4. Writing Rules

A rule appears in the makefile and says when and how to remake certain files, called the rule's targets (most often only one per rule). It lists the other files that are the prerequisites of the target, and 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. (See section 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 `all'). See section Arguments to Specify the Goals.

4.1 Rule Example

Here is an example of a rule:

foo.o : foo.c defs.h       # module for twiddling the frobs
        cc -c -g foo.c

Its target is `foo.o' and its prerequisites are `foo.c' and `defs.h'. It has one command, 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• How to decide whether `foo.o' is out of date: it is out of date if it does not exist, or if either `foo.c' or `defs.h' is more recent than it.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.

4.2 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 (see section Using Wildcard Characters in File Names) and a name of the form `a(m)' represents member m in archive file a (see section Archive Members as Targets). Usually there is only one target per rule, but occasionally there is a reason to have more (see section Multiple Targets in a Rule).

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. There are other differences in the syntax of command lines. See section Writing the Commands in Rules.

Because dollar signs are used to start make variable references, if you really want a dollar sign in a target or prerequisite you must write two of them, `$$' (see section How to Use Variables). If you have enabled secondary expansion (see section Secondary Expansion) and you want a literal dollar sign in the prerequisites lise, you must actually write four dollar signs (`$$$$').

You may split a long line by inserting a backslash followed by a newline, but this is not required, as make places no limit on the length of a line in a makefile.

A rule tells make two things: when the targets are out of date, and how to update them when necessary.

The criterion for being out of date is specified in terms of the prerequisites, which consist of file names separated by spaces. (Wildcards and archive members (see section Using make to Update Archive Files) 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 (see section Writing the Commands in Rules).

4.3 Types of Prerequisites

There are actually two different types of prerequisites understood by GNU make: normal prerequisites such as described in the previous section, and order-only prerequisites. A normal prerequisite makes two statements: first, it imposes an order 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).

4.4 Using Wildcard Characters in File Names

A single file name can specify many files using wildcard characters. The wildcard characters in make are `*', `?' and `[…]', the same as in the Bourne shell. For example, `*.c' specifies a list of all the files (in the working directory) whose names end in `.c'.

The character `~' at the beginning of a file name also has special significance. If alone, or followed by a slash, it represents your home directory. For example `~/bin' expands to `/home/you/bin'. If the `~' is followed by a word, the string represents the home directory of the user named by that word. For example `~john/bin' expands to `/home/john/bin'. On systems which don't have a home directory for each user (such as MS-DOS or MS-Windows), this functionality can be simulated by setting the environment variable HOME.

Wildcard expansion is performed by make automatically in targets and in prerequisites. In commands the shell is responsible for wildcard expansion. In other contexts, wildcard expansion happens only if you request it explicitly with the wildcard function.

The special significance of a wildcard character can be turned off by preceding it with a backslash. Thus, `foo\*bar' would refer to a specific file whose name consists of `foo', an asterisk, and `bar'.

4.4.1 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 Empty Target Files to Record Events. (The automatic variable `$?' is used to print only those files that have changed; see Automatic Variables.)

Wildcard expansion does not happen when you define a variable. Thus, if you write this:

objects = *.o

then the value of the variable objects is the actual string `*.o'. However, if you use the value of objects in a target, prerequisite or command, wildcard expansion will take place at that time. To set objects to the expansion, instead use:

objects := $(wildcard *.o)

See section The Function wildcard.

4.4.2 Pitfalls of Using Wildcards

Now here is an example of a naive way of using wildcard expansion, that does not do what you would intend. Suppose you would like to say that the executable file `foo' is made from all the object files in the directory, and you write this:

objects = *.o

foo : $(objects)
        cc -o foo $(CFLAGS) $(objects)

The value of objects is the actual string `*.o'. Wildcard expansion happens in the rule for `foo', so that each existing `.o' file becomes a prerequisite of `foo' and will be recompiled if necessary.

But what if you delete all the `.o' files? When a wildcard matches no files, it is left as it is, so then `foo' will depend on the oddly-named file `*.o'. Since no such file is likely to exist, make will give you an error saying it cannot figure out how to make `*.o'. This is not what you want!

Actually it is possible to obtain the desired result with wildcard expansion, but you need more sophisticated techniques, including the wildcard function and string substitution. See section The Function wildcard.

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.

4.4.3 The Function wildcard

Wildcard expansion happens automatically in rules. But wildcard expansion does not normally take place when a variable is set, or inside the arguments of a function. If you want to do wildcard expansion in such places, you need to use the wildcard function, like this:

$(wildcard pattern…)

This string, used anywhere in a makefile, is replaced by a space-separated list of names of existing files that match one of the given file name patterns. If no existing file name matches a pattern, then that pattern is omitted from the output of the wildcard function. Note that this is different from how unmatched wildcards behave in rules, where they are used verbatim rather than ignored (see section Pitfalls of Using Wildcards).

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. See section Functions for String Substitution and Analysis.)

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. See section The Two Flavors of Variables, for an explanation of `:=', which is a variant of `='.)

4.5 Searching Directories for Prerequisites

For large systems, it is often desirable to put sources in a separate directory from the binaries. The directory search features of make facilitate this by searching several directories automatically to find a prerequisite. When you redistribute the files among directories, you do not need to change the individual rules, just the search paths.

4.5.1 VPATH: Search Path for All Prerequisites

The value of the make variable VPATH specifies a list of directories that make should search. Most often, the directories are expected to contain prerequisite files that are not in the current directory; however, make uses VPATH as a search list for both prerequisites and targets of rules.

Thus, if a file that is listed as a target or prerequisite does not exist in the current directory, make searches the directories listed in VPATH for a file with that name. If a file is found in one of them, that file may become the prerequisite (see below). Rules may then specify the names of files in the prerequisite list as if they all existed in the current directory. See section Writing Shell Commands with Directory 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'.

4.5.2 The vpath Directive

Similar to the VPATH variable, but more selective, is the vpath directive (note lower case), which allows you to specify a search path for a particular class of file names: those that match a particular pattern. Thus you can supply certain search directories for one class of file names and other directories (or none) for other file names.

There are three forms of the vpath directive:

vpath pattern directories

Specify the search path directories for file names that match pattern.

The search path, directories, is a list of directories to be searched, separated by colons (semi-colons on MS-DOS and MS-Windows) or blanks, just like the search path used in the VPATH variable.

vpath pattern

Clear out the search path associated with pattern.

vpath

Clear all search paths previously specified with vpath directives.

A vpath pattern is a string containing a `%' character. The string must match the file name of a prerequisite that is being searched for, the `%' character matching any sequence of zero or more characters (as in pattern rules; see section Defining and Redefining 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'.

4.5.3 How Directory Searches are Performed

When a prerequisite is found through directory search, regardless of type (general or selective), the pathname located may not be the one that make actually provides you in the prerequisite list. Sometimes the path discovered through directory search is thrown away.

The algorithm make uses to decide whether to keep or abandon a path found via directory search is as follows:

1. If a target file does not exist at the path specified in the makefile, directory search is performed.

2. If the directory search is successful, that path is kept and this file is tentatively stored as the target.

3. All prerequisites of this target are examined using this same method.

4. After processing the prerequisites, the target may or may not need to be rebuilt:

   1. 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.

   2. 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.

4.5.4 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 `$^' (see section Automatic Variables). For instance, the value of `$^' is a list of all the prerequisites of the rule, including the names of the directories in which they were found, and the value of `$@' is the target. Thus:

foo.o : foo.c
        cc -c $(CFLAGS) $^ -o $@

(The variable CFLAGS exists so you can specify flags for C compilation by implicit rules; we use it here for consistency so it will affect all C compilations uniformly; see section Variables Used by Implicit Rules.)

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 $@

4.5.5 Directory Search and Implicit Rules

The search through the directories specified in VPATH or with vpath also happens during consideration of implicit rules (see section Using 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.

4.5.6 Directory Search for Link Libraries

Directory search applies in a special way to libraries used with the linker. This special feature comes into play when you write a prerequisite whose name is of the form `-lname'. (You can tell something strange is going on here because the prerequisite is normally the name of a file, and the file name of a library generally looks like `libname.a', not like `-lname'.)

When a prerequisite's name has the form `-lname', make handles it specially by searching for the file `libname.so' 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.

4.6 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 (see section Special Built-in Target Names) 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 (see section Using 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 Recursive Use of make). 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 make's ability to build targets in parallel (see section Parallel Execution), 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 (see section 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 `./Makefile'. Since the target remade by default will be the first one in the makefile, it is common to make this a phony target named `all' and give it, as prerequisites, all the individual programs. For example:

all : prog1 prog2 prog3
.PHONY : all

prog1 : prog1.o utils.o
        cc -o prog1 prog1.o utils.o

prog2 : prog2.o
        cc -o prog2 prog2.o

prog3 : prog3.o sort.o utils.o
        cc -o prog3 prog3.o sort.o utils.o

Now you can say just `make' to remake all three programs, or specify as arguments the ones to remake (as in `make prog1 prog3'). Phoniness is not inherited: the prerequisites of a phony target are not themselves phony, unless explicitly declared to be so.

When one phony target is a prerequisite of another, it serves as a subroutine of the other. For example, here `make cleanall' will delete the object files, the difference files, and the file `program':

.PHONY: cleanall cleanobj cleandiff

cleanall : cleanobj cleandiff
        rm program

cleanobj :
        rm *.o

cleandiff :
        rm *.diff

4.7 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. See section Phony Targets.

4.8 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 (see section Automatic Variables).

4.9 Special Built-in Target Names

Certain names have special meanings if they appear as targets.

.PHONY

The prerequisites of the special target .PHONY are considered to be phony targets. When it is time to consider such a target, make will run its commands unconditionally, regardless of whether a file with that name exists or what its last-modification time is. See section Phony Targets.

.SUFFIXES

The prerequisites of the special target .SUFFIXES are the list of suffixes to be used in checking for suffix rules. See section Old-Fashioned 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). See section Defining Last-Resort Default Rules. 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. See section Implicit Rule Search Algorithm.

.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. See section Interrupting or Killing make. Also, if the target is an intermediate file, it will not be deleted after it is no longer needed, as is normally done. See section Chains of Implicit 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. See section Chains of Implicit 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. See section Chains of Implicit Rules.

.SECONDARY with no prerequisites causes all targets to be treated as secondary (i.e., no target is removed because it is considered intermediate).

.SECONDEXPANSION

If .SECONDEXPANSION is mentioned as a target anywhere in the makefile, then all prerequisite lists defined after it appears will be expanded a second time after all makefiles have been read in. See section Secondary Expansion.

The prerequisites of the special target .SUFFIXES are the list of suffixes to be used in checking for suffix rules. See section Old-Fashioned Suffix Rules.

.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. See section Errors in Commands.

.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. See section Errors in Commands.

.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. See section Command Echoing. If you want to silence all commands for a particular run of make, use the `-s' or `--silent' option (see section Summary of Options).

.EXPORT_ALL_VARIABLES

Simply by being mentioned as a target, this tells make to export all variables to child processes by default. See section Communicating Variables to a Sub-make.

.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 `.'. See section Old-Fashioned Suffix Rules.

4.10 Multiple Targets in a Rule

A rule with multiple targets is equivalent to writing many rules, each with one target, and all identical aside from that. The same commands apply to all the targets, but their effects may vary because you can substitute the actual target name into the command using `$@'. The rule contributes the same prerequisites to all the targets also.

This is useful in two cases.

• You want just prerequisites, no commands. For example:

  kbd.o command.o files.o: command.h

  gives an additional prerequisite to each of the three object files mentioned.

• Similar commands work for all the targets. The commands do not need to be absolutely identical, since the automatic variable `$@' can be used to substitute the particular target to be remade into the commands (see section Automatic Variables). For example:

  bigoutput littleoutput : text.g generate text.g -$(subst output,,$@) > $@

  is equivalent to

  bigoutput : text.g generate text.g -big > bigoutput littleoutput : text.g generate text.g -little > littleoutput

  Here we assume the hypothetical program generate makes two types of output, one if given `-big' and one if given `-little'. See section Functions for String Substitution and Analysis, for an explanation of the subst function.

Suppose you would like to vary the prerequisites according to the target, much as the variable `$@' allows you to vary the commands. You cannot do this with multiple targets in an ordinary rule, but you can do it with a static pattern rule. See section Static Pattern Rules.

4.11 Multiple Rules for One Target

One file can be the target of several rules. All the prerequisites mentioned in all the rules are merged into one list of prerequisites for the target. If the target is older than any prerequisite from any rule, the commands are executed.

There can only be one set of commands to be executed for a file. If more than one rule gives commands for the same file, make uses the last set given and prints an error message. (As a special case, if the file's name begins with a dot, no error message is printed. This odd behavior is only for compatibility with other implementations of make... you should avoid using it). Occasionally it is useful to have the same target invoke multiple commands which are defined in different parts of your makefile; you can use double-colon rules (see section Double-Colon Rules) for this.

An extra rule with just prerequisites can be used to give a few extra prerequisites to many files at once. For example, makefiles often have a variable, such as objects, containing a list of all the compiler output files in the system being made. An easy way to say that all of them must be recompiled if `config.h' changes is to write the following:

objects = foo.o bar.o
foo.o : defs.h
bar.o : defs.h test.h
$(objects) : config.h

This could be inserted or taken out without changing the rules that really specify how to make the object files, making it a convenient form to use if you wish to add the additional prerequisite intermittently.

Another wrinkle is that the additional prerequisites could be specified with a variable that you set with a command argument to make (see section Overriding Variables). For example,

extradeps=
$(objects) : $(extradeps)

means that the command `make extradeps=foo.h' will consider `foo.h' as a prerequisite of each object file, but plain `make' will not.

If none of the explicit rules for a target has commands, then make searches for an applicable implicit rule to find some commands see section Using Implicit Rules).

4.12 Static Pattern Rules

Static pattern rules are rules which specify multiple targets and construct the prerequisite names for each target based on the target name. They are more general than ordinary rules with multiple targets because the targets do not have to have identical prerequisites. Their prerequisites must be analogous, but not necessarily identical.

4.12.1 Syntax of Static Pattern Rules

Here is the syntax of a static pattern rule:

targets …: target-pattern: prereq-patterns …
        commands
        …

The targets list specifies the targets that the rule applies to. The targets can contain wildcard characters, just like the targets of ordinary rules (see section Using Wildcard Characters in File Names).

The target-pattern and prereq-patterns say how to compute the prerequisites of each target. Each target is matched against the target-pattern to extract a part of the target name, called the stem. This stem is substituted into each of the prereq-patterns to make the prerequisite names (one from each prereq-pattern).

Each pattern normally contains the character `%' just once. When the target-pattern matches a target, the `%' can match any part of the target name; this part is called the stem. The rest of the pattern must match exactly. For example, the target `foo.o' matches the pattern `%.o', with `foo' as the stem. The targets `foo.c' and `foo.out' do not match that pattern.

The prerequisite names for each target are made by substituting the stem for the `%' in each prerequisite pattern. For example, if one prerequisite pattern is `%.c', then substitution of the stem `foo' gives the prerequisite name `foo.c'. It is legitimate to write a prerequisite pattern that does not contain `%'; then this prerequisite is the same for all targets.

`%' characters in pattern rules can be quoted with preceding backslashes (`\'). Backslashes that would otherwise quote `%' characters can be quoted with more backslashes. Backslashes that quote `%' characters or other backslashes are removed from the pattern before it is compared to file names or has a stem substituted into it. Backslashes that are not in danger of quoting `%' characters go unmolested. For example, the pattern `the\%weird\\%pattern\\' has `the%weird\' preceding the operative `%' character, and `pattern\\' following it. The final two backslashes are left alone because they cannot affect any `%' character.

Here is an example, which compiles each of `foo.o' and `bar.o' from the corresponding `.c' file:

objects = foo.o bar.o

all: $(objects)

$(objects): %.o: %.c
        $(CC) -c $(CFLAGS) $< -o $@

Here `$<' is the automatic variable that holds the name of the prerequisite and `$@' is the automatic variable that holds the name of the target; see Automatic Variables.

Each target specified must match the target pattern; a warning is issued for each target that does not. If you have a list of files, only some of which will match the pattern, you can use the filter function to remove nonmatching file names (see section Functions for String Substitution and Analysis):

files = foo.elc bar.o lose.o

$(filter %.o,$(files)): %.o: %.c
        $(CC) -c $(CFLAGS) $< -o $@
$(filter %.elc,$(files)): %.elc: %.el
        emacs -f batch-byte-compile $<

In this example the result of `$(filter %.o,$(files))' is `bar.o lose.o', and the first static pattern rule causes each of these object files to be updated by compiling the corresponding C source file. The result of `$(filter %.elc,$(files))' is `foo.elc', so that file is made from `foo.el'.

Another example shows how to use $* in static pattern rules:

bigoutput littleoutput : %output : text.g
        generate text.g -$* > $@

When the generate command is run, $* will expand to the stem, either `big' or `little'.

4.12.2 Static Pattern Rules versus Implicit Rules

A static pattern rule has much in common with an implicit rule defined as a pattern rule (see section 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 make decides when the rule applies.

An implicit rule can apply to any target that matches its pattern, but it does apply only when the target has no commands 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 commands, that's an error.

The static pattern rule can be better than an implicit rule for these reasons:

• You may wish to override the usual implicit rule for a few files whose names cannot be categorized syntactically but can be given in an explicit list.

• If you cannot be sure of the precise contents of the directories you are using, you may not be sure which other irrelevant files might lead make to use the wrong implicit rule. The choice might depend on the order in which the implicit rule search is done. With static pattern rules, there is no uncertainty: each rule applies to precisely the targets specified.

4.13 Double-Colon Rules

Double-colon rules are rules written with `::' instead of `:' after the target names. They are handled differently from ordinary rules when the same target appears in more than one rule.

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 commands are executed if the target is older than any prerequisites of that rule. If there are no prerequisites for that rule, its commands are 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 commands 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 commands; if it does not, an implicit rule will be used if one applies. See section Using Implicit Rules.

4.14 Generating Prerequisites Automatically

In the makefile for a program, many of the rules you need to write often say only that some object file depends on some header file. For example, if `main.c' uses `defs.h' via an #include, you would write:

main.o: defs.h

You need this rule so that make knows that it must remake `main.o' whenever `defs.h' changes. You can see that for a large program you would have to write dozens of such rules in your makefile. And, you must always be very careful to update the makefile every time you add or remove an #include.

To avoid this hassle, most modern C compilers can write these rules for you, by looking at the #include lines in the source files. Usually this is done with the `-M' option to the compiler. For example, the command:

cc -M main.c

generates the output:

main.o : main.c defs.h

Thus you no longer have to write all those rules yourself. The compiler will do it for you.

Note that such a prerequisite constitutes mentioning `main.o' in a makefile, so it can never be considered an intermediate file by implicit rule search. This means that make won't ever remove the file after using it; see section Chains of Implicit Rules.

With old make programs, it was traditional practice to use this compiler feature to generate prerequisites on demand with a command like `make depend'. That command would create a file `depend' containing all the automatically-generated prerequisites; then the makefile could use include to read them in (see section Including Other Makefiles).

In GNU make, the feature of remaking makefiles makes this practice obsolete--you need never tell make explicitly to regenerate the prerequisites, because it always regenerates any makefile that is out of date. See section How Makefiles Are Remade.

The practice we recommend for automatic prerequisite generation is to have one makefile corresponding to each source file. For each source file `name.c' there is a makefile `name.d' which lists what files the object file `name.o' depends on. That way only the source files that have changed need to be rescanned to produce the new prerequisites.

Here is the pattern rule to generate a file of prerequisites (i.e., a makefile) called `name.d' from a C source file called `name.c':

%.d: %.c
        @set -e; rm -f $@; \
         $(CC) -M $(CPPFLAGS) $< > $@.$$$$; \
         sed 's,\($*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@; \
         rm -f $@.$$$$

See section Defining and Redefining Pattern Rules, for information on defining pattern rules. The `-e' flag to the shell causes it to exit immediately if the $(CC) command (or any other command) fails (exits with a nonzero status).

With the GNU C compiler, you may wish to use the `-MM' flag instead of `-M'. This omits prerequisites on system header files. See (gcc.info)Preprocessor Options section `Options Controlling the Preprocessor' in Using GNU CC, for details.

The purpose of the sed command is to translate (for example):

main.o : main.c defs.h

into:

main.o main.d : main.c defs.h

This makes each `.d' file depend on all the source and header files that the corresponding `.o' file depends on. make then knows it must regenerate the prerequisites whenever any of the source or header files changes.

Once you've defined the rule to remake the `.d' files, you then use the include directive to read them all in. See section Including Other Makefiles. For example:

sources = foo.c bar.c

include $(sources:.c=.d)

(This example uses a substitution variable reference to translate the list of source files `foo.c bar.c' into a list of prerequisite makefiles, `foo.d bar.d'. See section Substitution References, for full information on substitution references.) Since the `.d' files are makefiles like any others, make will remake them as necessary with no further work from you. See section How Makefiles Are Remade.

Note that the `.d' files contain target definitions; you should be sure to place the include directive after the first, default goal in your makefiles or run the risk of having a random object file become the default goal. See section How make Processes a Makefile.

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