GNU make - 4. Writing Rules

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# 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 dependencies 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 10.5 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 9.2 Arguments to Specify the Goals.

## 4.1 Rule Syntax

In general, a rule looks like this:

    targets : dependencies
            command
            ...

or like this:

    targets : dependencies ; command
            command
            ...

The targets are file names, separated by spaces. Wildcard characters may be used (see section 4.2 Using Wildcard Characters in File Names) and a name of the form ``a(m)'` represents member m in archive file a (see section 11.1 Archive Members as Targets). Usually there is only one target per rule, but occasionally there is a reason to have more (see section 4.8 Multiple Targets in a Rule).

The command lines start with a tab character. The first command may appear on the line after the dependencies, with a tab character, or may appear on the same line, with a semicolon. Either way, the effect is the same. See section 5. Writing the Commands in Rules.

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, ``$$'` (see section 6. How to Use 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A rule tells `make` two things: when the targets are out of date, and how to update them when necessary.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 dependencies, which consist of file names separated by spaces. (Wildcards and archive members (see section 11. 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 dependencies (by comparison of last-modification times). The idea is that the contents of the target file are computed based on information in the dependencies, so if any of the dependencies 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 5. Writing the Commands in Rules).

## 4.2 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 dependencies, 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'`.

### 4.2.1 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 dependencies 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 section 4.6 Empty Target Files to Record Events. (The automatic variable ``$?'` is used to print only those files that have changed; see section 10.5.3 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, dependency or command, wildcard expansion will take place at that time. To set `objects` to the expansion, instead use:

    objects := $(wildcard *.o)

See section 4.2.3 The Function `wildcard`.

### 4.2.2 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 dependency 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. These are described in the following section.

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.

### 4.2.3 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 4.2.2 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 8.2 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 6.2 The Two Flavors of Variables, for an explanation of ``:='`, which is a variant of ``='`.)

## 4.3 Searching Directories for Dependencies

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 dependency. When you redistribute the files among directories, you do not need to change the individual rules, just the search paths.

### 4.3.1 `VPATH`: Search Path for All Dependencies

The value of the `make` variable `VPATH` specifies a list of directories that `make` should search. Most often, the directories are expected to contain dependency 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 dependency 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 dependency (see below). Rules may then specify the names of files in the dependency list as if they all existed in the current directory. See section 4.3.4 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'`.

### 4.3.2 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 dependency that is being searched for, the ``%'` character matching any sequence of zero or more characters (as in pattern rules; see section 10.5 Defining and Redefining Pattern Rules). For example, `%.h` matches files that end in `.h`. (If there is no ``%'`, the pattern must match the dependency 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 dependency fails to exist in the current directory, if the pattern in a `vpath` directive matches the name of the dependency 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 dependency 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 dependency 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'`.

### 4.3.3 How Directory Searches are Performed

When a dependency 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 dependency 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 dependencies of this target are examined using this same method.
4. After processing the dependencies, 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 dependency 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.

### 4.3.4 Writing Shell Commands with Directory Search

When a dependency 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 dependency in the directory where `make` finds it.

This is done with the automatic variables such as ``$^'` (see section 10.5.3 Automatic Variables). For instance, the value of ``$^'` is a list of all the dependencies 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 10.3 Variables Used by Implicit Rules.)

Often the dependencies include header files as well, which you do not want to mention in the commands. The automatic variable ``$<'` is just the first dependency:

    VPATH = src:../headers
    foo.o : foo.c defs.h hack.h
            cc -c $(CFLAGS) $< -o $@

### 4.3.5 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 10. 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.

### 4.3.6 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 dependency whose name is of the form ``-lname'`. (You can tell something strange is going on here because the dependency is normally the name of a file, and the file name of the library looks like ``libname.a'`, not like ``-lname'`.)

When a dependency's name has the form ``-lname'`, `make` handles it specially by searching for the file ``libname.a'` in the current directory, in directories specified by matching `vpath` search paths and the `VPATH` search path, and then in the directories ``/lib'`, ``/usr/lib'`, and ``prefix/lib'` (normally ``/usr/local/lib'`, but MS-DOS/MS-Windows versions of `make` behave as if prefix is defined to be the root of the DJGPP installation tree).

For example,

    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'`.

## 4.4 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 dependencies, 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 4.7 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 10. 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

A phony target should not be a dependency 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 dependency of a real target, the phony target commands will be executed only when the phony target is a specified goal (see section 9.2 Arguments to Specify the Goals).

Phony targets can have dependencies. 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 dependencies, 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 dependency 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

## 4.5 Rules without Commands or Dependencies

If a rule has no dependencies 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 4.4 Phony Targets.

## 4.6 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 must have some dependencies. When you ask to remake the empty target, the commands are executed if any dependency is more recent than the target; in other words, if a dependency 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 10.5.3 Automatic Variables).

## 4.7 Special Built-in Target Names

Certain names have special meanings if they appear as targets.

`.PHONY`
  The dependencies 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 4.4 Phony Targets.
`.SUFFIXES`
  The dependencies of the special target `.SUFFIXES` are the list of suffixes to be used in checking for suffix rules. See section 10.7 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 10.6 Defining Last-Resort Default Rules. If `.DEFAULT` commands are specified, every file mentioned as a dependency, but not as a target in a rule, will have these commands executed on its behalf. See section 10.8 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 5.5 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 10.4 Chains of Implicit Rules. You can also list the target pattern of an implicit rule (such as ``%.o'`) as a dependency 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 10.4 Chains of Implicit Rules. `.INTERMEDIATE` with no dependencies marks all file targets mentioned in the makefile as intermediate.
`.SECONDARY`
  The targets which `.SECONDARY` depends on are treated as intermediate files, except that they are never automatically deleted. See section 10.4 Chains of Implicit Rules. `.SECONDARY` with no dependencies marks all file targets mentioned in the makefile as secondary.
`.IGNORE`
  If you specify dependencies 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 dependencies, `.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 5.4 Errors in Commands.
`.SILENT`
  If you specify dependencies for `.SILENT`, then `make` will not the print commands to remake those particular files before executing them. The commands for `.SILENT` are not meaningful. If mentioned as a target with no dependencies, `.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 5.1 Command Echoing. If you want to silence all commands for a particular run of `make`, use the ``-s'` or ``--silent'` option (see section 9.7 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 5.6.2 Communicating Variables to a Sub-`make`.

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 10.7 Old-Fashioned Suffix Rules.

## 4.8 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 dependencies to all the targets also.

This is useful in two cases.

- You want just dependencies, no commands. For example:

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

  gives an additional dependency 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 10.5.3 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 8.2 Functions for String Substitution and Analysis, for an explanation of the `subst` function.

Suppose you would like to vary the dependencies 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 4.10 Static Pattern Rules.

## 4.9 Multiple Rules for One Target

One file can be the target of several rules. All the dependencies mentioned in all the rules are merged into one list of dependencies for the target. If the target is older than any dependency 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`.) There is no reason to write your makefiles this way; that is why `make` gives you an error message.

An extra rule with just dependencies can be used to give a few extra dependencies to many files at once. For example, one usually has a variable named `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 dependency intermittently.

Another wrinkle is that the additional dependencies could be specified with a variable that you set with a command argument to `make` (see section 9.5 Overriding Variables). For example,

    extradeps=
    $(objects) : $(extradeps)

means that the command ``make extradeps=foo.h'` will consider ``foo.h'` as a dependency 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 10. Using Implicit Rules).

## 4.10 Static Pattern Rules

Static pattern rules are rules which specify multiple targets and construct the dependency 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 dependencies. Their dependencies must be analogous, but not necessarily identical.

### 4.10.1 Syntax of Static Pattern Rules

Here is the syntax of a static pattern rule:

    targets ...: target-pattern: dep-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 4.2 Using Wildcard Characters in File Names).

The target-pattern and dep-patterns say how to compute the dependencies 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 dep-patterns to make the dependency names (one from each dep-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 dependency names for each target are made by substituting the stem for the ``%'` in each dependency pattern. For example, if one dependency pattern is ``%.c'`, then substitution of the stem ``foo'` gives the dependency name ``foo.c'`. It is legitimate to write a dependency pattern that does not contain ``%'`; then this dependency 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 ``$<'` is the automatic variable that holds the name of the dependency and ``$@'` is the automatic variable that holds the name of the target; see section 10.5.3 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 8.2 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'`.

### 4.10.2 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 10.5 Defining and Redefining Pattern Rules). Both have a pattern for the target and patterns for constructing the names of dependencies. 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 dependencies 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.

## 4.11 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 dependencies of that rule. 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 dependency 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 10. Using Implicit Rules.

## 4.12 Generating Dependencies 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 dependency 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 10.4 Chains of Implicit Rules.

With old `make` programs, it was traditional practice to use this compiler feature to generate dependencies on demand with a command like ``make depend'`. That command would create a file ``depend'` containing all the automatically-generated dependencies; then the makefile could use `include` to read them in (see section 3.3 Including Other Makefiles).

In GNU `make`, the feature of remaking makefiles makes this practice obsolete--you need never tell `make` explicitly to regenerate the dependencies, because it always regenerates any makefile that is out of date. See section 3.5 How Makefiles Are Remade.

The practice we recommend for automatic dependency 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 dependencies.

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

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

See section 10.5 Defining and Redefining Pattern Rules, for information on defining pattern rules. The ``-e'` flag to the shell makes it exit immediately if the `$(CC)` command fails (exits with a nonzero status). Normally the shell exits with the status of the last command in the pipeline (`sed` in this case), so `make` would not notice a nonzero status from the compiler.

With the GNU C compiler, you may wish to use the ``-MM'` flag instead of ``-M'`. This omits dependencies on system header files. See 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 dependencies 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 3.3 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 dependency makefiles, ``foo.d bar.d'`. See section 6.3.1 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 3.5 How Makefiles Are Remade.

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