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Chapter 3: Writing Makeﬁles 15

you can write an explicit rule with the makeﬁle as the target, and an empty recipe (see

Section 5.9 [Using Empty Recipes], page 57).

If the makeﬁles specify a double-colon rule to remake a ﬁle with a recipe but no prereq-

uisites, that ﬁle will always be remade (see Section 4.12 [Double-Colon], page 38). 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 never do anything else. So, to avoid this, make will not attempt to re-

make makeﬁles which are speciﬁed as targets of a double-colon rule with a recipe but no

prerequisites.

If you do not specify any makeﬁles to be read with ‘-f’ or ‘--file’ options, make will try

the default makeﬁle names; see Section 3.2 [What Name to Give Your Makeﬁle], page 12.

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 in

the same order in which they are searched for (see Section 3.2 [What Name to Give Your

Makeﬁle], page 12) 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 Section 9.3 [Instead of Executing Recipes],

page 101), you would not want to use an out-of-date makeﬁle to decide which targets to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prevent updating of makeﬁles, because an out-of-date makeﬁle would result in the wrong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.

3.6 Overriding Part of Another Makeﬁle

Sometimes it is useful to have a makeﬁle that is mostly just like another makeﬁle. You

can often use the ‘include’ directive to include one in the other, and add more targets or

variable deﬁnitions. However, it is invalid for two makeﬁles to give diﬀerent recipes for the

same target. But there is another way.

In the containing makeﬁle (the one that wants to include the other), you can use a

match-anything pattern rule to say that to remake any target that cannot be made from

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