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make
A makefile that says how to recompile a program can be used in more
than one way. The simplest use is to recompile every file that is out
of date. Usually, makefiles are written so that if you run
make
with no arguments, it does just that.
But you might want to update only some of the files; you might want to use a different compiler or different compiler options; you might want just to find out which files are out of date without changing them.
By giving arguments when you run make
, you can do any of these
things and many others.
The exit status of make
is always one of three values:
0
The exit status is zero if make
is successful.
2
The exit status is two if make
encounters any errors.
It will print messages describing the particular errors.
1
The exit status is one if you use the `-q' flag and make
determines that some target is not already up to date.
See section Instead of Executing the Commands.
9.1 Arguments to Specify the Makefile | How to specify which makefile to use. | |
9.2 Arguments to Specify the Goals | How to use goal arguments to specify which parts of the makefile to use. | |
9.3 Instead of Executing the Commands | How to use mode flags to specify what kind of thing to do with the commands in the makefile other than simply execute them. | |
9.4 Avoiding Recompilation of Some Files | How to avoid recompiling certain files. | |
9.5 Overriding Variables | How to override a variable to specify an alternate compiler and other things. | |
9.6 Testing the Compilation of a Program | How to proceed past some errors, to test compilation. | |
9.7 Summary of Options |
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The way to specify the name of the makefile is with the `-f' or `--file' option (`--makefile' also works). For example, `-f altmake' says to use the file `altmake' as the makefile.
If you use the `-f' flag several times and follow each `-f' with an argument, all the specified files are used jointly as makefiles.
If you do not use the `-f' or `--file' flag, the default is to try `GNUmakefile', `makefile', and `Makefile', in that order, and use the first of these three which exists or can be made (see section Writing Makefiles).
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The goals are the targets that make
should strive ultimately
to update. Other targets are updated as well if they appear as
prerequisites of goals, or prerequisites of prerequisites of goals, etc.
By default, the goal is the first target in the makefile (not counting
targets that start with a period). Therefore, makefiles are usually
written so that the first target is for compiling the entire program or
programs they describe. If the first rule in the makefile has several
targets, only the first target in the rule becomes the default goal, not
the whole list. You can manage the selection of the default goal from
within your makefile using the .DEFAULT_GOAL
variable
(see section Other Special Variables).
You can also specify a different goal or goals with command-line
arguments to make
. Use the name of the goal as an argument.
If you specify several goals, make
processes each of them in
turn, in the order you name them.
Any target in the makefile may be specified as a goal (unless it
starts with `-' or contains an `=', in which case it will be
parsed as a switch or variable definition, respectively). Even
targets not in the makefile may be specified, if make
can find
implicit rules that say how to make them.
Make
will set the special variable MAKECMDGOALS
to the
list of goals you specified on the command line. If no goals were given
on the command line, this variable is empty. Note that this variable
should be used only in special circumstances.
An example of appropriate use is to avoid including `.d' files
during clean
rules (see section Generating Prerequisites Automatically), so
make
won't create them only to immediately remove them
again:
sources = foo.c bar.c ifneq ($(MAKECMDGOALS),clean) include $(sources:.c=.d) endif |
One use of specifying a goal is if you want to compile only a part of the program, or only one of several programs. Specify as a goal each file that you wish to remake. For example, consider a directory containing several programs, with a makefile that starts like this:
.PHONY: all all: size nm ld ar as |
If you are working on the program size
, you might want to say
`make size' so that only the files of that program are recompiled.
Another use of specifying a goal is to make files that are not normally made. For example, there may be a file of debugging output, or a version of the program that is compiled specially for testing, which has a rule in the makefile but is not a prerequisite of the default goal.
Another use of specifying a goal is to run the commands associated with a phony target (see section Phony Targets) or empty target (see section Empty Target Files to Record Events). Many makefiles contain a phony target named `clean' which deletes everything except source files. Naturally, this is done only if you request it explicitly with `make clean'. Following is a list of typical phony and empty target names. See section Standard Targets for Users, for a detailed list of all the standard target names which GNU software packages use.
Make all the top-level targets the makefile knows about.
Delete all files that are normally created by running make
.
Like `clean', but may refrain from deleting a few files that people normally don't want to recompile. For example, the `mostlyclean' target for GCC does not delete `libgcc.a', because recompiling it is rarely necessary and takes a lot of time.
Any of these targets might be defined to delete more files than `clean' does. For example, this would delete configuration files or links that you would normally create as preparation for compilation, even if the makefile itself cannot create these files.
Copy the executable file into a directory that users typically search for commands; copy any auxiliary files that the executable uses into the directories where it will look for them.
Print listings of the source files that have changed.
Create a tar file of the source files.
Create a shell archive (shar file) of the source files.
Create a distribution file of the source files. This might be a tar file, or a shar file, or a compressed version of one of the above, or even more than one of the above.
Update a tags table for this program.
Perform self tests on the program this makefile builds.
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The makefile tells make
how to tell whether a target is up to date,
and how to update each target. But updating the targets is not always
what you want. Certain options specify other activities for make
.
"No-op". The activity is to print what commands would be used to make the targets up to date, but not actually execute them.
"Touch". The activity is to mark the targets as up to date without
actually changing them. In other words, make
pretends to compile
the targets but does not really change their contents.
"Question". The activity is to find out silently whether the targets are up to date already; but execute no commands in either case. In other words, neither compilation nor output will occur.
"What if". Each `-W' flag is followed by a file name. The given
files' modification times are recorded by make
as being the present
time, although the actual modification times remain the same.
You can use the `-W' flag in conjunction with the `-n' flag
to see what would happen if you were to modify specific files.
With the `-n' flag, make
prints the commands that it would
normally execute but does not execute them.
With the `-t' flag, make
ignores the commands in the rules
and uses (in effect) the command touch
for each target that needs to
be remade. The touch
command is also printed, unless `-s' or
.SILENT
is used. For speed, make
does not actually invoke
the program touch
. It does the work directly.
With the `-q' flag, make
prints nothing and executes no
commands, but the exit status code it returns is zero if and only if the
targets to be considered are already up to date. If the exit status is
one, then some updating needs to be done. If make
encounters an
error, the exit status is two, so you can distinguish an error from a
target that is not up to date.
It is an error to use more than one of these three flags in the same
invocation of make
.
The `-n', `-t', and `-q' options do not affect command
lines that begin with `+' characters or contain the strings
`$(MAKE)' or `${MAKE}'. Note that only the line containing
the `+' character or the strings `$(MAKE)' or `${MAKE}'
is run regardless of these options. Other lines in the same rule are
not run unless they too begin with `+' or contain `$(MAKE)' or
`${MAKE}' (See section How the MAKE
Variable Works.)
The `-W' flag provides two features:
If you also use the `-n' or `-q' flag, you can see what
make
would do if you were to modify some files.
Without the `-n' or `-q' flag, when make
is actually
executing commands, the `-W' flag can direct make
to act
as if some files had been modified, without actually modifying the
files.
Note that the options `-p' and `-v' allow you to obtain other
information about make
or about the makefiles in use
(see section Summary of Options).
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Sometimes you may have changed a source file but you do not want to
recompile all the files that depend on it. For example, suppose you add
a macro or a declaration to a header file that many other files depend
on. Being conservative, make
assumes that any change in the
header file requires recompilation of all dependent files, but you know
that they do not need to be recompiled and you would rather not waste
the time waiting for them to compile.
If you anticipate the problem before changing the header file, you can
use the `-t' flag. This flag tells make
not to run the
commands in the rules, but rather to mark the target up to date by
changing its last-modification date. You would follow this procedure:
Use the command `make' to recompile the source files that really need recompilation, ensuring that the object files are up-to-date before you begin.
Make the changes in the header files.
Use the command `make -t' to mark all the object files as
up to date. The next time you run make
, the changes in the
header files will not cause any recompilation.
If you have already changed the header file at a time when some files do need recompilation, it is too late to do this. Instead, you can use the `-o file' flag, which marks a specified file as "old" (see section Summary of Options). This means that the file itself will not be remade, and nothing else will be remade on its account. Follow this procedure:
Recompile the source files that need compilation for reasons independent of the particular header file, with `make -o headerfile'. If several header files are involved, use a separate `-o' option for each header file.
Touch all the object files with `make -t'.
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An argument that contains `=' specifies the value of a variable: `v=x' sets the value of the variable v to x. If you specify a value in this way, all ordinary assignments of the same variable in the makefile are ignored; we say they have been overridden by the command line argument.
The most common way to use this facility is to pass extra flags to
compilers. For example, in a properly written makefile, the variable
CFLAGS
is included in each command that runs the C compiler, so a
file `foo.c' would be compiled something like this:
cc -c $(CFLAGS) foo.c |
Thus, whatever value you set for CFLAGS
affects each compilation
that occurs. The makefile probably specifies the usual value for
CFLAGS
, like this:
CFLAGS=-g |
Each time you run make
, you can override this value if you
wish. For example, if you say `make CFLAGS='-g -O'', each C
compilation will be done with `cc -c -g -O'. (This also
illustrates how you can use quoting in the shell to enclose spaces and
other special characters in the value of a variable when you override
it.)
The variable CFLAGS
is only one of many standard variables that
exist just so that you can change them this way. See section Variables Used by Implicit Rules, for a complete list.
You can also program the makefile to look at additional variables of your own, giving the user the ability to control other aspects of how the makefile works by changing the variables.
When you override a variable with a command argument, you can define either a recursively-expanded variable or a simply-expanded variable. The examples shown above make a recursively-expanded variable; to make a simply-expanded variable, write `:=' instead of `='. But, unless you want to include a variable reference or function call in the value that you specify, it makes no difference which kind of variable you create.
There is one way that the makefile can change a variable that you have
overridden. This is to use the override
directive, which is a line
that looks like this: `override variable = value'
(see section The override
Directive).
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Normally, when an error happens in executing a shell command, make
gives up immediately, returning a nonzero status. No further commands are
executed for any target. The error implies that the goal cannot be
correctly remade, and make
reports this as soon as it knows.
When you are compiling a program that you have just changed, this is not
what you want. Instead, you would rather that make
try compiling
every file that can be tried, to show you as many compilation errors
as possible.
On these occasions, you should use the `-k' or
`--keep-going' flag. This tells make
to continue to
consider the other prerequisites of the pending targets, remaking them
if necessary, before it gives up and returns nonzero status. For
example, after an error in compiling one object file, `make -k'
will continue compiling other object files even though it already
knows that linking them will be impossible. In addition to continuing
after failed shell commands, `make -k' will continue as much as
possible after discovering that it does not know how to make a target
or prerequisite file. This will always cause an error message, but
without `-k', it is a fatal error (see section Summary of Options).
The usual behavior of make
assumes that your purpose is to get the
goals up to date; once make
learns that this is impossible, it might
as well report the failure immediately. The `-k' flag says that the
real purpose is to test as much as possible of the changes made in the
program, perhaps to find several independent problems so that you can
correct them all before the next attempt to compile. This is why Emacs'
M-x compile command passes the `-k' flag by default.
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Here is a table of all the options make
understands:
These options are ignored for compatibility with other versions of make
.
Consider all targets out-of-date. GNU make
proceeds to
consider targets and their prerequisites using the normal algorithms;
however, all targets so considered are always remade regardless of the
status of their prerequisites. To avoid infinite recursion, if
MAKE_RESTARTS
(see section Other Special Variables) is set to a number greater than 0 this option is disabled
when considering whether to remake makefiles (see section How Makefiles Are Remade).
Change to directory dir before reading the makefiles. If multiple
`-C' options are specified, each is interpreted relative to the
previous one: `-C / -C etc' is equivalent to `-C /etc'.
This is typically used with recursive invocations of make
(see section Recursive Use of make
).
Print debugging information in addition to normal processing. The
debugging information says which files are being considered for
remaking, which file-times are being compared and with what results,
which files actually need to be remade, which implicit rules are
considered and which are applied--everything interesting about how
make
decides what to do. The -d
option is equivalent to
`--debug=a' (see below).
Print debugging information in addition to normal processing. Various levels and types of output can be chosen. With no arguments, print the "basic" level of debugging. Possible arguments are below; only the first character is considered, and values must be comma- or space-separated.
a (all)
All types of debugging output are enabled. This is equivalent to using `-d'.
b (basic)
Basic debugging prints each target that was found to be out-of-date, and whether the build was successful or not.
v (verbose)
A level above `basic'; includes messages about which makefiles were parsed, prerequisites that did not need to be rebuilt, etc. This option also enables `basic' messages.
i (implicit)
Prints messages describing the implicit rule searches for each target. This option also enables `basic' messages.
j (jobs)
Prints messages giving details on the invocation of specific subcommands.
m (makefile)
By default, the above messages are not enabled while trying to remake the makefiles. This option enables messages while rebuilding makefiles, too. Note that the `all' option does enable this option. This option also enables `basic' messages.
Give variables taken from the environment precedence over variables from makefiles. See section Variables from the Environment.
Read the file named file as a makefile. See section Writing Makefiles.
Remind you of the options that make
understands and then exit.
Ignore all errors in commands executed to remake files. See section Errors in Commands.
Specifies a directory dir to search for included makefiles. See section Including Other Makefiles. If several `-I' options are used to specify several directories, the directories are searched in the order specified.
Specifies the number of jobs (commands) to run simultaneously. With no
argument, make
runs as many jobs simultaneously as possible. If
there is more than one `-j' option, the last one is effective.
See section Parallel Execution,
for more information on how commands are run.
Note that this option is ignored on MS-DOS.
Continue as much as possible after an error. While the target that failed, and those that depend on it, cannot be remade, the other prerequisites of these targets can be processed all the same. See section Testing the Compilation of a Program.
Specifies that no new jobs (commands) should be started if there are other jobs running and the load average is at least load (a floating-point number). With no argument, removes a previous load limit. See section Parallel Execution.
On systems that support symbolic links, this option causes make
to consider the timestamps on any symbolic links in addition to the
timestamp on the file referenced by those links. When this option is
provided, the most recent timestamp among the file and the symbolic
links is taken as the modification time for this target file.
Print the commands that would be executed, but do not execute them. See section Instead of Executing the Commands.
Do not remake the file file even if it is older than its prerequisites, and do not remake anything on account of changes in file. Essentially the file is treated as very old and its rules are ignored. See section Avoiding Recompilation of Some Files.
Print the data base (rules and variable values) that results from reading the makefiles; then execute as usual or as otherwise specified. This also prints the version information given by the `-v' switch (see below). To print the data base without trying to remake any files, use `make -qp'. To print the data base of predefined rules and variables, use `make -p -f /dev/null'. The data base output contains filename and linenumber information for command and variable definitions, so it can be a useful debugging tool in complex environments.
"Question mode". Do not run any commands, or print anything; just return an exit status that is zero if the specified targets are already up to date, one if any remaking is required, or two if an error is encountered. See section Instead of Executing the Commands.
Eliminate use of the built-in implicit rules (see section Using Implicit Rules). You can still define your own by writing
pattern rules (see section Defining and Redefining Pattern Rules). The `-r' option also clears out the default list of
suffixes for suffix rules (see section Old-Fashioned Suffix Rules). But you can still define your own suffixes with a rule for
.SUFFIXES
, and then define your own suffix rules. Note that only
rules are affected by the -r
option; default variables
remain in effect (see section Variables Used by Implicit Rules); see the `-R' option below.
Eliminate use of the built-in rule-specific variables (see section Variables Used by Implicit Rules). You can still define your own, of course. The `-R' option also automatically enables the `-r' option (see above), since it doesn't make sense to have implicit rules without any definitions for the variables that they use.
Silent operation; do not print the commands as they are executed. See section Command Echoing.
Cancel the effect of the `-k' option. This is never necessary
except in a recursive make
where `-k' might be inherited
from the top-level make
via MAKEFLAGS
(see section Recursive Use of make
)
or if you set `-k' in MAKEFLAGS
in your environment.
Touch files (mark them up to date without really changing them)
instead of running their commands. This is used to pretend that the
commands were done, in order to fool future invocations of
make
. See section Instead of Executing the Commands.
Print the version of the make
program plus a copyright, a list
of authors, and a notice that there is no warranty; then exit.
Print a message containing the working directory both before and after
executing the makefile. This may be useful for tracking down errors
from complicated nests of recursive make
commands.
See section Recursive Use of make
. (In practice, you
rarely need to specify this option since `make' does it for you;
see The `--print-directory' Option.)
Disable printing of the working directory under -w
.
This option is useful when -w
is turned on automatically,
but you do not want to see the extra messages.
See section The `--print-directory' Option.
Pretend that the target file has just been modified. When used
with the `-n' flag, this shows you what would happen if you were
to modify that file. Without `-n', it is almost the same as
running a touch
command on the given file before running
make
, except that the modification time is changed only in the
imagination of make
.
See section Instead of Executing the Commands.
Issue a warning message whenever make
sees a reference to an
undefined variable. This can be helpful when you are trying to debug
makefiles which use variables in complex ways.
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