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This is Info file make.info, produced by Makeinfo-1.54 from the input file make.texinfo. This file documents the GNU Make utility, which determines automatically which pieces of a large program need to be recompiled, and issues the commands to recompile them. This is Edition 0.42, last updated 14 May 1993, of `The GNU Make Manual', for `make', Version 3.66 Beta. Copyright (C) 1988, '89, '90, '91, '92, '93 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the text of the translations of the section entitled "GNU General Public License" must be approved for accuracy by the Foundation. File: make.info, Node: Top, Next: Copying, Prev: (dir), Up: (dir) Make **** The GNU `make' utility automatically determines which pieces of a large program need to be recompiled, and issues the commands to recompile them. This is Edition 0.42 of the `GNU Make Manual', last updated 14 May 1993 for `make' Version 3.66 Beta. This manual describes `make' and contains the following chapters: * Menu: * Copying:: * Overview:: Overview of `make'. * Introduction:: An introduction to `make'. * Makefiles:: Makefiles tell `make' what to do. * Rules:: Rules describe when a file must be remade. * Commands:: Commands 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. * make Invocation: Running. How to invoke `make' on the command line. * Implicit Rules:: Use implicit rules to treat many files alike, based on their file names. * Archives:: How `make' can update library archives. * Features:: Features GNU `make' has over other `make's. * Missing:: What GNU `make' lacks from other `make's. * Makefile Conventions:: Conventions for makefiles in GNU programs. * Quick Reference:: A quick reference for experienced users. * Complex Makefile:: A real example of a straightforward, but nontrivial, makefile. * Concept Index:: Index of Concepts * Name Index:: Index of Functions, Variables, & Directives -- The Detailed Node Listing -- Overview of `make' * Preparing:: Preparing and Running Make * Reading:: On Reading this Text * Bugs:: Problems and Bugs An Introduction to Makefiles * Rule Introduction:: What a rule looks like. * Simple Makefile:: A Simple Makefile * How Make Works:: How `make' Processes This Makefile * Variables Simplify:: Variables Make Makefiles Simpler * make Deduces:: Letting `make' Deduce the Commands * Combine By Dependency:: 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. Writing Rules * Rule Example:: An example explained. * Rule Syntax:: General syntax explained. * 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 commands or dependencies 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 dependencies according to the target name. * Double-Colon:: How to use a special kind of rule to allow several independent rules for one target. * Automatic Dependencies:: How to automatically generate rules giving dependencies from the 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 Dependencies * General Search:: Specifying a search path that applies to every dependency. * Selective Search:: Specifying a search path for a specified class of names. * Commands/Search:: How to write shell commands 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 the Commands in Rules * Echoing:: How to control when commands are echoed. * Execution:: How commands are executed. * Parallel:: How commands can be executed in parallel. * Errors:: What happens after a command execution error. * Interrupts:: What happens when a command is interrupted. * Recursion:: Invoking `make' from makefiles. * Sequences:: Defining canned sequences of commands. * Empty Commands:: Defining useful, do-nothing commands. Recursive Use of `make' * MAKE Variable:: The special effects of using `$(MAKE)'. * Variables/Recursion:: How to communicate variables to a sub-`make'. * Options/Recursion:: How to communicate options to a sub-`make'. * -w Option:: How the `-w' or `--print-directory' option helps debug use of recursive `make' commands. How to Use Variables * Reference:: How to use the value of a variable. * Flavors:: Variables come in two flavors. * Advanced:: Advanced features for referencing a variable. * Values:: All the ways variables get their values. * Setting:: How to set a variable in the makefile. * Appending:: How to append more text to the old value of a variable. * Override Directive:: How to set a variable in the makefile even if the user has set it with a command argument. * Defining:: An alternate way to set a variable to a verbatim string. * Environment:: Variable values can come from the environment. 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. * Filename Functions:: Functions for manipulating file names. * Foreach Function:: Repeat some text with controlled variation. * Origin Function:: Find where a variable got its value. * Shell Function:: Substitute the output of a shell command. How to Run `make' * Makefile Arguments:: How to specify which makefile to use. * Goals:: How to use goal arguments to specify which parts of the makefile to use. * Instead of Execution:: How to use mode flags to specify what kind of thing to do with the commands 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 commands for updating a file. * Catalogue of Rules:: A list of built-in implicit 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 defining commands for rules which cannot find any. * Suffix Rules:: The old-fashioned style of implicit rule. * Search Algorithm:: 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:: How to use automatic variables in the commands of implicit rules. * Pattern Match:: How patterns match. * Match-Anything Rules:: Precautions you should take prior to defining rules that can match any target file whatever. * Canceling Rules:: How to override or cancel built-in rules. Using `make' to Update Archive Files * Archive Members:: Archive members as targets. * Archive Update:: The implicit rule for archive member targets. * Archive 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. File: make.info, Node: Copying, Next: Overview, Prev: Top, Up: Top GNU GENERAL PUBLIC LICENSE ************************** Version 2, June 1991 Copyright (C) 1989, 1991 Free Software Foundation, Inc. 675 Mass Ave, Cambridge, MA 02139, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Preamble ======== The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too. When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things. To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it. For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights. We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software. Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations. Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all. The precise terms and conditions for copying, distribution and modification follow. TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION 1. This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The "Program", below, refers to any such program or work, and a "work based on the Program" means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language. (Hereinafter, translation is included without limitation in the term "modification".) Each licensee is addressed as "you". Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. 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Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance. 6. You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it. 7. 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For example, if a patent license would not permit royalty-free redistribution of the Program by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Program. If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances. It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices. 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The Free Software Foundation may publish revised and/or new versions of the General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and "any later version", you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation. 11. If you wish to incorporate parts of the Program into other free programs whose distribution conditions are different, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally. NO WARRANTY 12. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION. 13. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. END OF TERMS AND CONDITIONS How to Apply These Terms to Your New Programs ============================================= If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms. To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found. ONE LINE TO GIVE THE PROGRAM'S NAME AND AN IDEA OF WHAT IT DOES. Copyright (C) 19YY NAME OF AUTHOR This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. Also add information on how to contact you by electronic and paper mail. If the program is interactive, make it output a short notice like this when it starts in an interactive mode: Gnomovision version 69, Copyright (C) 19YY NAME OF AUTHOR Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details. The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse-clicks or menu items--whatever suits your program. You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names: Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker. SIGNATURE OF TY COON, 1 April 1989 Ty Coon, President of Vice This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License. File: make.info, Node: Overview, Next: Introduction, Prev: Copying, Up: Top Overview of `make' ****************** The `make' utility automatically determines which pieces of a large program need to be recompiled, and issues commands to recompile them. This manual describes GNU `make', which was implemented by Richard Stallman and Roland McGrath. GNU `make' conforms to section 6.2 of `IEEE Standard 1003.2-1992' (POSIX.2). Our examples show C programs, since they are most common, but you can use `make' with any programming language whose compiler can be run with a shell command. Indeed, `make' is not limited to programs. You can use it to describe any task where some files must be updated automatically from others whenever the others change. * Menu: * Preparing:: Preparing and Running Make * Reading:: On Reading this Text * Bugs:: Problems and Bugs File: make.info, Node: Preparing, Next: Reading, Up: Overview Preparing and Running Make ========================== To prepare to use `make', you must write a file called the "makefile" that describes the relationships among files in your program and provides commands for updating each file. In a program, typically, the executable file is updated from object files, which are in turn made by compiling source files. Once a suitable makefile exists, each time you change some source files, this simple shell command: make suffices to perform all necessary recompilations. The `make' program uses the makefile data base and the last-modification times of the files to decide which of the files need to be updated. For each of those files, it issues the commands recorded in the data base. You can provide command line arguments to `make' to control which files should be recompiled, or how. *Note How to Run `make': Running. File: make.info, Node: Reading, Next: Bugs, Prev: Preparing, Up: Overview How to Read This Manual ======================= If you are new to `make', or are looking for a general introduction, read the first few sections of each chapter, skipping the later sections. In each chapter, the first few sections contain introductory or general information and the later sections contain specialized or technical information. The exception is the second chapter, *Note An Introduction to Makefiles: Introduction, all of which is introductory. If you are familiar with other `make' programs, see *Note Features of GNU `make': Features, which lists the enhancements GNU `make' has, and *Note Incompatibilities and Missing Features: Missing, which explains the few things GNU `make' lacks that others have. For a quick summary, see *Note Options Summary::, *Note Quick Reference::, and *Note Special Targets::. File: make.info, Node: Bugs, Prev: Reading, Up: Overview Problems and Bugs ================= If you have problems with GNU `make' or think you've found a bug, please report it to the developers; we cannot promise to do anything but we might well want to fix it. Before reporting a bug, make sure you've actually found a real bug. Carefully reread the documentation and see if it really says you can do what you're trying to do. If it's not clear whether you should be able to do something or not, report that too; it's a bug in the documentation! Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest possible makefile that reproduces the problem. Then send us the makefile and the exact results `make' gave you. Also say what you expected to occur; this will help us decide whether the problem was really in the documentation. Once you've got a precise problem, please send electronic mail either through the Internet or via UUCP: Internet address: bug-gnu-utils@prep.ai.mit.edu UUCP path: mit-eddie!prep.ai.mit.edu!bug-gnu-utils Please include the version number of `make' you are using. You can get this information with the command `make --version'. Be sure also to include the type of machine and operating system you are using. If possible, include the contents of the file `config.h' that is generated by the configuration process. Non-bug suggestions are always welcome as well. If you have questions about things that are unclear in the documentation or are just obscure features, contact Roland McGrath; he will try to help you out, although he may not have time to fix the problem. You can send electronic mail to Roland McGrath either through the Internet or via UUCP: Internet address: roland@prep.ai.mit.edu UUCP path: mit-eddie!prep.ai.mit.edu!roland File: make.info, Node: Introduction, Next: Makefiles, Prev: Overview, Up: Top An Introduction to Makefiles **************************** You need a file called a "makefile" to tell `make' what to do. Most often, the makefile tells `make' how to compile and link a program. In this chapter, we will discuss a simple makefile that describes how to compile and link a text editor which consists of eight C source files and three header files. The makefile can also tell `make' how to run miscellaneous commands when explicitly asked (for example, to remove certain files as a clean-up operation). To see a more complex example of a makefile, see *Note Complex Makefile::. When `make' recompiles the editor, each changed C source file must be recompiled. If a header file has changed, each C source file that includes the header file must be recompiled to be safe. Each compilation produces an object file corresponding to the source file. Finally, if any source file has been recompiled, all the object files, whether newly made or saved from previous compilations, must be linked together to produce the new executable editor. * Menu: * Rule Introduction:: What a rule looks like. * Simple Makefile:: A Simple Makefile * How Make Works:: How `make' Processes This Makefile * Variables Simplify:: Variables Make Makefiles Simpler * make Deduces:: Letting `make' Deduce the Commands * Combine By Dependency:: Another Style of Makefile * Cleanup:: Rules for Cleaning the Directory File: make.info, Node: Rule Introduction, Next: Simple Makefile, Up: Introduction What a Rule Looks Like ====================== A simple makefile consists of "rules" with the following shape: TARGET ... : DEPENDENCIES ... COMMAND ... ... A "target" is usually the name of a file that is generated by a program; examples of targets are executable or object files. A target can also be the name of an action to carry out, such as `clean' (*note Phony Targets::.). A "dependency" is a file that is used as input to create the target. A target often depends on several files. A "command" is an action that `make' carries out. A rule may have more than one command, each on its own line. *Please note:* you need to put a tab character at the beginning of every command line! This is an obscurity that catches the unwary. Usually a command is in a rule with dependencies and serves to create a target file if any of the dependencies change. However, the rule that specifies commands for the target need not have dependencies. For example, the rule containing the delete command associated with the target `clean' does not have dependencies. A "rule", then, explains how and when to remake certain files which are the targets of the particular rule. `make' carries out the commands on the dependencies to create or update the target. A rule can also explain how and when to carry out an action. *Note Writing Rules: Rules. A makefile may contain other text besides rules, but a simple makefile need only contain rules. Rules may look somewhat more complicated than shown in this template, but all fit the pattern more or less. File: make.info, Node: Simple Makefile, Next: How Make Works, Prev: Rule Introduction, Up: Introduction A Simple Makefile ================= Here is a straightforward makefile that describes the way an executable file called `edit' depends on eight object files which, in turn, depend on eight C source and three header files. In this example, all the C files include `defs.h', but only those defining editing commands include `command.h', and only low level files that change the editor buffer include `buffer.h'. edit : main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o cc -o edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o main.o : main.c defs.h cc -c main.c kbd.o : kbd.c defs.h command.h cc -c kbd.c command.o : command.c defs.h command.h cc -c command.c display.o : display.c defs.h buffer.h cc -c display.c insert.o : insert.c defs.h buffer.h cc -c insert.c search.o : search.c defs.h buffer.h cc -c search.c files.o : files.c defs.h buffer.h command.h cc -c files.c utils.o : utils.c defs.h cc -c utils.c clean : rm edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o We split each long line into two lines using backslash-newline; this is like using one long line, but is easier to read. To use this makefile to create the executable file called `edit', type: make To use this makefile to delete the executable file and all the object files from the directory, type: make clean In the example makefile, the targets include the executable file `edit', and the object files `main.o' and `kbd.o'. The dependencies are files such as `main.c' and `defs.h'. In fact, each `.o' file is both a target and a dependency. Commands include `cc -c main.c' and `cc -c kbd.c'. When a target is a file, it needs to be recompiled or relinked if any of its dependencies change. In addition, any dependencies that are themselves automatically generated should be updated first. In this example, `edit' depends on each of the eight object files; the object file `main.o' depends on the source file `main.c' and on the header file `defs.h'. A shell command follows each line that contains a target and dependencies. These shell commands say how to update the target file. A tab character must come at the beginning of every command line to distinguish commands lines from other lines in the makefile. (Bear in mind that `make' does not know anything about how the commands work. It is up to you to supply commands that will update the target file properly. All `make' does is execute the commands in the rule you have specified when the target file needs to be updated.) The target `clean' is not a file, but merely the name of an action. Since you normally do not want to carry out the actions in this rule, `clean' is not a dependency of any other rule. Consequently, `make' never does anything with it unless you tell it specifically. Note that this rule not only is not a dependency, it also does not have any dependencies, so the only purpose of the rule is to run the specified commands. Targets that do not refer to files but are just actions are called "phony targets". *Note Phony Targets::, for information about this kind of target. *Note Errors in Commands: Errors, to see how to cause `make' to ignore errors from `rm' or any other command. File: make.info, Node: How Make Works, Next: Variables Simplify, Prev: Simple Makefile, Up: Introduction How `make' Processes a Makefile =============================== By default, `make' starts with the first rule (not counting rules whose target names start with `.'). This is called the "default goal". ("Goals" are the targets that `make' strives ultimately to update. *Note Arguments to Specify the Goals: Goals.) In the simple example of the previous section, the default goal is to update the executable program `edit'; therefore, we put that rule first. Thus, when you give the command: make `make' reads the makefile in the current directory and begins by processing the first rule. In the example, this rule is for relinking `edit'; but before `make' can fully process this rule, it must process the rules for the files that `edit' depends on, which in this case are the object files. Each of these files is processed according to its own rule. These rules say to update each `.o' file by compiling its source file. The recompilation must be done if the source file, or any of the header files named as dependencies, 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 dependencies of the goal. If some other rule is not depended on by the goal (or anything it depends on, etc.), that rule is not processed, unless you tell `make' to do so (with a command such as `make clean'). Before recompiling an object file, `make' considers updating its dependencies, the source file and header files. This makefile does not specify anything to be done for them--the `.c' and `.h' files are not the targets of any rules--so `make' does nothing for these files. But `make' would update automatically generated C programs, such as those made by Bison or Yacc, by their own rules at this time. After recompiling whichever object files need it, `make' decides whether to relink `edit'. This must be done if the file `edit' does not exist, or if any of the object files are newer than it. If an object file was just recompiled, it is now newer than `edit', so `edit' is relinked. Thus, if we change the file `insert.c' and run `make', `make' will compile that file to update `insert.o', and then link `edit'. If we change the file `command.h' and run `make', `make' will recompile the object files `kbd.o', `command.o' and `files.o' and then link the file `edit'. File: make.info, Node: Variables Simplify, Next: make Deduces, Prev: How Make Works, Up: Introduction Variables Make Makefiles Simpler ================================ In our example, we had to list all the object files twice in the rule for `edit' (repeated here): edit : main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o cc -o edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o Such duplication is error-prone; if a new object file is added to the system, we might add it to one list and forget the other. We can eliminate the risk and simplify the makefile by using a variable. "Variables" allow a text string to be defined once and substituted in multiple places later (*note How to Use Variables: Using Variables.). It is standard practice for every makefile to have a variable named `objects', `OBJECTS', `objs', `OBJS', `obj', or `OBJ' which is a list of all object file names. We would define such a variable `objects' with a line like this in the makefile: objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o Then, each place we want to put a list of the object file names, we can substitute the variable's value by writing `$(objects)' (*note How to Use Variables: Using Variables.). Here is how the complete simple makefile looks when you use a variable for the object files: objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) main.o : main.c defs.h cc -c main.c kbd.o : kbd.c defs.h command.h cc -c kbd.c command.o : command.c defs.h command.h cc -c command.c display.o : display.c defs.h buffer.h cc -c display.c insert.o : insert.c defs.h buffer.h cc -c insert.c search.o : search.c defs.h buffer.h cc -c search.c files.o : files.c defs.h buffer.h command.h cc -c files.c utils.o : utils.c defs.h cc -c utils.c clean : rm edit $(objects) File: make.info, Node: make Deduces, Next: Combine By Dependency, Prev: Variables Simplify, Up: Introduction Letting `make' Deduce the Commands ================================== It is not necessary to spell out the commands for compiling the individual C source files, because `make' can figure them out: it has an "implicit rule" for updating a `.o' file from a correspondingly named `.c' file using a `cc -c' command. For example, it will use the command `cc -c main.c -o main.o' to compile `main.c' into `main.o'. We can therefore omit the commands from the rules for the object files. *Note Using Implicit Rules: Implicit Rules. When a `.c' file is used automatically in this way, it is also automatically added to the list of dependencies. We can therefore omit the `.c' files from the dependencies, provided we omit the commands. Here is the entire example, with both of these changes, and a variable `objects' as suggested above: objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) main.o : defs.h kbd.o : defs.h command.h command.o : defs.h command.h display.o : defs.h buffer.h insert.o : defs.h buffer.h search.o : defs.h buffer.h files.o : defs.h buffer.h command.h utils.o : defs.h .PHONY : clean clean : -rm edit $(objects) This is how we would write the makefile in actual practice. (The complications associated with `clean' are described elsewhere. See *Note Phony Targets::, and *Note Errors in Commands: Errors.) Because implicit rules are so convenient, they are important. You will see them used frequently. File: make.info, Node: Combine By Dependency, Next: Cleanup, Prev: make Deduces, Up: Introduction Another Style of Makefile ========================= When the objects of a makefile are created only by implicit rules, an alternative style of makefile is possible. In this style of makefile, you group entries by their dependencies instead of by their targets. Here is what one looks like: objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) $(objects) : defs.h kbd.o command.o files.o : command.h display.o insert.o search.o files.o : buffer.h Here `defs.h' is given as a dependency of all the object files; `command.h' and `buffer.h' are dependencies 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.