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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 An example explained. 4.2 Rule Syntax General syntax explained. 4.3 Using Wildcard Characters in File Names Using wildcard characters such as `*'. 4.4 Searching Directories for Prerequisites Searching other directories for source files. 4.5 Phony Targets Using a target that is not a real file's name. 4.6 Rules without Commands or Prerequisites You can use a target without commands or prerequisites to mark other targets as phony. 4.7 Empty Target Files to Record Events When only the date matters and the files are empty. 4.8 Special Built-in Target Names Targets with special built-in meanings. 4.9 Multiple Targets in a Rule When to make use of several targets in a rule. 4.10 Multiple Rules for One Target How to use several rules with the same target. 4.11 Static Pattern Rules Static pattern rules apply to multiple targets and can vary the prerequisites according to the target name. 4.12 Double-Colon Rules How to use a special kind of rule to allow several independent rules for one target. 4.13 Generating Prerequisites Automatically How to automatically generate rules giving prerequisites from source files themselves.
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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:
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.
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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. See section 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 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
prerequisites, 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 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).
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A single file name can specify many files using wildcard characters.
The wildcard characters in make
are `*', `?' and
`[...]', the same as in the Bourne shell. For example, `*.c'
specifies a list of all the files (in the working directory) whose names
end in `.c'.
The character `~' at the beginning of a file name also has special significance. If alone, or followed by a slash, it represents your home directory. For example `~/bin' expands to `/home/you/bin'. If the `~' is followed by a word, the string represents the home directory of the user named by that word. For example `~john/bin' expands to `/home/john/bin'. On systems which don't have a home directory for each user (such as MS-DOS or MS-Windows), this functionality can be simulated by setting the environment variable HOME.
Wildcard expansion happens automatically in targets, in prerequisites,
and in commands (where the shell does the expansion). In other
contexts, wildcard expansion happens only if you request it explicitly
with the wildcard
function.
The special significance of a wildcard character can be turned off by preceding it with a backslash. Thus, `foo\*bar' would refer to a specific file whose name consists of `foo', an asterisk, and `bar'.
4.3.1 Wildcard Examples Several examples 4.3.2 Pitfalls of Using Wildcards Problems to avoid. 4.3.3 The Function wildcard
How to cause wildcard expansion where it does not normally take place.
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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 4.3.3 The Function wildcard
.
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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.
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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 4.3.2 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 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 `='.)
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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.
4.4.1 VPATH
: Search Path for All PrerequisitesSpecifying a search path that applies to every prerequisite. 4.4.2 The vpath
DirectiveSpecifying a search path for a specified class of names. 4.4.3 How Directory Searches are Performed When and how search paths are applied. 4.4.4 Writing Shell Commands with Directory Search How to write shell commands that work together with search paths. 4.4.5 Directory Search and Implicit Rules How search paths affect implicit rules. 4.4.6 Directory Search for Link Libraries Directory search for link libraries.
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VPATH
: Search Path for All Prerequisites
The value of the make
variable VPATH
specifies a list of
directories that make
should search. Most often, the
directories are expected to contain prerequisite files that are not in the
current directory; however, VPATH
specifies a search list that
make
applies for all files, including files which are targets of
rules.
Thus, if a file that is listed as a target or prerequisite does not exist
in the current directory, make
searches the directories listed in
VPATH
for a file with that name. If a file is found in one of
them, that file may become the prerequisite (see below). Rules may then
specify the names of files in the prerequisite list as if they all
existed in the current directory. 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'.
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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
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
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'.
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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:
make
doesn't need to rebuild
the target then you use the path found via directory search.
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.
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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 $@ |
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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.
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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.
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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 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 |
Another example of the usefulness of phony targets is in conjunction
with recursive invocations 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 of problems with this method, however. First, any error
detected in a submake is not noted by this rule, so it will continue to
build the rest of the directories even when one fails. This can be
overcome by adding shell commands to note the error and exit, but then
it will do so even if make
is invoked with the -k
option,
which is unfortunate. Second, and perhaps more importantly, you cannot
take advantage of the parallel build capabilities of make using this
method, since there is only one rule.
By declaring the subdirectories as phony targets (you must do this as the subdirectory obviously always exists; otherwise it won't be built) you can remove these problems:
SUBDIRS = foo bar baz .PHONY: subdirs $(SUBDIRS) subdirs: $(SUBDIRS) $(SUBDIRS): $(MAKE) -C $ foo: baz |
Here we've also declared that the `foo' subdirectory cannot be built until after the `baz' subdirectory is complete; this kind of relationship declaration is particularly important when attempting parallel builds.
A phony target should not be a prerequisite of a real target file; if it
is, its commands are run every time make
goes to update that
file. As long as a phony target is never a prerequisite of a real
target, the phony target commands will be executed only when the phony
target is a specified goal (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').
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 |
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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 4.5 Phony Targets.
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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).
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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 10.6 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).
.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.
.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 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 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.
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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 `$@'. The rule contributes the same prerequisites to all the targets also.
This is useful in two cases.
kbd.o command.o files.o: command.h |
gives an additional prerequisite to each of the three object files mentioned.
bigoutput littleoutput : text.g generate text.g -$(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.
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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
.)
There is no reason to
write your makefiles this way; that is why make
gives you
an error message.
An extra rule with just prerequisites can be used to give a few extra
prerequisites 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 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).
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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.
4.11.1 Syntax of Static Pattern Rules The syntax of static pattern rules. 4.11.2 Static Pattern Rules versus Implicit Rules When are they better than implicit rules?
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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 Using Wildcard Characters in File Names).
The target-pattern and dep-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 dep-patterns to make the prerequisite 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 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'.
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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:
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.
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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. 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.
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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 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 prerequisites, 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 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; $(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 prerequisites 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 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 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 prerequisite
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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