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16 GNU make

the information in the containing makeﬁle, make should look in another makeﬁle. See

Section 10.5 [Pattern Rules], page 118, for more information on pattern rules.

For example, if you have a makeﬁle called Makefile that says how to make the target

‘foo’ (and other targets), you can write a makeﬁle called GNUmakefile that contains:

foo:

frobnicate > foo

%: force

@$(MAKE) -f Makefile $@

force: ;

If you say ‘make foo’, make will ﬁnd GNUmakefile, read it, and see that to make foo,

it needs to run the recipe ‘frobnicate > foo’. If you say ‘make bar’, make will ﬁnd no

way to make bar in GNUmakefile, so it will use the recipe from the pattern rule: ‘make

-f Makefile bar’. If Makefile provides a rule for updating bar, make will apply the rule.

And likewise for any other target that GNUmakefile does not say how to make.

The way this works is that the pattern rule has a pattern of just ‘%’, so it matches any

target whatever. The rule speciﬁes a prerequisite force, to guarantee that the recipe will

be run even if the target ﬁle already exists. We give the force target an empty recipe to

prevent make from searching for an implicit rule to build it—otherwise it would apply the

same match-anything rule to force itself and create a prerequisite loop!

3.7 How make Reads a Makeﬁle

GNU make does its work in two distinct phases. During the ﬁrst phase it reads all the make-

ﬁles, included makeﬁles, etc. and internalizes all the variables and their values, implicit and

explicit rules, and constructs a dependency graph of all the targets and their prerequisites.

During the second phase, make uses these internal structures to determine what targets will

need to be rebuilt and to invoke the rules necessary to do so.

It’s important to understand this two-phase approach because it has a direct impact

on how variable and function expansion happens; this is often a source of some confusion

when writing makeﬁles. Here we will present a summary of the phases in which expansion

happens for diﬀerent constructs within the makeﬁle. We say that expansion is immediate if

it happens during the ﬁrst phase: in this case make will expand any variables or functions

in that section of a construct as the makeﬁle is parsed. We say that expansion is deferred if

expansion is not performed immediately. Expansion of a deferred construct is not performed

until either the construct appears later in an immediate context, or until the second phase.

You may not be familiar with some of these constructs yet. You can reference this section

as you become familiar with them, in later chapters.

Variable Assignment

Variable deﬁnitions are parsed as follows:

immediate = deferred

immediate ?= deferred

immediate := immediate

immediate ::= immediate

immediate += deferred or immediate

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