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 The simplest use is to recompile every file that is out
of date.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might want to use a different compiler or different compiler options; you might want just to find out which files are out of date without changing them.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
make is successful.
2
make encounters any errors.
It will print messages describing the particular errors.
1
make
determines that some target is not already up to date.
See section Instead of Executing the Commands.
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).
The goals are the targets that make should strive ultimately
to update. Other targets are updated as well if they appear as
dependencies of goals, or dependencies of dependencies 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 specify a different goal or goals with 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.
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 dependency 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.
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.
make pretends to compile
the targets but does not really change their contents.
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:
make would do if you were to modify some files.
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).
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:
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:
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 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).
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 dependencies 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 dependency 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.
Here is a table of all the options make understands:
make.
make
(see section Recursive Use of make).
make decides what to do.
make understands and then exit.
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.
.SUFFIXES, and then define your own suffix rules.
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.
make. See section Instead of Executing the Commands.
make program plus a copyright, a list
of authors, and a notice that there is no warranty; then exit.
make commands.
See section Recursive Use of make. (In practice, you
rarely need to specify this option since `make' does it for you;
see section The `--print-directory' Option.)
-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.
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.
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.