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18 GNU make

That is, the target and prerequisite sections are expanded immediately, and the recipe

used to construct the target is always deferred. This general rule is true for explicit rules,

pattern rules, suﬃx rules, static pattern rules, and simple prerequisite deﬁnitions.

3.8 Secondary Expansion

In the previous section we learned that GNU make works in two distinct phases: a read-in

phase and a target-update phase (see Section 3.7 [How make Reads a Makeﬁle], page 16).

GNU make also has the ability to enable a second expansion of the prerequisites (only) for

some or all targets deﬁned in the makeﬁle. In order for this second expansion to occur,

the special target .SECONDEXPANSION must be deﬁned before the ﬁrst prerequisite list that

makes use of this feature.

If that special target is deﬁned then in between the two phases mentioned above, right

at the end of the read-in phase, all the prerequisites of the targets deﬁned after the special

target are expanded a second time. In most circumstances this secondary expansion will

have no eﬀect, since all variable and function references will have been expanded during the

initial parsing of the makeﬁles. In order to take advantage of the secondary expansion phase

of the parser, then, it’s necessary to escape the variable or function reference in the makeﬁle.

In this case the ﬁrst expansion merely un-escapes the reference but doesn’t expand it, and

expansion is left to the secondary expansion phase. For example, consider this makeﬁle:

.SECONDEXPANSION:

ONEVAR = onefile

TWOVAR = twofile

myfile: $(ONEVAR) $$(TWOVAR)

After the ﬁrst expansion phase the prerequisites list of the myfile target will be onefile

and $(TWOVAR); the ﬁrst (unescaped) variable reference to ONEVAR is expanded, while

the second (escaped) variable reference is simply unescaped, without being recognized as a

variable reference. Now during the secondary expansion the ﬁrst word is expanded again

but since it contains no variable or function references it remains the value onefile, while

the second word is now a normal reference to the variable TWOVAR, which is expanded

to the value twofile. The ﬁnal result is that there are two prerequisites, onefile and

twofile.

Obviously, this is not a very interesting case since the same result could more easily have

been achieved simply by having both variables appear, unescaped, in the prerequisites list.

One diﬀerence becomes apparent if the variables are reset; consider this example:

.SECONDEXPANSION:

AVAR = top

onefile: $(AVAR)

twofile: $$(AVAR)

AVAR = bottom

Here the prerequisite of onefile will be expanded immediately, and resolve to the value

top, while the prerequisite of twofile will not be full expanded until the secondary expan-

sion and yield a value of bottom.

This is marginally more exciting, but the true power of this feature only becomes ap-

parent when you discover that secondary expansions always take place within the scope of

the automatic variables for that target. This means that you can use variables such as $@,

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