This is make.info, produced by makeinfo version 4.2 from make.texi. INFO-DIR-SECTION GNU Packages START-INFO-DIR-ENTRY * Make: (make). Remake files automatically. END-INFO-DIR-ENTRY 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 0.60, last updated 08 July 2002, of `The GNU Make Manual', for `make', Version 3.80. Copyright 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".  File: make.info, Node: Remaking Makefiles, Next: Overriding Makefiles, Prev: Special Variables, Up: Makefiles How Makefiles Are Remade ======================== Sometimes makefiles can be remade from other files, such as RCS or SCCS files. If a makefile can be remade from other files, you probably want `make' to get an up-to-date version of the makefile to read in. To this end, after reading in all makefiles, `make' will consider each as a goal target and attempt to update it. If a makefile has a rule which says how to update it (found either in that very makefile or in another one) or if an implicit rule applies to it (*note Using Implicit Rules: Implicit Rules.), it will be updated if necessary. After all makefiles have been checked, if any have actually been changed, `make' starts with a clean slate and reads all the makefiles over again. (It will also attempt to update each of them over again, but normally this will not change them again, since they are already up to date.) If you know that one or more of your makefiles cannot be remade and you want to keep `make' from performing an implicit rule search on them, perhaps for efficiency reasons, you can use any normal method of preventing implicit rule lookup to do so. For example, you can write an explicit rule with the makefile as the target, and an empty command string (*note Using Empty Commands: Empty Commands.). If the makefiles specify a double-colon rule to remake a file with commands but no prerequisites, that file will always be remade (*note Double-Colon::). In the case of makefiles, a makefile that has a double-colon rule with commands but no prerequisites will be remade every time `make' is run, and then again after `make' starts over and reads the makefiles in again. This would cause an infinite loop: `make' would constantly remake the makefile, and never do anything else. So, to avoid this, `make' will *not* attempt to remake makefiles which are specified as targets of a double-colon rule with commands but no prerequisites. If you do not specify any makefiles to be read with `-f' or `--file' options, `make' will try the default makefile names; *note What Name to Give Your Makefile: Makefile Names.. Unlike makefiles explicitly requested with `-f' or `--file' options, `make' is not certain that these makefiles should exist. However, if a default makefile does not exist but can be created by running `make' rules, you probably want the rules to be run so that the makefile can be used. Therefore, if none of the default makefiles exists, `make' will try to make each of them in the same order in which they are searched for (*note What Name to Give Your Makefile: Makefile Names.) until it succeeds in making one, or it runs out of names to try. Note that it is not an error if `make' cannot find or make any makefile; a makefile is not always necessary. When you use the `-t' or `--touch' option (*note Instead of Executing the Commands: Instead of Execution.), you would not want to use an out-of-date makefile to decide which targets to touch. So the `-t' option has no effect on updating makefiles; they are really updated even if `-t' is specified. Likewise, `-q' (or `--question') and `-n' (or `--just-print') do not prevent updating of makefiles, because an out-of-date makefile would result in the wrong output for other targets. Thus, `make -f mfile -n foo' will update `mfile', read it in, and then print the commands to update `foo' and its prerequisites without running them. The commands printed for `foo' will be those specified in the updated contents of `mfile'. However, on occasion you might actually wish to prevent updating of even the makefiles. You can do this by specifying the makefiles as goals in the command line as well as specifying them as makefiles. When the makefile name is specified explicitly as a goal, the options `-t' and so on do apply to them. Thus, `make -f mfile -n mfile foo' would read the makefile `mfile', print the commands needed to update it without actually running them, and then print the commands needed to update `foo' without running them. The commands for `foo' will be those specified by the existing contents of `mfile'.  File: make.info, Node: Overriding Makefiles, Next: Reading Makefiles, Prev: Remaking Makefiles, Up: Makefiles Overriding Part of Another Makefile =================================== Sometimes it is useful to have a makefile that is mostly just like another makefile. You can often use the `include' directive to include one in the other, and add more targets or variable definitions. However, if the two makefiles give different commands for the same target, `make' will not let you just do this. But there is another way. In the containing makefile (the one that wants to include the other), you can use a match-anything pattern rule to say that to remake any target that cannot be made from the information in the containing makefile, `make' should look in another makefile. *Note Pattern Rules::, for more information on pattern rules. For example, if you have a makefile called `Makefile' that says how to make the target `foo' (and other targets), you can write a makefile called `GNUmakefile' that contains: foo: frobnicate > foo %: force @$(MAKE) -f Makefile $@ force: ; If you say `make foo', `make' will find `GNUmakefile', read it, and see that to make `foo', it needs to run the command `frobnicate > foo'. If you say `make bar', `make' will find no way to make `bar' in `GNUmakefile', so it will use the commands from the pattern rule: `make -f Makefile bar'. If `Makefile' provides a rule for updating `bar', `make' will apply the rule. And likewise for any other target that `GNUmakefile' does not say how to make. The way this works is that the pattern rule has a pattern of just `%', so it matches any target whatever. The rule specifies a prerequisite `force', to guarantee that the commands will be run even if the target file already exists. We give `force' target empty commands to prevent `make' from searching for an implicit rule to build it--otherwise it would apply the same match-anything rule to `force' itself and create a prerequisite loop!  File: make.info, Node: Reading Makefiles, Prev: Overriding Makefiles, Up: Makefiles How `make' Reads a Makefile =========================== GNU `make' does its work in two distinct phases. During the first phase it reads all the makefiles, included makefiles, etc. and internalizes all the variables and their values, implicit and explicit rules, and constructs a dependency graph of all the targets and their prerequisites. During the second phase, `make' uses these internal structures to determine what targets will need to be rebuilt and to invoke the rules necessary to do so. It's important to understand this two-phase approach because it has a direct impact on how variable and function expansion happens; this is often a source of some confusion when writing makefiles. Here we will present a summary of the phases in which expansion happens for different constructs within the makefile. We say that expansion is "immediate" if it happens during the first phase: in this case `make' will expand any variables or functions in that section of a construct as the makefile is parsed. We say that expansion is "deferred" if expansion is not performed immediately. Expansion of deferred construct is not performed until either the construct appears later in an immediate context, or until the second phase. You may not be familiar with some of these constructs yet. You can reference this section as you become familiar with them, in later chapters. Variable Assignment ------------------- Variable definitions are parsed as follows: IMMEDIATE = DEFERRED IMMEDIATE ?= DEFERRED IMMEDIATE := IMMEDIATE IMMEDIATE += DEFERRED or IMMEDIATE define IMMEDIATE DEFERRED endef For the append operator, `+=', the right-hand side is considered immediate if the variable was previously set as a simple variable (`:='), and deferred otherwise. Conditional Statements ---------------------- All instances of conditional syntax are parsed immediately, in their entirety; this includes the `ifdef', `ifeq', `ifndef', and `ifneq' forms. Rule Definition --------------- A rule is always expanded the same way, regardless of the form: IMMEDIATE : IMMEDIATE ; DEFERRED DEFERRED That is, the target and prerequisite sections are expanded immediately, and the commands used to construct the target are always deferred. This general rule is true for explicit rules, pattern rules, suffix rules, static pattern rules, and simple prerequisite definitions.  File: make.info, Node: Rules, Next: Commands, Prev: Makefiles, Up: Top Writing Rules ************* A "rule" appears in the makefile and says when and how to remake certain files, called the rule's "targets" (most often only one per rule). It lists the other files that are the "prerequisites" of the target, and "commands" to use to create or update the target. The order of rules is not significant, except for determining the "default goal": the target for `make' to consider, if you do not otherwise specify one. The default goal is the target of the first rule in the first makefile. If the first rule has multiple targets, only the first target is taken as the default. There are two exceptions: a target starting with a period is not a default unless it contains one or more slashes, `/', as well; and, a target that defines a pattern rule has no effect on the default goal. (*Note Defining and Redefining Pattern Rules: Pattern Rules.) Therefore, we usually write the makefile so that the first rule is the one for compiling the entire program or all the programs described by the makefile (often with a target called `all'). *Note Arguments to Specify the Goals: Goals. * Menu: * Rule Example:: An example explained. * Rule Syntax:: General syntax explained. * Prerequisite Types:: There are two types of prerequisites. * Wildcards:: Using wildcard characters such as `*'. * Directory Search:: Searching other directories for source files. * Phony Targets:: Using a target that is not a real file's name. * Force Targets:: You can use a target without commands or prerequisites to mark other targets as phony. * Empty Targets:: When only the date matters and the files are empty. * Special Targets:: Targets with special built-in meanings. * Multiple Targets:: When to make use of several targets in a rule. * Multiple Rules:: How to use several rules with the same target. * Static Pattern:: Static pattern rules apply to multiple targets and can vary the prerequisites according to the target name. * Double-Colon:: How to use a special kind of rule to allow several independent rules for one target. * Automatic Prerequisites:: How to automatically generate rules giving prerequisites from source files themselves.  File: make.info, Node: Rule Example, Next: Rule Syntax, Prev: Rules, Up: Rules Rule Example ============ Here is an example of a rule: foo.o : foo.c defs.h # module for twiddling the frobs cc -c -g foo.c Its target is `foo.o' and its prerequisites are `foo.c' and `defs.h'. It has one command, which is `cc -c -g foo.c'. The command line starts with a tab to identify it as a command. This rule says two things: * How to decide whether `foo.o' is out of date: it is out of date if it does not exist, or if either `foo.c' or `defs.h' is more recent than it. * How to update the file `foo.o': by running `cc' as stated. The command does not explicitly mention `defs.h', but we presume that `foo.c' includes it, and that that is why `defs.h' was added to the prerequisites.  File: make.info, Node: Rule Syntax, Next: Prerequisite Types, Prev: Rule Example, Up: Rules Rule Syntax =========== In general, a rule looks like this: TARGETS : PREREQUISITES COMMAND ... or like this: TARGETS : PREREQUISITES ; COMMAND COMMAND ... The TARGETS are file names, separated by spaces. Wildcard characters may be used (*note Using Wildcard Characters in File Names: Wildcards.) and a name of the form `A(M)' represents member M in archive file A (*note Archive Members as Targets: Archive Members.). Usually there is only one target per rule, but occasionally there is a reason to have more (*note Multiple Targets in a Rule: Multiple Targets.). The COMMAND lines start with a tab character. The first command 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. *Note Writing the Commands in Rules: Commands. Because dollar signs are used to start variable references, if you really want a dollar sign in a rule you must write two of them, `$$' (*note How to Use Variables: Using Variables.). You may split a long line by inserting a backslash followed by a newline, but this is not required, as `make' places no limit on the length of a line in a makefile. A rule tells `make' two things: when the targets are out of date, and how to update them when necessary. The criterion for being out of date is specified in terms of the PREREQUISITES, which consist of file names separated by spaces. (Wildcards and archive members (*note Archives::) are allowed here too.) A target is out of date if it does not exist or if it is older than any of the prerequisites (by comparison of last-modification times). The idea is that the contents of the target file are computed based on information in the prerequisites, so if any of the prerequisites changes, the contents of the existing target file are no longer necessarily valid. How to update is specified by COMMANDS. These are lines to be executed by the shell (normally `sh'), but with some extra features (*note Writing the Commands in Rules: Commands.).  File: make.info, Node: Prerequisite Types, Next: Wildcards, Prev: Rule Syntax, Up: Rules Types of Prerequisites ====================== There are actually two different types of prerequisites understood by GNU `make': normal prerequisites such as described in the previous section, and "order-only" prerequisites. A normal prerequisite actually makes two statements: first, it imposes an order of execution of build commands: any commands necessary to build any of a target's prerequisites will be fully executed before any commands necessary to build the target. Second, it imposes a dependency relationship: if any prerequisite is newer than the target, then the target is considered out-of-date and must be rebuilt. Normally, this is exactly what you want: if a target's prerequisite is updated, then the target should also be updated. Occasionally, however, you have a situation where you want to impose a specific ordering on the rules to be invoked _without_ forcing the target to be updated if one of those rules is executed. In that case, you want to define "order-only" prerequisites. Order-only prerequisites can be specified by placing a pipe symbol (`|') in the prerequisites list: any prerequisites to the left of the pipe symbol are normal; any prerequisites to the right are order-only: TARGETS : NORMAL-PREREQUISITES | ORDER-ONLY-PREREQUISITES The normal prerequisites section may of course be empty. Also, you may still declare multiple lines of prerequisites for the same target: they are appended appropriately. 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 are a strict superset of the behavior of an order-only prerequisite).  File: make.info, Node: Wildcards, Next: Directory Search, Prev: Prerequisite Types, Up: Rules Using Wildcard Characters in File Names ======================================= A single file name can specify many files using "wildcard characters". The wildcard characters in `make' are `*', `?' and `[...]', the same as in the Bourne shell. For example, `*.c' specifies a list of all the files (in the working directory) whose names end in `.c'. The character `~' at the beginning of a file name also has special significance. If alone, or followed by a slash, it represents your home directory. For example `~/bin' expands to `/home/you/bin'. If the `~' is followed by a word, the string represents the home directory of the user named by that word. For example `~john/bin' expands to `/home/john/bin'. On systems which don't have a home directory for each user (such as MS-DOS or MS-Windows), this functionality can be simulated by setting the environment variable HOME. Wildcard expansion happens automatically in targets, in prerequisites, and in commands (where the shell does the expansion). In other contexts, wildcard expansion happens only if you request it explicitly with the `wildcard' function. The special significance of a wildcard character can be turned off by preceding it with a backslash. Thus, `foo\*bar' would refer to a specific file whose name consists of `foo', an asterisk, and `bar'. * Menu: * Wildcard Examples:: Several examples * Wildcard Pitfall:: Problems to avoid. * Wildcard Function:: How to cause wildcard expansion where it does not normally take place.  File: make.info, Node: Wildcard Examples, Next: Wildcard Pitfall, Prev: Wildcards, Up: Wildcards Wildcard Examples ----------------- Wildcards can be used in the commands of a rule, where they are expanded by the shell. For example, here is a rule to delete all the object files: clean: rm -f *.o Wildcards are also useful in the prerequisites of a rule. With the following rule in the makefile, `make print' will print all the `.c' files that have changed since the last time you printed them: print: *.c lpr -p $? touch print This rule uses `print' as an empty target file; see *Note Empty Target Files to Record Events: Empty Targets. (The automatic variable `$?' is used to print only those files that have changed; see *Note Automatic Variables: Automatic.) Wildcard expansion does not happen when you define a variable. Thus, if you write this: objects = *.o then the value of the variable `objects' is the actual string `*.o'. However, if you use the value of `objects' in a target, prerequisite or command, wildcard expansion will take place at that time. To set `objects' to the expansion, instead use: objects := $(wildcard *.o) *Note Wildcard Function::.  File: make.info, Node: Wildcard Pitfall, Next: Wildcard Function, Prev: Wildcard Examples, Up: Wildcards Pitfalls of Using Wildcards --------------------------- Now here is an example of a naive way of using wildcard expansion, that does not do what you would intend. Suppose you would like to say that the executable file `foo' is made from all the object files in the directory, and you write this: objects = *.o foo : $(objects) cc -o foo $(CFLAGS) $(objects) The value of `objects' is the actual string `*.o'. Wildcard expansion happens in the rule for `foo', so that each _existing_ `.o' file becomes a prerequisite of `foo' and will be recompiled if necessary. But what if you delete all the `.o' files? When a wildcard matches no files, it is left as it is, so then `foo' will depend on the oddly-named file `*.o'. Since no such file is likely to exist, `make' will give you an error saying it cannot figure out how to make `*.o'. This is not what you want! Actually it is possible to obtain the desired result with wildcard expansion, but you need more sophisticated techniques, including the `wildcard' function and string substitution. *Note The Function `wildcard': Wildcard Function. Microsoft operating systems (MS-DOS and MS-Windows) use backslashes to separate directories in pathnames, like so: c:\foo\bar\baz.c This is equivalent to the Unix-style `c:/foo/bar/baz.c' (the `c:' part is the so-called drive letter). When `make' runs on these systems, it supports backslashes as well as the Unix-style forward slashes in pathnames. However, this support does _not_ include the wildcard expansion, where backslash is a quote character. Therefore, you _must_ use Unix-style slashes in these cases.  File: make.info, Node: Wildcard Function, Prev: Wildcard Pitfall, Up: Wildcards The Function `wildcard' ----------------------- Wildcard expansion happens automatically in rules. But wildcard expansion does not normally take place when a variable is set, or inside the arguments of a function. If you want to do wildcard expansion in such places, you need to use the `wildcard' function, like this: $(wildcard PATTERN...) This string, used anywhere in a makefile, is replaced by a space-separated list of names of existing files that match one of the given file name patterns. If no existing file name matches a pattern, then that pattern is omitted from the output of the `wildcard' function. Note that this is different from how unmatched wildcards behave in rules, where they are used verbatim rather than ignored (*note Wildcard Pitfall::). One use of the `wildcard' function is to get a list of all the C source files in a directory, like this: $(wildcard *.c) We can change the list of C source files into a list of object files by replacing the `.c' suffix with `.o' in the result, like this: $(patsubst %.c,%.o,$(wildcard *.c)) (Here we have used another function, `patsubst'. *Note Functions for String Substitution and Analysis: Text Functions.) Thus, a makefile to compile all C source files in the directory and then link them together could be written as follows: objects := $(patsubst %.c,%.o,$(wildcard *.c)) foo : $(objects) cc -o foo $(objects) (This takes advantage of the implicit rule for compiling C programs, so there is no need to write explicit rules for compiling the files. *Note The Two Flavors of Variables: Flavors, for an explanation of `:=', which is a variant of `='.)  File: make.info, Node: Directory Search, Next: Phony Targets, Prev: Wildcards, Up: Rules Searching Directories for Prerequisites ======================================= For large systems, it is often desirable to put sources in a separate directory from the binaries. The "directory search" features of `make' facilitate this by searching several directories automatically to find a prerequisite. When you redistribute the files among directories, you do not need to change the individual rules, just the search paths. * Menu: * General Search:: Specifying a search path that applies to every prerequisite. * Selective Search:: Specifying a search path for a specified class of names. * Search Algorithm:: When and how search paths are applied. * 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.  File: make.info, Node: General Search, Next: Selective Search, Prev: Directory Search, Up: Directory Search `VPATH': Search Path for All Prerequisites ------------------------------------------ The value of the `make' variable `VPATH' specifies a list of directories that `make' should search. Most often, the directories are expected to contain prerequisite files that are not in the current directory; however, `VPATH' specifies a search list that `make' applies for all files, including files which are targets of rules. Thus, if a file that is listed as a target or prerequisite does not exist in the current directory, `make' searches the directories listed in `VPATH' for a file with that name. If a file is found in one of them, that file may become the prerequisite (see below). Rules may then specify the names of files in the prerequisite list as if they all existed in the current directory. *Note Writing Shell Commands with Directory Search: Commands/Search. In the `VPATH' variable, directory names are separated by colons or blanks. The order in which directories are listed is the order followed by `make' in its search. (On MS-DOS and MS-Windows, semi-colons are used as separators of directory names in `VPATH', since the colon can be used in the pathname itself, after the drive letter.) For example, VPATH = src:../headers specifies a path containing two directories, `src' and `../headers', which `make' searches in that order. With this value of `VPATH', the following rule, foo.o : foo.c is interpreted as if it were written like this: foo.o : src/foo.c assuming the file `foo.c' does not exist in the current directory but is found in the directory `src'.  File: make.info, Node: Selective Search, Next: Search Algorithm, Prev: General Search, Up: Directory Search The `vpath' Directive --------------------- Similar to the `VPATH' variable, but more selective, is the `vpath' directive (note lower case), which allows you to specify a search path for a particular class of file names: those that match a particular pattern. Thus you can supply certain search directories for one class of file names and other directories (or none) for other file names. There are three forms of the `vpath' directive: `vpath PATTERN DIRECTORIES' Specify the search path DIRECTORIES for file names that match PATTERN. The search path, DIRECTORIES, is a list of directories to be searched, separated by colons (semi-colons on MS-DOS and MS-Windows) or blanks, just like the search path used in the `VPATH' variable. `vpath PATTERN' Clear out the search path associated with PATTERN. `vpath' Clear all search paths previously specified with `vpath' directives. A `vpath' pattern is a string containing a `%' character. The string must match the file name of a prerequisite that is being searched for, the `%' character matching any sequence of zero or more characters (as in pattern rules; *note Defining and Redefining Pattern Rules: Pattern Rules.). For example, `%.h' matches files that end in `.h'. (If there is no `%', the pattern must match the prerequisite exactly, which is not useful very often.) `%' characters in a `vpath' directive's pattern can be quoted with preceding backslashes (`\'). Backslashes that would otherwise quote `%' characters can be quoted with more backslashes. Backslashes that quote `%' characters or other backslashes are removed from the pattern before it is compared to file names. Backslashes that are not in danger of quoting `%' characters go unmolested. When a prerequisite fails to exist in the current directory, if the PATTERN in a `vpath' directive matches the name of the prerequisite file, then the DIRECTORIES in that directive are searched just like (and before) the directories in the `VPATH' variable. For example, vpath %.h ../headers tells `make' to look for any prerequisite whose name ends in `.h' in the directory `../headers' if the file is not found in the current directory. If several `vpath' patterns match the prerequisite file's name, then `make' processes each matching `vpath' directive one by one, searching all the directories mentioned in each directive. `make' handles multiple `vpath' directives in the order in which they appear in the makefile; multiple directives with the same pattern are independent of each other. Thus, vpath %.c foo vpath % blish vpath %.c bar will look for a file ending in `.c' in `foo', then `blish', then `bar', while vpath %.c foo:bar vpath % blish will look for a file ending in `.c' in `foo', then `bar', then `blish'.  File: make.info, Node: Search Algorithm, Next: Commands/Search, Prev: Selective Search, Up: Directory Search How Directory Searches are Performed ------------------------------------ When a prerequisite is found through directory search, regardless of type (general or selective), the pathname located may not be the one that `make' actually provides you in the prerequisite list. Sometimes the path discovered through directory search is thrown away. The algorithm `make' uses to decide whether to keep or abandon a path found via directory search is as follows: 1. If a target file does not exist at the path specified in the makefile, directory search is performed. 2. If the directory search is successful, that path is kept and this file is tentatively stored as the target. 3. All prerequisites of this target are examined using this same method. 4. After processing the prerequisites, the target may or may not need to be rebuilt: a. If the target does _not_ need to be rebuilt, the path to the file found during directory search is used for any prerequisite lists which contain this target. In short, if `make' doesn't need to rebuild the target then you use the path found via directory search. b. If the target _does_ need to be rebuilt (is out-of-date), the pathname found during directory search is _thrown away_, and the target is rebuilt using the file name specified in the makefile. In short, if `make' must rebuild, then the target is rebuilt locally, not in the directory found via directory search. This algorithm may seem complex, but in practice it is quite often exactly what you want. Other versions of `make' use a simpler algorithm: if the file does not exist, and it is found via directory search, then that pathname is always used whether or not the target needs to be built. Thus, if the target is rebuilt it is created at the pathname discovered during directory search. If, in fact, this is the behavior you want for some or all of your directories, you can use the `GPATH' variable to indicate this to `make'. `GPATH' has the same syntax and format as `VPATH' (that is, a space- or colon-delimited list of pathnames). If an out-of-date target is found by directory search in a directory that also appears in `GPATH', then that pathname is not thrown away. The target is rebuilt using the expanded path.  File: make.info, Node: Commands/Search, Next: Implicit/Search, Prev: Search Algorithm, Up: Directory Search Writing Shell Commands with Directory Search -------------------------------------------- When a prerequisite is found in another directory through directory search, this cannot change the commands of the rule; they will execute as written. Therefore, you must write the commands with care so that they will look for the prerequisite in the directory where `make' finds it. This is done with the "automatic variables" such as `$^' (*note Automatic Variables: Automatic.). For instance, the value of `$^' is a list of all the prerequisites of the rule, including the names of the directories in which they were found, and the value of `$@' is the target. Thus: foo.o : foo.c cc -c $(CFLAGS) $^ -o $@ (The variable `CFLAGS' exists so you can specify flags for C compilation by implicit rules; we use it here for consistency so it will affect all C compilations uniformly; *note Variables Used by Implicit Rules: Implicit Variables..) Often the prerequisites include header files as well, which you do not want to mention in the commands. The automatic variable `$<' is just the first prerequisite: VPATH = src:../headers foo.o : foo.c defs.h hack.h cc -c $(CFLAGS) $< -o $@  File: make.info, Node: Implicit/Search, Next: Libraries/Search, Prev: Commands/Search, Up: Directory Search Directory Search and Implicit Rules ----------------------------------- The search through the directories specified in `VPATH' or with `vpath' also happens during consideration of implicit rules (*note Using Implicit Rules: Implicit Rules.). For example, when a file `foo.o' has no explicit rule, `make' considers implicit rules, such as the built-in rule to compile `foo.c' if that file exists. If such a file is lacking in the current directory, the appropriate directories are searched for it. If `foo.c' exists (or is mentioned in the makefile) in any of the directories, the implicit rule for C compilation is applied. The commands of implicit rules normally use automatic variables as a matter of necessity; consequently they will use the file names found by directory search with no extra effort.  File: make.info, Node: Libraries/Search, Prev: Implicit/Search, Up: Directory Search Directory Search for Link Libraries ----------------------------------- Directory search applies in a special way to libraries used with the linker. This special feature comes into play when you write a prerequisite whose name is of the form `-lNAME'. (You can tell something strange is going on here because the prerequisite is normally the name of a file, and the _file name_ of a library generally looks like `libNAME.a', not like `-lNAME'.) When a prerequisite's name has the form `-lNAME', `make' handles it specially by searching for the file `libNAME.so' in the current directory, in directories specified by matching `vpath' search paths and the `VPATH' search path, and then in the directories `/lib', `/usr/lib', and `PREFIX/lib' (normally `/usr/local/lib', but MS-DOS/MS-Windows versions of `make' behave as if PREFIX is defined to be the root of the DJGPP installation tree). If that file is not found, then the file `libNAME.a' is searched for, in the same directories as above. For example, if there is a `/usr/lib/libcurses.a' library on your system (and no `/usr/lib/libcurses.so' file), then foo : foo.c -lcurses cc $^ -o $@ would cause the command `cc foo.c /usr/lib/libcurses.a -o foo' to be executed when `foo' is older than `foo.c' or than `/usr/lib/libcurses.a'. Although the default set of files to be searched for is `libNAME.so' and `libNAME.a', this is customizable via the `.LIBPATTERNS' variable. Each word in the value of this variable is a pattern string. When a prerequisite like `-lNAME' is seen, `make' will replace the percent in each pattern in the list with NAME and perform the above directory searches using that library filename. If no library is found, the next word in the list will be used. The default value for `.LIBPATTERNS' is "`lib%.so lib%.a'", which provides the default behavior described above. You can turn off link library expansion completely by setting this variable to an empty value.  File: make.info, Node: Phony Targets, Next: Force Targets, Prev: Directory Search, Up: Rules Phony Targets ============= A phony target is one that is not really the name of a file. It is just a name for some commands 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 commands will not create the target file, the commands will be executed every time the target comes up for remaking. Here is an example: clean: rm *.o temp Because the `rm' command does not create a file named `clean', probably no such file will ever exist. Therefore, the `rm' command will be executed every time you say `make clean'. The phony target will cease to work if anything ever does create a file named `clean' in this directory. Since it has no prerequisites, the file `clean' would inevitably be considered up to date, and its commands would not be executed. To avoid this problem, you can explicitly declare the target to be phony, using the special target `.PHONY' (*note Special Built-in Target Names: Special Targets.) as follows: .PHONY : clean Once this is done, `make clean' will run the commands regardless of whether there is a file named `clean'. Since it knows that phony targets do not name actual files that could be remade from other files, `make' skips the implicit rule search for phony targets (*note Implicit Rules::). This is why declaring a target phony is good for performance, even if you are not worried about the actual file existing. Thus, you first write the line that states that `clean' is a phony target, then you write the rule, like this: .PHONY: clean clean: rm *.o temp Another example of the usefulness of phony targets is in conjunction with recursive invocations of `make' (for more information, see *Note Recursive Use of `make': Recursion). In this case the makefile will often contain a variable which lists a number of subdirectories to be built. One way to handle this is with one rule whose command is a shell loop over the subdirectories, like this: SUBDIRS = foo bar baz subdirs: for dir in $(SUBDIRS); do \ $(MAKE) -C $$dir; \ done There are a few problems with this method, however. First, any error detected in a submake is not noted by this rule, so it will continue to build the rest of the directories even when one fails. This can be overcome by adding shell commands to note the error and exit, but then it will do so even if `make' is invoked with the `-k' option, which is unfortunate. Second, and perhaps more importantly, you cannot take advantage of the parallel build capabilities of make using this method, since there is only one rule. By declaring the subdirectories as phony targets (you must do this as the subdirectory obviously always exists; otherwise it won't be built) you can remove these problems: SUBDIRS = foo bar baz .PHONY: subdirs $(SUBDIRS) subdirs: $(SUBDIRS) $(SUBDIRS): $(MAKE) -C $@ foo: baz Here we've also declared that the `foo' subdirectory cannot be built until after the `baz' subdirectory is complete; this kind of relationship declaration is particularly important when attempting parallel builds. A phony target should not be a prerequisite of a real target file; if it is, its commands are run every time `make' goes to update that file. As long as a phony target is never a prerequisite of a real target, the phony target commands will be executed only when the phony target is a specified goal (*note Arguments to Specify the Goals: Goals.). Phony targets can have prerequisites. When one directory contains multiple programs, it is most convenient to describe all of the programs in one makefile `./Makefile'. Since the target remade by default will be the first one in the makefile, it is common to make this a phony target named `all' and give it, as prerequisites, all the individual programs. For example: all : prog1 prog2 prog3 .PHONY : all prog1 : prog1.o utils.o cc -o prog1 prog1.o utils.o prog2 : prog2.o cc -o prog2 prog2.o prog3 : prog3.o sort.o utils.o cc -o prog3 prog3.o sort.o utils.o Now you can say just `make' to remake all three programs, or specify as arguments the ones to remake (as in `make prog1 prog3'). When one phony target is a prerequisite of another, it serves as a subroutine of the other. For example, here `make cleanall' will delete the object files, the difference files, and the file `program': .PHONY: cleanall cleanobj cleandiff cleanall : cleanobj cleandiff rm program cleanobj : rm *.o cleandiff : rm *.diff  File: make.info, Node: Force Targets, Next: Empty Targets, Prev: Phony Targets, Up: Rules Rules without Commands or Prerequisites ======================================= If a rule has no prerequisites or commands, and the target of the rule is a nonexistent file, then `make' imagines this target to have been updated whenever its rule is run. This implies that all targets depending on this one will always have their commands run. An example will illustrate this: clean: FORCE rm $(objects) FORCE: Here the target `FORCE' satisfies the special conditions, so the target `clean' that depends on it is forced to run its commands. There is nothing special about the name `FORCE', but that is one name commonly used this way. As you can see, using `FORCE' this way has the same results as using `.PHONY: clean'. Using `.PHONY' is more explicit and more efficient. However, other versions of `make' do not support `.PHONY'; thus `FORCE' appears in many makefiles. *Note Phony Targets::.  File: make.info, Node: Empty Targets, Next: Special Targets, Prev: Force Targets, Up: Rules Empty Target Files to Record Events =================================== The "empty target" is a variant of the phony target; it is used to hold commands 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 commands were last executed. It does so because one of the commands is a `touch' command to update the target file. The empty target file should have some prerequisites (otherwise it doesn't make sense). When you ask to remake the empty target, the commands are executed if any prerequisite is more recent than the target; in other words, if a prerequisite has changed since the last time you remade the target. Here is an example: print: foo.c bar.c lpr -p $? touch print With this rule, `make print' will execute the `lpr' command if either source file has changed since the last `make print'. The automatic variable `$?' is used to print only those files that have changed (*note Automatic Variables: Automatic.).  File: make.info, Node: Special Targets, Next: Multiple Targets, Prev: Empty Targets, Up: Rules Special Built-in Target Names ============================= Certain names have special meanings if they appear as targets. `.PHONY' The prerequisites of the special target `.PHONY' are considered to be phony targets. When it is time to consider such a target, `make' will run its commands unconditionally, regardless of whether a file with that name exists or what its last-modification time is. *Note Phony Targets: Phony Targets. `.SUFFIXES' The prerequisites of the special target `.SUFFIXES' are the list of suffixes to be used in checking for suffix rules. *Note Old-Fashioned Suffix Rules: Suffix Rules. `.DEFAULT' The commands specified for `.DEFAULT' are used for any target for which no rules are found (either explicit rules or implicit rules). *Note Last Resort::. If `.DEFAULT' commands are specified, every file mentioned as a prerequisite, but not as a target in a rule, will have these commands executed on its behalf. *Note Implicit Rule Search Algorithm: Implicit Rule Search. `.PRECIOUS' The targets which `.PRECIOUS' depends on are given the following special treatment: if `make' is killed or interrupted during the execution of their commands, the target is not deleted. *Note Interrupting or Killing `make': Interrupts. Also, if the target is an intermediate file, it will not be deleted after it is no longer needed, as is normally done. *Note Chains of Implicit Rules: Chained Rules. In this latter respect it overlaps with the `.SECONDARY' special target. You can also list the target pattern of an implicit rule (such as `%.o') as a prerequisite file of the special target `.PRECIOUS' to preserve intermediate files created by rules whose target patterns match that file's name. `.INTERMEDIATE' The targets which `.INTERMEDIATE' depends on are treated as intermediate files. *Note Chains of Implicit Rules: Chained Rules. `.INTERMEDIATE' with no prerequisites has no effect. `.SECONDARY' The targets which `.SECONDARY' depends on are treated as intermediate files, except that they are never automatically deleted. *Note Chains of Implicit Rules: Chained Rules. `.SECONDARY' with no prerequisites causes all targets to be treated as secondary (i.e., no target is removed because it is considered intermediate). `.DELETE_ON_ERROR' If `.DELETE_ON_ERROR' is mentioned as a target anywhere in the makefile, then `make' will delete the target of a rule if it has changed and its commands exit with a nonzero exit status, just as it does when it receives a signal. *Note Errors in Commands: Errors. `.IGNORE' If you specify prerequisites for `.IGNORE', then `make' will ignore errors in execution of the commands run for those particular files. The commands for `.IGNORE' are not meaningful. If mentioned as a target with no prerequisites, `.IGNORE' says to ignore errors in execution of commands for all files. This usage of `.IGNORE' is supported only for historical compatibility. Since this affects every command in the makefile, it is not very useful; we recommend you use the more selective ways to ignore errors in specific commands. *Note Errors in Commands: Errors. `.LOW_RESOLUTION_TIME' If you specify prerequisites for `.LOW_RESOLUTION_TIME', `make' assumes that these files are created by commands that generate low resolution time stamps. The commands for `.LOW_RESOLUTION_TIME' are not meaningful. The high resolution file time stamps of many modern hosts lessen the chance of `make' incorrectly concluding that a file is up to date. Unfortunately, these hosts provide no way to set a high resolution file time stamp, so commands like `cp -p' that explicitly set a file's time stamp must discard its subsecond part. If a file is created by such a command, you should list it as a prerequisite of `.LOW_RESOLUTION_TIME' so that `make' does not mistakenly conclude that the file is out of date. For example: .LOW_RESOLUTION_TIME: dst dst: src cp -p src dst Since `cp -p' discards the subsecond part of `src''s time stamp, `dst' is typically slightly older than `src' even when it is up to date. The `.LOW_RESOLUTION_TIME' line causes `make' to consider `dst' to be up to date if its time stamp is at the start of the same second that `src''s time stamp is in. Due to a limitation of the archive format, archive member time stamps are always low resolution. You need not list archive members as prerequisites of `.LOW_RESOLUTION_TIME', as `make' does this automatically. `.SILENT' If you specify prerequisites for `.SILENT', then `make' will not print the commands to remake those particular files before executing them. The commands for `.SILENT' are not meaningful. If mentioned as a target with no prerequisites, `.SILENT' says not to print any commands before executing them. This usage of `.SILENT' is supported only for historical compatibility. We recommend you use the more selective ways to silence specific commands. *Note Command Echoing: Echoing. If you want to silence all commands for a particular run of `make', use the `-s' or `--silent' option (*note Options Summary::). `.EXPORT_ALL_VARIABLES' Simply by being mentioned as a target, this tells `make' to export all variables to child processes by default. *Note Communicating Variables to a Sub-`make': Variables/Recursion. `.NOTPARALLEL' If `.NOTPARALLEL' is mentioned as a target, then this invocation of `make' will be run serially, even if the `-j' option is given. Any recursively invoked `make' command will still be run in parallel (unless its makefile contains this target). Any prerequisites on this target are ignored. Any defined implicit rule suffix also counts as a special target if it appears as a target, and so does the concatenation of two suffixes, such as `.c.o'. These targets are suffix rules, an obsolete way of defining implicit rules (but a way still widely used). In principle, any target name could be special in this way if you break it in two and add both pieces to the suffix list. In practice, suffixes normally begin with `.', so these special target names also begin with `.'. *Note Old-Fashioned Suffix Rules: Suffix Rules.