3.5 How Makefiles Are Remade

Sometimes makefiles can be remade from other files, such as RCS or SCCS files. If a makefile can be remade from other files, you probably want make to get an up-to-date version of the makefile to read in.

To this end, after reading in all makefiles make will consider each as a goal target, in the order in which they were processed, and attempt to update it. If parallel builds (see Parallel Execution) are enabled then makefiles will be rebuilt in parallel as well.

If a makefile has a rule which says how to update it (found either in that very makefile or in another one) or if an implicit rule applies to it (see Using Implicit Rules), it will be updated if necessary. After all makefiles have been checked, if any have actually been changed, make starts with a clean slate and reads all the makefiles over again. (It will also attempt to update each of them over again, but normally this will not change them again, since they are already up to date.) Each restart will cause the special variable MAKE_RESTARTS to be updated (see Other Special Variables).

If you know that one or more of your makefiles cannot be remade and you want to keep make from performing an implicit rule search on them, perhaps for efficiency reasons, you can use any normal method of preventing implicit rule look-up to do so. For example, you can write an explicit rule with the makefile as the target, and an empty recipe (see Using Empty Recipes).

If the makefiles specify a double-colon rule to remake a file with a recipe but no prerequisites, that file will always be remade (see Double-Colon Rules). In the case of makefiles, a makefile that has a double-colon rule with a recipe but no prerequisites will be remade every time make is run, and then again after make starts over and reads the makefiles in again. This would cause an infinite loop: make would constantly remake the makefile and restart, and never do anything else. So, to avoid this, make will not attempt to remake makefiles which are specified as targets of a double-colon rule with a recipe but no prerequisites.

Phony targets (see Phony Targets) have the same effect: they are never considered up-to-date and so an included file marked as phony would cause make to restart continuously. To avoid this make will not attempt to remake makefiles which are marked phony.

You can take advantage of this to optimize startup time: if you know you don’t need your Makefile to be remade you can prevent make from trying to remake it by adding either:

.PHONY: Makefile

or:

Makefile:: ;

If you do not specify any makefiles to be read with ‘-f’ or ‘--file’ options, make will try the default makefile names; see What Name to Give Your Makefile. Unlike makefiles explicitly requested with ‘-f’ or ‘--file’ options, make is not certain that these makefiles should exist. However, if a default makefile does not exist but can be created by running make rules, you probably want the rules to be run so that the makefile can be used.

Therefore, if none of the default makefiles exists, make will try to make each of them until it succeeds in making one, or it runs out of names to try. Note that it is not an error if make cannot find or make any makefile; a makefile is not always necessary.

When you use the ‘-t’ or ‘--touch’ option (see Instead of Executing Recipes), you would not want to use an out-of-date makefile to decide which targets to touch. So the ‘-t’ option has no effect on updating makefiles; they are really updated even if ‘-t’ is specified. Likewise, ‘-q’ (or ‘--question’) and ‘-n’ (or ‘--just-print’) do not prevent updating of makefiles, because an out-of-date makefile would result in the wrong output for other targets. Thus, ‘make -f mfile -n foo’ will update mfile, read it in, and then print the recipe to update foo and its prerequisites without running it. The recipe printed for foo will be the one specified in the updated contents of mfile.

However, on occasion you might actually wish to prevent updating of even the makefiles. You can do this by specifying the makefiles as goals in the command line as well as specifying them as makefiles. When the makefile name is specified explicitly as a goal, the options ‘-t’ and so on do apply to them.

Thus, ‘make -f mfile -n mfile foo’ would read the makefile mfile, print the recipe needed to update it without actually running it, and then print the recipe needed to update foo without running that. The recipe for foo will be the one specified by the existing contents of mfile.