GNU make - 9. How to Run make

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# 9. How to Run `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 The simplest use is to recompile every file that is out of date.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might want to use a different compiler or different compiler options; you might want just to find out which files are out of date without changing them.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 9.3 Instead of Executing the Commands.

## 9.1 Arguments to Specify the Makefile

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 3. Writing Makefiles).

## 9.2 Arguments to Specify the Goals

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.

`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 4.12 Generating Dependencies 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 dependency of the default goal.

Another use of specifying a goal is to run the commands associated with a phony target (see section 4.4 Phony Targets) or empty target (see section 4.6 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 14.5 Standard Targets for Users, for a detailed list of all the standard target names which GNU software packages use.

``all'`
  Make all the top-level targets the makefile knows about.
``clean'`
  Delete all files that are normally created by running `make`.
``mostlyclean'`
  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.
``distclean'`
``realclean'`
``clobber'`
  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.
``install'`
  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'`
  Print listings of the source files that have changed.
``tar'`
  Create a tar file of the source files.
``shar'`
  Create a shell archive (shar file) of the source files.
``dist'`
  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.
``TAGS'`
  Update a tags table for this program.
``check'`
``test'`
  Perform self tests on the program this makefile builds.

## 9.3 Instead of Executing the Commands

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`.

``-n'`
``--just-print'`
``--dry-run'`
``--recon'`
  "No-op". The activity is to print what commands would be used to make the targets up to date, but not actually execute them.
``-t'`
``--touch'`
  "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.
``-q'`
``--question'`
  "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.
``-W file'`
``--what-if=file'`
``--assume-new=file'`
``--new-file=file'`
  "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 5.6.1 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 9.7 Summary of Options).

## 9.4 Avoiding Recompilation of Some Files

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:

1. Use the command ``make'` to recompile the source files that really need recompilation.
2. Make the changes in the header files.
3. 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 9.7 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:

1. 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.
2. Touch all the object files with ``make -t'`.

## 9.5 Overriding Variables

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 10.3 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 6.7 The `override` Directive).

## 9.6 Testing the Compilation of a Program

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 9.7 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.

## 9.7 Summary of Options

Here is a table of all the options `make` understands:

``-b'`
``-m'`
  These options are ignored for compatibility with other versions of `make`.
``-C dir'`
``--directory=dir'`
  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 5.6 Recursive Use of `make`).
``-d'`
``--debug'`
  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.
``-e'`
``--environment-overrides'`
  Give variables taken from the environment precedence over variables from makefiles. See section 6.9 Variables from the Environment.
``-f file'`
``--file=file'`
``--makefile=file'`
  Read the file named file as a makefile. See section 3. Writing Makefiles.
``-h'`
``--help'`
  Remind you of the options that `make` understands and then exit.
``-i'`
``--ignore-errors'`
  Ignore all errors in commands executed to remake files. See section 5.4 Errors in Commands.
``-I dir'`
``--include-dir=dir'`
  Specifies a directory dir to search for included makefiles. See section 3.3 Including Other Makefiles. If several ``-I'` options are used to specify several directories, the directories are searched in the order specified.
``-j [jobs]'`
``--jobs=[jobs]'`
  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 5.3 Parallel Execution, for more information on how commands are run. Note that this option is ignored on MS-DOS.
``-k'`
``--keep-going'`
  Continue as much as possible after an error. While the target that failed, and those that depend on it, cannot be remade, the other dependencies of these targets can be processed all the same. See section 9.6 Testing the Compilation of a Program.
``-l [load]'`
``--load-average[=load]'`
``--max-load[=load]'`
  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 5.3 Parallel Execution.
``-n'`
``--just-print'`
``--dry-run'`
``--recon'`
  Print the commands that would be executed, but do not execute them. See section 9.3 Instead of Executing the Commands.
``-o file'`
``--old-file=file'`
``--assume-old=file'`
  Do not remake the file file even if it is older than its dependencies, 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 9.4 Avoiding Recompilation of Some Files.
``-p'`
``--print-data-base'`
  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 -p -f /dev/null'`.
``-q'`
``--question'`
  "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 9.3 Instead of Executing the Commands.
``-r'`
``--no-builtin-rules'`
  Eliminate use of the built-in implicit rules (see section 10. Using Implicit Rules). You can still define your own by writing pattern rules (see section 10.5 Defining and Redefining Pattern Rules). The ``-r'` option also clears out the default list of suffixes for suffix rules (see section 10.7 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 10.3 Variables Used by Implicit Rules).
``-s'`
``--silent'`
``--quiet'`
  Silent operation; do not print the commands as they are executed. See section 5.1 Command Echoing.
``-S'`
``--no-keep-going'`
``--stop'`
  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 5.6 Recursive Use of `make`) or if you set ``-k'` in `MAKEFLAGS` in your environment.
``-t'`
``--touch'`
  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 9.3 Instead of Executing the Commands.
``-v'`
``--version'`
  Print the version of the `make` program plus a copyright, a list of authors, and a notice that there is no warranty; then exit.
``-w'`
``--print-directory'`
  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 5.6 Recursive Use of `make`. (In practice, you rarely need to specify this option since ``make'` does it for you; see section 5.6.4 The ``--print-directory'` Option.)
``--no-print-directory'`
  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 5.6.4 The ``--print-directory'` Option.
``-W file'`
``--what-if=file'`
``--new-file=file'`
``--assume-new=file'`
  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 9.3 Instead of Executing the Commands.
``--warn-undefined-variables'`
  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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