syntax @cindex rule syntax @cindex syntax of rules In general, a rule looks like this: @example @var{targets} : @var{prerequisites} @var{recipe} @dots{} @end example @noindent or like this: @example @var{targets} : @var{prerequisites} ; @var{recipe} @var{recipe} @dots{} @end example @cindex targets @cindex rule targets The @var{targets} are file names, separated by spaces. Wildcard characters may be used (@pxref{Wildcards, ,Using Wildcard Characters in File Names}) and a name of the form @file{@var{a}(@var{m})} represents member @var{m} in archive file @var{a} (@pxref{Archive Members, ,Archive Members as Targets}). Usually there is only one target per rule, but occasionally there is a reason to have more (@pxref{Multiple Targets, , Multiple Targets in a Rule}).@refill @cindex recipes @cindex tab character (in commands) The @var{recipe} lines start with a tab character (or the first character in the value of the @code{.RECIPEPREFIX} variable; @pxref{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. @xref{Recipes, ,Writing Recipes in Rules}. @cindex dollar sign (@code{}), in rules @cindex @code{}, in rules @cindex rules, and @code{} Because dollar signs are used to start @code{make} variable references, if you really want a dollar sign in a target or prerequisite you must write two of them, @samp{$$} (@pxref{Using Variables, ,How to Use Variables}). If you have enabled secondary expansion (@pxref{Secondary Expansion}) and you want a literal dollar sign in the prerequisites list, you must actually write @emph{four} dollar signs (@samp{}). You may split a long line by inserting a backslash followed by a newline, but this is not required, as @code{make} places no limit on the length of a line in a makefile. A rule tells @code{make} two things: when the targets are out of date, and how to update them when necessary. @cindex prerequisites @cindex rule prerequisites The criterion for being out of date is specified in terms of the @var{prerequisites}, which consist of file names separated by spaces. (Wildcards and archive members (@pxref{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 @var{recipe}. This is one or more lines to be executed by the shell (normally @samp{sh}), but with some extra features (@pxref{Recipes, ,Writing Recipes in Rules}). @node Prerequisite Types, Wildcards, Rule Syntax, Rules @comment node-name, next, previous, up @section Types of Prerequisites @cindex prerequisite types @cindex types of prerequisites @cindex prerequisites, normal @cindex normal prerequisites @cindex prerequisites, order-only @cindex order-only prerequisites There are actually two different types of prerequisites understood by GNU @code{make}: normal prerequisites such as described in the previous section, and @dfn{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 @emph{without} forcing the target to be updated if one of those rules is executed. In that case, you want to define @dfn{order-only} prerequisites. Order-only prerequisites can be specified by placing a pipe symbol (@code{|}) in the prerequisites list: any prerequisites to the left of the pipe symbol are normal; any prerequisites to the right are order-only: @example @var{targets} : @var{normal-prerequisites} | @var{order-only-prerequisites} @end example 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 @code{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: @example 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) @end example Now the rule to create the @file{objdir} directory will be run, if needed, before any @samp{.o} is built, but no @samp{.o} will be built because the @file{objdir} directory timestamp changed. @node Wildcards, Directory Search, Prerequisite Types, Rules @section Using Wildcard Characters in File Names @cindex wildcard @cindex file name with wildcards @cindex globbing (wildcards) @cindex @code{*} (wildcard character) @cindex @code{?} (wildcard character) @cindex @code{[@dots{}]} (wildcard characters) A single file name can specify many files using @dfn{wildcard characters}. The wildcard characters in @code{make} are @samp{*}, @samp{?} and @samp{[@dots{}]}, the same as in the Bourne shell. For example, @file{*.c} specifies a list of all the files (in the working directory) whose names end in @samp{.c}.@refill @cindex @code{~} (tilde) @cindex tilde (@code{~}) @cindex home directory The character @samp{~} 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 @file{~/bin} expands to @file{/home/you/bin}. If the @samp{~} is followed by a word, the string represents the home directory of the user named by that word. For example @file{~john/bin} expands to @file{/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 @var{HOME}.@refill Wildcard expansion is performed by @code{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 @code{wildcard} function. The special significance of a wildcard character can be turned off by preceding it with a backslash. Thus, @file{foo\*bar} would refer to a specific file whose name consists of @samp{foo}, an asterisk, and @samp{bar}.@refill @menu * Wildcard Examples:: Several examples. * Wildcard Pitfall:: Problems to avoid. * Wildcard Function:: How to cause wildcard expansion where it does not normally take place. @end menu @node Wildcard Examples, Wildcard Pitfall, Wildcards, Wildcards @subsection 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: @example @group clean: rm -f *.o @end group @end example @cindex @code{rm} (shell command) Wildcards are also useful in the prerequisites of a rule. With the following rule in the makefile, @samp{make print} will print all the @samp{.c} files that have changed since the last time you printed them: @example print: *.c lpr -p$? touch print @end example @cindex @code{print} target @cindex @code{lpr} (shell command) @cindex @code{touch} (shell command) @noindent This rule uses @file{print} as an empty target file; see @ref{Empty Targets, ,Empty Target Files to Record Events}. (The automatic variable @samp{$?} is used to print only those files that have changed; see @ref{Automatic Variables}.)@refill Wildcard expansion does not happen when you define a variable. Thus, if you write this: @example objects = *.o @end example @noindent then the value of the variable @code{objects} is the actual string @samp{*.o}. However, if you use the value of @code{objects} in a target or prerequisite, wildcard expansion will take place there. If you use the value of @code{objects} in a recipe, the shell may perform wildcard expansion when the recipe runs. To set @code{objects} to the expansion, instead use: @example objects :=$(wildcard *.o) @end example @noindent @xref{Wildcard Function}. @node Wildcard Pitfall, Wildcard Function, Wildcard Examples, Wildcards @subsection Pitfalls of Using Wildcards @cindex wildcard pitfalls @cindex pitfalls of wildcards @cindex mistakes with wildcards @cindex errors with wildcards @cindex problems with 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 @file{foo} is made from all the object files in the directory, and you write this: @example objects = *.o foo : $(objects) cc -o foo$(CFLAGS) $(objects) @end example @noindent The value of @code{objects} is the actual string @samp{*.o}. Wildcard expansion happens in the rule for @file{foo}, so that each @emph{existing} @samp{.o} file becomes a prerequisite of @file{foo} and will be recompiled if necessary. But what if you delete all the @samp{.o} files? When a wildcard matches no files, it is left as it is, so then @file{foo} will depend on the oddly-named file @file{*.o}. Since no such file is likely to exist, @code{make} will give you an error saying it cannot figure out how to make @file{*.o}. This is not what you want! Actually it is possible to obtain the desired result with wildcard expansion, but you need more sophisticated techniques, including the @code{wildcard} function and string substitution. @ifnottex @xref{Wildcard Function, ,The Function @code{wildcard}}. @end ifnottex @iftex These are described in the following section. @end iftex @cindex wildcards and MS-DOS/MS-Windows backslashes @cindex backslashes in pathnames and wildcard expansion Microsoft operating systems (MS-DOS and MS-Windows) use backslashes to separate directories in pathnames, like so: @example c:\foo\bar\baz.c @end example This is equivalent to the Unix-style @file{c:/foo/bar/baz.c} (the @file{c:} part is the so-called drive letter). When @code{make} runs on these systems, it supports backslashes as well as the Unix-style forward slashes in pathnames. However, this support does @emph{not} include the wildcard expansion, where backslash is a quote character. Therefore, you @emph{must} use Unix-style slashes in these cases. @node Wildcard Function, , Wildcard Pitfall, Wildcards @subsection The Function @code{wildcard} @findex wildcard Wildcard expansion happens automatically in rules. But wildcard expansion does not normally take place when a variable is set, or inside the arguments of a function. If you want to do wildcard expansion in such places, you need to use the @code{wildcard} function, like this: @example$(wildcard @var{pattern}@dots{}) @end example @noindent 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 @code{wildcard} function. Note that this is different from how unmatched wildcards behave in rules, where they are used verbatim rather than ignored (@pxref{Wildcard Pitfall}). One use of the @code{wildcard} function is to get a list of all the C source files in a directory, like this: @example $(wildcard *.c) @end example We can change the list of C source files into a list of object files by replacing the @samp{.c} suffix with @samp{.o} in the result, like this: @example$(patsubst %.c,%.o,$(wildcard *.c)) @end example @noindent (Here we have used another function, @code{patsubst}. @xref{Text Functions, ,Functions for String Substitution and Analysis}.)@refill Thus, a makefile to compile all C source files in the directory and then link them together could be written as follows: @example objects :=$(patsubst %.c,%.o,$(wildcard *.c)) foo :$(objects) cc -o foo $(objects) @end example @noindent (This takes advantage of the implicit rule for compiling C programs, so there is no need to write explicit rules for compiling the files. @xref{Flavors, ,The Two Flavors of Variables}, for an explanation of @samp{:=}, which is a variant of @samp{=}.) @node Directory Search, Phony Targets, Wildcards, Rules @section Searching Directories for Prerequisites @vindex VPATH @findex vpath @cindex vpath @cindex search path for prerequisites (@code{VPATH}) @cindex directory search (@code{VPATH}) For large systems, it is often desirable to put sources in a separate directory from the binaries. The @dfn{directory search} features of @code{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. @end menu @node General Search, Selective Search, Directory Search, Directory Search @subsection @code{VPATH}: Search Path for All Prerequisites @vindex VPATH The value of the @code{make} variable @code{VPATH} specifies a list of directories that @code{make} should search. Most often, the directories are expected to contain prerequisite files that are not in the current directory; however, @code{make} uses @code{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, @code{make} searches the directories listed in @code{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. @xref{Recipes/Search, ,Writing Recipes with Directory Search}. In the @code{VPATH} variable, directory names are separated by colons or blanks. The order in which directories are listed is the order followed by @code{make} in its search. (On MS-DOS and MS-Windows, semi-colons are used as separators of directory names in @code{VPATH}, since the colon can be used in the pathname itself, after the drive letter.) For example, @example VPATH = src:../headers @end example @noindent specifies a path containing two directories, @file{src} and @file{../headers}, which @code{make} searches in that order. With this value of @code{VPATH}, the following rule, @example foo.o : foo.c @end example @noindent is interpreted as if it were written like this: @example foo.o : src/foo.c @end example @noindent assuming the file @file{foo.c} does not exist in the current directory but is found in the directory @file{src}. @node Selective Search, Search Algorithm, General Search, Directory Search @subsection The @code{vpath} Directive @findex vpath Similar to the @code{VPATH} variable, but more selective, is the @code{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 @code{vpath} directive: @table @code @item vpath @var{pattern} @var{directories} Specify the search path @var{directories} for file names that match @var{pattern}. The search path, @var{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 @code{VPATH} variable. @item vpath @var{pattern} Clear out the search path associated with @var{pattern}. @c Extra blank line makes sure this gets two lines. @item vpath Clear all search paths previously specified with @code{vpath} directives. @end table A @code{vpath} pattern is a string containing a @samp{%} character. The string must match the file name of a prerequisite that is being searched for, the @samp{%} character matching any sequence of zero or more characters (as in pattern rules; @pxref{Pattern Rules, ,Defining and Redefining Pattern Rules}). For example, @code{%.h} matches files that end in @code{.h}. (If there is no @samp{%}, the pattern must match the prerequisite exactly, which is not useful very often.) @cindex @code{%}, quoting in @code{vpath} @cindex @code{%}, quoting with @code{\} (backslash) @cindex @code{\} (backslash), to quote @code{%} @cindex backslash (@code{\}), to quote @code{%} @cindex quoting @code{%}, in @code{vpath} @samp{%} characters in a @code{vpath} directive's pattern can be quoted with preceding backslashes (@samp{\}). Backslashes that would otherwise quote @samp{%} characters can be quoted with more backslashes. Backslashes that quote @samp{%} characters or other backslashes are removed from the pattern before it is compared to file names. Backslashes that are not in danger of quoting @samp{%} characters go unmolested.@refill When a prerequisite fails to exist in the current directory, if the @var{pattern} in a @code{vpath} directive matches the name of the prerequisite file, then the @var{directories} in that directive are searched just like (and before) the directories in the @code{VPATH} variable. For example, @example vpath %.h ../headers @end example @noindent tells @code{make} to look for any prerequisite whose name ends in @file{.h} in the directory @file{../headers} if the file is not found in the current directory. If several @code{vpath} patterns match the prerequisite file's name, then @code{make} processes each matching @code{vpath} directive one by one, searching all the directories mentioned in each directive. @code{make} handles multiple @code{vpath} directives in the order in which they appear in the makefile; multiple directives with the same pattern are independent of each other. @need 750 Thus, @example @group vpath %.c foo vpath % blish vpath %.c bar @end group @end example @noindent will look for a file ending in @samp{.c} in @file{foo}, then @file{blish}, then @file{bar}, while @example @group vpath %.c foo:bar vpath % blish @end group @end example @noindent will look for a file ending in @samp{.c} in @file{foo}, then @file{bar}, then @file{blish}. @node Search Algorithm, Recipes/Search, Selective Search, Directory Search @subsection How Directory Searches are Performed @cindex algorithm for directory search @cindex directory search algorithm When a prerequisite is found through directory search, regardless of type (general or selective), the pathname located may not be the one that @code{make} actually provides you in the prerequisite list. Sometimes the path discovered through directory search is thrown away. The algorithm @code{make} uses to decide whether to keep or abandon a path found via directory search is as follows: @enumerate @item If a target file does not exist at the path specified in the makefile, directory search is performed. @item If the directory search is successful, that path is kept and this file is tentatively stored as the target. @item All prerequisites of this target are examined using this same method. @item After processing the prerequisites, the target may or may not need to be rebuilt: @enumerate a @item If the target does @emph{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 @code{make} doesn't need to rebuild the target then you use the path found via directory search. @item If the target @emph{does} need to be rebuilt (is out-of-date), the pathname found during directory search is @emph{thrown away}, and the target is rebuilt using the file name specified in the makefile. In short, if @code{make} must rebuild, then the target is rebuilt locally, not in the directory found via directory search. @end enumerate @end enumerate This algorithm may seem complex, but in practice it is quite often exactly what you want. @cindex traditional directory search (GPATH) @cindex directory search, traditional (GPATH) Other versions of @code{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. @vindex GPATH If, in fact, this is the behavior you want for some or all of your directories, you can use the @code{GPATH} variable to indicate this to @code{make}. @code{GPATH} has the same syntax and format as @code{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 @code{GPATH}, then that pathname is not thrown away. The target is rebuilt using the expanded path. @node Recipes/Search, Implicit/Search, Search Algorithm, Directory Search @subsection Writing Recipes with Directory Search @cindex recipes, and directory search @cindex directory search (@code{VPATH}), and recipes 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 @code{make} finds it. This is done with the @dfn{automatic variables} such as @samp{$^} (@pxref{Automatic Variables}). For instance, the value of @samp{$^} is a list of all the prerequisites of the rule, including the names of the directories in which they were found, and the value of @samp{$@@} is the target. Thus:@refill @example foo.o : foo.c cc -c $(CFLAGS)$^ -o $@@ @end example @noindent (The variable @code{CFLAGS} exists so you can specify flags for C compilation by implicit rules; we use it here for consistency so it will affect all C compilations uniformly; @pxref{Implicit Variables, ,Variables Used by Implicit Rules}.) Often the prerequisites include header files as well, which you do not want to mention in the recipe. The automatic variable @samp{$<} is just the first prerequisite: @example VPATH = src:../headers foo.o : foo.c defs.h hack.h cc -c $(CFLAGS)$< -o $@@ @end example @node Implicit/Search, Libraries/Search, Recipes/Search, Directory Search @subsection Directory Search and Implicit Rules @cindex @code{VPATH}, and implicit rules @cindex directory search (@code{VPATH}), and implicit rules @cindex search path for prerequisites (@code{VPATH}), and implicit rules @cindex implicit rule, and directory search @cindex implicit rule, and @code{VPATH} @cindex rule, implicit, and directory search @cindex rule, implicit, and @code{VPATH} The search through the directories specified in @code{VPATH} or with @code{vpath} also happens during consideration of implicit rules (@pxref{Implicit Rules, ,Using Implicit Rules}). For example, when a file @file{foo.o} has no explicit rule, @code{make} considers implicit rules, such as the built-in rule to compile @file{foo.c} if that file exists. If such a file is lacking in the current directory, the appropriate directories are searched for it. If @file{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. @node Libraries/Search, , Implicit/Search, Directory Search @subsection Directory Search for Link Libraries @cindex link libraries, and directory search @cindex libraries for linking, directory search @cindex directory search (@code{VPATH}), and link libraries @cindex @code{VPATH}, and link libraries @cindex search path for prerequisites (@code{VPATH}), and link libraries @cindex @code{-l} (library search) @cindex link libraries, patterns matching @cindex @code{.LIBPATTERNS}, and link libraries @vindex .LIBPATTERNS 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 @samp{-l@var{name}}. (You can tell something strange is going on here because the prerequisite is normally the name of a file, and the @emph{file name} of a library generally looks like @file{lib@var{name}.a}, not like @samp{-l@var{name}}.)@refill When a prerequisite's name has the form @samp{-l@var{name}}, @code{make} handles it specially by searching for the file @file{lib@var{name}.so}, and, if it is not found, for the file @file{lib@var{name}.a} in the current directory, in directories specified by matching @code{vpath} search paths and the @code{VPATH} search path, and then in the directories @file{/lib}, @file{/usr/lib}, and @file{@var{prefix}/lib} (normally @file{/usr/local/lib}, but MS-DOS/MS-Windows versions of @code{make} behave as if @var{prefix} is defined to be the root of the DJGPP installation tree). For example, if there is a @file{/usr/lib/libcurses.a} library on your system (and no @file{/usr/lib/libcurses.so} file), then @example @group foo : foo.c -lcurses cc$^ -o $@@ @end group @end example @noindent would cause the command @samp{cc foo.c /usr/lib/libcurses.a -o foo} to be executed when @file{foo} is older than @file{foo.c} or than @file{/usr/lib/libcurses.a}.@refill Although the default set of files to be searched for is @file{lib@var{name}.so} and @file{lib@var{name}.a}, this is customizable via the @code{.LIBPATTERNS} variable. Each word in the value of this variable is a pattern string. When a prerequisite like @samp{-l@var{name}} is seen, @code{make} will replace the percent in each pattern in the list with @var{name} and perform the above directory searches using each library file name. The default value for @code{.LIBPATTERNS} is @samp{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. @node Phony Targets, Force Targets, Directory Search, Rules @section Phony Targets @cindex phony targets @cindex targets, phony @cindex targets without a file 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: @example @group clean: rm *.o temp @end group @end example @noindent Because the @code{rm} command does not create a file named @file{clean}, probably no such file will ever exist. Therefore, the @code{rm} command will be executed every time you say @samp{make clean}. @cindex @code{rm} (shell command) @findex .PHONY In this example, the @file{clean} target will not work properly if a file named @file{clean} is ever created in this directory. Since it has no prerequisites, @file{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 @code{.PHONY} (@pxref{Special Targets, ,Special Built-in Target Names}) as follows: @example @group .PHONY: clean clean: rm *.o temp @end group @end example @noindent Once this is done, @samp{make clean} will run the recipe regardless of whether there is a file named @file{clean}. Phony targets are also useful in conjunction with recursive invocations of @code{make} (@pxref{Recursion, ,Recursive Use of @code{make}}). 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: @example @group SUBDIRS = foo bar baz subdirs: for dir in$(SUBDIRS); do \ $(MAKE) -C $$dir; \ done @end group @end example 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 @code{make} is invoked with the @code{-k} option, which is unfortunate. Second, and perhaps more importantly, you cannot take advantage of @code{make}'s ability to build targets in parallel (@pxref{Parallel, ,Parallel Execution}), since there is only one rule. By declaring the sub-directories as @code{.PHONY} targets (you must do this as the sub-directory obviously always exists; otherwise it won't be built) you can remove these problems: @example @group SUBDIRS = foo bar baz .PHONY: subdirs (SUBDIRS) subdirs: (SUBDIRS) (SUBDIRS): (MAKE) -C @@ foo: baz @end group @end example Here we've also declared that the @file{foo} sub-directory cannot be built until after the @file{baz} sub-directory is complete; this kind of relationship declaration is particularly important when attempting parallel builds. The implicit rule search (@pxref{Implicit Rules}) is skipped for @code{.PHONY} targets. This is why declaring a target as @code{.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 @code{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 (@pxref{Goals, ,Arguments to Specify the 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 @file{./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 @samp{all} and give it, as prerequisites, all the individual programs. For example: @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 @end example @noindent Now you can say just @samp{make} to remake all three programs, or specify as arguments the ones to remake (as in @samp{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 @samp{make cleanall} will delete the object files, the difference files, and the file @file{program}: @example .PHONY: cleanall cleanobj cleandiff cleanall : cleanobj cleandiff rm program cleanobj : rm *.o cleandiff : rm *.diff @end example @node Force Targets, Empty Targets, Phony Targets, Rules @section Rules without Recipes or Prerequisites @cindex force targets @cindex targets, force @cindex @code{FORCE} @cindex rule, no recipe or prerequisites If a rule has no prerequisites or recipe, and the target of the rule is a nonexistent file, then @code{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: @example @group clean: FORCE rm (objects) FORCE: @end group @end example Here the target @samp{FORCE} satisfies the special conditions, so the target @file{clean} that depends on it is forced to run its recipe. There is nothing special about the name @samp{FORCE}, but that is one name commonly used this way. As you can see, using @samp{FORCE} this way has the same results as using @samp{.PHONY: clean}. Using @samp{.PHONY} is more explicit and more efficient. However, other versions of @code{make} do not support @samp{.PHONY}; thus @samp{FORCE} appears in many makefiles. @xref{Phony Targets}. @node Empty Targets, Special Targets, Force Targets, Rules @section Empty Target Files to Record Events @cindex empty targets @cindex targets, empty @cindex recording events with empty targets The @dfn{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 @code{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: @example print: foo.c bar.c lpr -p ? touch print @end example @cindex @code{print} target @cindex @code{lpr} (shell command) @cindex @code{touch} (shell command) @noindent With this rule, @samp{make print} will execute the @code{lpr} command if either source file has changed since the last @samp{make print}. The automatic variable @samp{?} is used to print only those files that have changed (@pxref{Automatic Variables}). @node Special Targets, Multiple Targets, Empty Targets, Rules @section Special Built-in Target Names @cindex special targets @cindex built-in special targets @cindex targets, built-in special Certain names have special meanings if they appear as targets. @table @code @findex .PHONY @item .PHONY The prerequisites of the special target @code{.PHONY} are considered to be phony targets. When it is time to consider such a target, @code{make} will run its recipe unconditionally, regardless of whether a file with that name exists or what its last-modification time is. @xref{Phony Targets, ,Phony Targets}. @findex .SUFFIXES @item .SUFFIXES The prerequisites of the special target @code{.SUFFIXES} are the list of suffixes to be used in checking for suffix rules. @xref{Suffix Rules, , Old-Fashioned Suffix Rules}. @findex .DEFAULT @item .DEFAULT The recipe specified for @code{.DEFAULT} is used for any target for which no rules are found (either explicit rules or implicit rules). @xref{Last Resort}. If a @code{.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. @xref{Implicit Rule Search, ,Implicit Rule Search Algorithm}. @findex .PRECIOUS @item .PRECIOUS @cindex precious targets @cindex preserving with @code{.PRECIOUS} The targets which @code{.PRECIOUS} depends on are given the following special treatment: if @code{make} is killed or interrupted during the execution of their recipes, the target is not deleted. @xref{Interrupts, ,Interrupting or Killing @code{make}}. Also, if the target is an intermediate file, it will not be deleted after it is no longer needed, as is normally done. @xref{Chained Rules, ,Chains of Implicit Rules}. In this latter respect it overlaps with the @code{.SECONDARY} special target. You can also list the target pattern of an implicit rule (such as @samp{%.o}) as a prerequisite file of the special target @code{.PRECIOUS} to preserve intermediate files created by rules whose target patterns match that file's name. @findex .INTERMEDIATE @item .INTERMEDIATE @cindex intermediate targets, explicit The targets which @code{.INTERMEDIATE} depends on are treated as intermediate files. @xref{Chained Rules, ,Chains of Implicit Rules}. @code{.INTERMEDIATE} with no prerequisites has no effect. @findex .SECONDARY @item .SECONDARY @cindex secondary targets @cindex preserving with @code{.SECONDARY} The targets which @code{.SECONDARY} depends on are treated as intermediate files, except that they are never automatically deleted. @xref{Chained Rules, ,Chains of Implicit Rules}. @code{.SECONDARY} with no prerequisites causes all targets to be treated as secondary (i.e., no target is removed because it is considered intermediate). @findex .SECONDEXPANSION @item .SECONDEXPANSION If @code{.SECONDEXPANSION} is mentioned as a target anywhere in the makefile, then all prerequisite lists defined @emph{after} it appears will be expanded a second time after all makefiles have been read in. @xref{Secondary Expansion, ,Secondary Expansion}. @findex .DELETE_ON_ERROR @item .DELETE_ON_ERROR @cindex removing targets on failure If @code{.DELETE_ON_ERROR} is mentioned as a target anywhere in the makefile, then @code{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. @xref{Errors, ,Errors in Recipes}. @findex .IGNORE @item .IGNORE If you specify prerequisites for @code{.IGNORE}, then @code{make} will ignore errors in execution of the recipe for those particular files. The recipe for @code{.IGNORE} (if any) is ignored. If mentioned as a target with no prerequisites, @code{.IGNORE} says to ignore errors in execution of recipes for all files. This usage of @samp{.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. @xref{Errors, ,Errors in Recipes}. @findex .LOW_RESOLUTION_TIME @item .LOW_RESOLUTION_TIME If you specify prerequisites for @code{.LOW_RESOLUTION_TIME}, @command{make} assumes that these files are created by commands that generate low resolution time stamps. The recipe for the @code{.LOW_RESOLUTION_TIME} target are ignored. The high resolution file time stamps of many modern file systems lessen the chance of @command{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 @samp{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 @code{.LOW_RESOLUTION_TIME} so that @command{make} does not mistakenly conclude that the file is out of date. For example: @example @group .LOW_RESOLUTION_TIME: dst dst: src cp -p src dst @end group @end example Since @samp{cp -p} discards the sub-second part of @file{src}'s time stamp, @file{dst} is typically slightly older than @file{src} even when it is up to date. The @code{.LOW_RESOLUTION_TIME} line causes @command{make} to consider @file{dst} to be up to date if its time stamp is at the start of the same second that @file{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 @code{.LOW_RESOLUTION_TIME}, as @command{make} does this automatically. @findex .SILENT @item .SILENT If you specify prerequisites for @code{.SILENT}, then @code{make} will not print the recipe used to remake those particular files before executing them. The recipe for @code{.SILENT} is ignored. If mentioned as a target with no prerequisites, @code{.SILENT} says not to print any recipes before executing them. You may also use more selective ways to silence specific recipe command lines. @xref{Echoing, ,Recipe Echoing}. If you want to silence all recipes for a particular run of @code{make}, use the @samp{-s} or @w{@samp{--silent}} option (@pxref{Options Summary}). @findex .EXPORT_ALL_VARIABLES @item .EXPORT_ALL_VARIABLES Simply by being mentioned as a target, this tells @code{make} to export all variables to child processes by default. @xref{Variables/Recursion, ,Communicating Variables to a Sub-@code{make}}. @findex .NOTPARALLEL @item .NOTPARALLEL @cindex parallel execution, overriding If @code{.NOTPARALLEL} is mentioned as a target, then this invocation of @code{make} will be run serially, even if the @samp{-j} option is given. Any recursively invoked @code{make} command will still run recipes in parallel (unless its makefile also contains this target). Any prerequisites on this target are ignored. @findex .ONESHELL @item .ONESHELL @cindex recipe execution, single invocation If @code{.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 (@pxref{Execution, ,Recipe Execution}). @findex .POSIX @item .POSIX @cindex POSIX-conforming mode, setting If @code{.POSIX} is mentioned as a target, then the makefile will be parsed and run in POSIX-conforming mode. This does @emph{not} mean that only POSIX-conforming makefiles will be accepted: all advanced GNU @code{make} features are still available. Rather, this target causes @code{make} to behave as required by POSIX in those areas where @code{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 @code{-e} flag: the first failing command in a recipe will cause the recipe to fail immediately. @end table 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 @samp{.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 @samp{.}, so these special target names also begin with @samp{.}. @xref{Suffix Rules, ,Old-Fashioned Suffix Rules}. @node Multiple Targets, Multiple Rules, Special Targets, Rules @section Multiple Targets in a Rule @cindex multiple targets @cindex several targets in a rule @cindex targets, multiple @cindex rule, with multiple targets When an explicit rule has multiple targets they can be treated in one of two possible ways: as independent targets or as grouped targets. The manner in which they are treated is determined by the separator that appears after the list of targets. @subsubheading Rules with Independent Targets @cindex independent targets @cindex targets, independent Rules that use the standard target separator, @code{:}, define independent targets. This is equivalent to writing the same rule once for each target, with duplicated prerequisites and recipes. Typically, the recipe would use automatic variables such as @samp{@@} to specify which target is being built. Rules with independent targets are useful in two cases: @itemize @bullet @item You want just prerequisites, no recipe. For example: @example kbd.o command.o files.o: command.h @end example @noindent gives an additional prerequisite to each of the three object files mentioned. It is equivalent to writing: @example kbd.o: command.h command.o: command.h files.o: command.h @end example @item Similar recipes work for all the targets. The automatic variable @samp{@@} can be used to substitute the particular target to be remade into the commands (@pxref{Automatic Variables}). For example: @example @group bigoutput littleoutput : text.g generate text.g -(subst output,,@@) > @@ @end group @end example @findex subst @noindent is equivalent to @example bigoutput : text.g generate text.g -big > bigoutput littleoutput : text.g generate text.g -little > littleoutput @end example @noindent Here we assume the hypothetical program @code{generate} makes two types of output, one if given @samp{-big} and one if given @samp{-little}. @xref{Text Functions, ,Functions for String Substitution and Analysis}, for an explanation of the @code{subst} function. @end itemize Suppose you would like to vary the prerequisites according to the target, much as the variable @samp{@@} allows you to vary the recipe. You cannot do this with multiple targets in an ordinary rule, but you can do it with a @dfn{static pattern rule}. @xref{Static Pattern, ,Static Pattern Rules}. @subsubheading Rules with Grouped Targets @cindex grouped targets @cindex targets, grouped If instead of independent targets you have a recipe that generates multiple files from a single invocation, you can express that relationship by declaring your rule to use @emph{grouped targets}. A grouped target rule uses the separator @code{&:} (the @samp{&} here is used to imply all''). When @code{make} builds any one of the grouped targets, it understands that all the other targets in the group are also created as a result of the invocation of the recipe. Furthermore, if only some of the grouped targets are out of date or missing @code{make} will realize that running the recipe will update all of the targets. As an example, this rule defines a grouped target: @example @group foo bar biz &: baz boz echo ^ > foo echo ^ > bar echo ^ > biz @end group @end example During the execution of a grouped target's recipe, the automatic variable @samp{@@} is set to the name of the particular target in the group which triggered the rule. Caution must be used if relying on this variable in the recipe of a grouped target rule. Unlike independent targets, a grouped target rule @emph{must} include a recipe. However, targets that are members of a grouped target may also appear in independent target rule definitions that do not have recipes. Each target may have only one recipe associated with it. If a grouped target appears in either an independent target rule or in another grouped target rule with a recipe, you will get a warning and the latter recipe will replace the former recipe. Additionally the target will be removed from the previous group and appear only in the new group. If you would like a target to appear in multiple groups, then you must use the double-colon grouped target separator, @code{&::} when declaring all of the groups containing that target. Grouped double-colon targets are each considered independently, and each grouped double-colon rule's recipe is executed at most once, if at least one of its multiple targets requires updating. @node Multiple Rules, Static Pattern, Multiple Targets, Rules @section Multiple Rules for One Target @cindex multiple rules for one target @cindex several rules for one target @cindex rule, multiple for one target @cindex target, multiple rules for one 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, @code{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 @code{make}@dots{} 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 @dfn{double-colon rules} (@pxref{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 @code{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 @file{config.h} changes is to write the following: @example objects = foo.o bar.o foo.o : defs.h bar.o : defs.h test.h (objects) : config.h @end example 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 @code{make} (@pxref{Overriding, ,Overriding Variables}). For example, @example @group extradeps= (objects) : (extradeps) @end group @end example @noindent means that the command @samp{make extradeps=foo.h} will consider @file{foo.h} as a prerequisite of each object file, but plain @samp{make} will not. If none of the explicit rules for a target has a recipe, then @code{make} searches for an applicable implicit rule to find one @pxref{Implicit Rules, ,Using Implicit Rules}). @node Static Pattern, Double-Colon, Multiple Rules, Rules @section Static Pattern Rules @cindex static pattern rule @cindex rule, static pattern @cindex pattern rules, static (not implicit) @cindex varying prerequisites @cindex prerequisites, varying (static pattern) @dfn{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 @emph{analogous}, but not necessarily @emph{identical}. @menu * Static Usage:: The syntax of static pattern rules. * Static versus Implicit:: When are they better than implicit rules? @end menu @node Static Usage, Static versus Implicit, Static Pattern, Static Pattern @subsection Syntax of Static Pattern Rules @cindex static pattern rule, syntax of @cindex pattern rules, static, syntax of Here is the syntax of a static pattern rule: @example @var{targets} @dots{}: @var{target-pattern}: @var{prereq-patterns} @dots{} @var{recipe} @dots{} @end example @noindent The @var{targets} list specifies the targets that the rule applies to. The targets can contain wildcard characters, just like the targets of ordinary rules (@pxref{Wildcards, ,Using Wildcard Characters in File Names}). @cindex target pattern, static (not implicit) @cindex stem The @var{target-pattern} and @var{prereq-patterns} say how to compute the prerequisites of each target. Each target is matched against the @var{target-pattern} to extract a part of the target name, called the @dfn{stem}. This stem is substituted into each of the @var{prereq-patterns} to make the prerequisite names (one from each @var{prereq-pattern}). Each pattern normally contains the character @samp{%} just once. When the @var{target-pattern} matches a target, the @samp{%} can match any part of the target name; this part is called the @dfn{stem}. The rest of the pattern must match exactly. For example, the target @file{foo.o} matches the pattern @samp{%.o}, with @samp{foo} as the stem. The targets @file{foo.c} and @file{foo.out} do not match that pattern.@refill @cindex prerequisite pattern, static (not implicit) The prerequisite names for each target are made by substituting the stem for the @samp{%} in each prerequisite pattern. For example, if one prerequisite pattern is @file{%.c}, then substitution of the stem @samp{foo} gives the prerequisite name @file{foo.c}. It is legitimate to write a prerequisite pattern that does not contain @samp{%}; then this prerequisite is the same for all targets. @cindex @code{%}, quoting in static pattern @cindex @code{%}, quoting with @code{\} (backslash) @cindex @code{\} (backslash), to quote @code{%} @cindex backslash (@code{\}), to quote @code{%} @cindex quoting @code{%}, in static pattern @samp{%} characters in pattern rules can be quoted with preceding backslashes (@samp{\}). Backslashes that would otherwise quote @samp{%} characters can be quoted with more backslashes. Backslashes that quote @samp{%} 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 @samp{%} characters go unmolested. For example, the pattern @file{the\%weird\\%pattern\\} has @samp{the%weird\} preceding the operative @samp{%} character, and @samp{pattern\\} following it. The final two backslashes are left alone because they cannot affect any @samp{%} character.@refill Here is an example, which compiles each of @file{foo.o} and @file{bar.o} from the corresponding @file{.c} file: @example @group objects = foo.o bar.o all: (objects) (objects): %.o: %.c (CC) -c (CFLAGS) < -o @@ @end group @end example @noindent Here @samp{<} is the automatic variable that holds the name of the prerequisite and @samp{@@} is the automatic variable that holds the name of the target; see @ref{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 @code{filter} function to remove non-matching file names (@pxref{Text Functions, ,Functions for String Substitution and Analysis}): @example 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 < @end example @noindent In this example the result of @samp{(filter %.o,(files))} is @file{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 @w{@samp{(filter %.elc,(files))}} is @file{foo.elc}, so that file is made from @file{foo.el}.@refill Another example shows how to use @code{*} in static pattern rules: @vindex *@r{, and static pattern} @example @group bigoutput littleoutput : %output : text.g generate text.g -* > @@ @end group @end example @noindent When the @code{generate} command is run, @code{*} will expand to the stem, either @samp{big} or @samp{little}. @node Static versus Implicit, , Static Usage, Static Pattern @subsection Static Pattern Rules versus Implicit Rules @cindex rule, static pattern versus implicit @cindex static pattern rule, versus implicit A static pattern rule has much in common with an implicit rule defined as a pattern rule (@pxref{Pattern Rules, ,Defining and Redefining Pattern Rules}). Both have a pattern for the target and patterns for constructing the names of prerequisites. The difference is in how @code{make} decides @emph{when} the rule applies. An implicit rule @emph{can} apply to any target that matches its pattern, but it @emph{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: @itemize @bullet @item 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. @item 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 @code{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. @end itemize @node Double-Colon, Automatic Prerequisites, Static Pattern, Rules @section Double-Colon Rules @cindex double-colon rules @cindex rule, double-colon (@code{::}) @cindex multiple rules for one target (@code{::}) @cindex @code{::} rules (double-colon) @dfn{Double-colon} rules are explicit rules written with @samp{::} instead of @samp{:} 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 (@pxref{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. @xref{Implicit Rules, ,Using Implicit Rules}. @node Automatic Prerequisites, , Double-Colon, Rules @section Generating Prerequisites Automatically @cindex prerequisites, automatic generation @cindex automatic generation of prerequisites @cindex 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 @file{main.c} uses @file{defs.h} via an @code{#include}, you would write: @example main.o: defs.h @end example @noindent You need this rule so that @code{make} knows that it must remake @file{main.o} whenever @file{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 @code{#include}. @cindex @code{#include} @cindex @code{-M} (to compiler) To avoid this hassle, most modern C compilers can write these rules for you, by looking at the @code{#include} lines in the source files. Usually this is done with the @samp{-M} option to the compiler. For example, the command: @example cc -M main.c @end example @noindent generates the output: @example main.o : main.c defs.h @end example @noindent 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 @file{main.o} in a makefile, so it can never be considered an intermediate file by implicit rule search. This means that @code{make} won't ever remove the file after using it; @pxref{Chained Rules, ,Chains of Implicit Rules}. @cindex @code{make depend} With old @code{make} programs, it was traditional practice to use this compiler feature to generate prerequisites on demand with a command like @samp{make depend}. That command would create a file @file{depend} containing all the automatically-generated prerequisites; then the makefile could use @code{include} to read them in (@pxref{Include}). In GNU @code{make}, the feature of remaking makefiles makes this practice obsolete---you need never tell @code{make} explicitly to regenerate the prerequisites, because it always regenerates any makefile that is out of date. @xref{Remaking Makefiles}. The practice we recommend for automatic prerequisite generation is to have one makefile corresponding to each source file. For each source file @file{@var{name}.c} there is a makefile @file{@var{name}.d} which lists what files the object file @file{@var{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 @file{@var{name}.d} from a C source file called @file{@var{name}.c}: @smallexample @group %.d: %.c @@set -e; rm -f @@; \ (CC) -M (CPPFLAGS) < > @@.$$$$; \ sed 's,$$*$$\.o[ :]*,\1.o @@ : ,g' < @@.$$$$> @@; \ rm -f @@.$$$$@end group @end smallexample @noindent @xref{Pattern Rules}, for information on defining pattern rules. The @samp{-e} flag to the shell causes it to exit immediately if the @code{$(CC)} command (or any other command) fails (exits with a nonzero status). @cindex @code{-e} (shell flag) @cindex @code{-MM} (to GNU compiler) With the GNU C compiler, you may wish to use the @samp{-MM} flag instead of @samp{-M}. This omits prerequisites on system header files. @xref{Preprocessor Options, , Options Controlling the Preprocessor, gcc, Using GNU CC}, for details. @cindex @code{sed} (shell command) The purpose of the @code{sed} command is to translate (for example): @example main.o : main.c defs.h @end example @noindent into: @example main.o main.d : main.c defs.h @end example @noindent @cindex @code{.d} This makes each @samp{.d} file depend on all the source and header files that the corresponding @samp{.o} file depends on. @code{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 @samp{.d} files, you then use the @code{include} directive to read them all in. @xref{Include}. For example: @example @group sources = foo.c bar.c include \$(sources:.c=.d) @end group @end example @noindent (This example uses a substitution variable reference to translate the list of source files @samp{foo.c bar.c} into a list of prerequisite makefiles, @samp{foo.d bar.d}. @xref{Substitution Refs}, for full information on substitution references.) Since the @samp{.d} files are makefiles like any others, @code{make} will remake them as necessary with no further work from you. @xref{Remaking Makefiles}. Note that the @samp{.d} files contain target definitions; you should be sure to place the @code{include} directive @emph{after} the first, default goal in your makefiles or run the risk of having a random object file become the default goal. @xref{How Make Works}. @node Recipes, Using Variables, Rules, Top @chapter Writing Recipes in Rules @cindex recipes @cindex recipes, how to write @cindex writing recipes 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 t