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118 GNU make

Once make has decided to use the intermediate ﬁle, it is entered in the data base as if it had

been mentioned in the makeﬁle, along with the implicit rule that says how to create it.

Intermediate ﬁles are remade using their rules just like all other ﬁles. But intermediate

ﬁles are treated diﬀerently in two ways.

The ﬁrst diﬀerence is what happens if the intermediate ﬁle does not exist. If an ordinary

ﬁle b does not exist, and make considers a target that depends on b, it invariably creates b

and then updates the target from b. But if b is an intermediate ﬁle, then make can leave well

enough alone. It won’t bother updating b, or the ultimate target, unless some prerequisite

of b is newer than that target or there is some other reason to update that target.

The second diﬀerence is that if make does create b in order to update something else,

it deletes b later on after it is no longer needed. Therefore, an intermediate ﬁle which did

not exist before make also does not exist after make. make reports the deletion to you by

printing a ‘rm -f’ command showing which ﬁle it is deleting.

Ordinarily, a ﬁle cannot be intermediate if it is mentioned in the makeﬁle as a target

or prerequisite. However, you can explicitly mark a ﬁle as intermediate by listing it as

a prerequisite of the special target .INTERMEDIATE. This takes eﬀect even if the ﬁle is

mentioned explicitly in some other way.

You can prevent automatic deletion of an intermediate ﬁle by marking it as a secondary

ﬁle. To do this, list it as a prerequisite of the special target .SECONDARY. When a ﬁle

is secondary, make will not create the ﬁle merely because it does not already exist, but

make does not automatically delete the ﬁle. Marking a ﬁle as secondary also marks it as

intermediate.

You can list the target pattern of an implicit rule (such as ‘%.o’) as a prerequisite of the

special target .PRECIOUS to preserve intermediate ﬁles made by implicit rules whose target

patterns match that ﬁle’s name; see Section 5.6 [Interrupts], page 50.

A chain can involve more than two implicit rules. For example, it is possible to make a

ﬁle foo from RCS/foo.y,v by running RCS, Yacc and cc. Then both foo.y and foo.c are

intermediate ﬁles that are deleted at the end.

No single implicit rule can appear more than once in a chain. This means that make

will not even consider such a ridiculous thing as making foo from foo.o.o by running the

linker twice. This constraint has the added beneﬁt of preventing any inﬁnite loop in the

search for an implicit rule chain.

There are some special implicit rules to optimize certain cases that would otherwise

be handled by rule chains. For example, making foo from foo.c could be handled by

compiling and linking with separate chained rules, using foo.o as an intermediate ﬁle. But

what actually happens is that a special rule for this case does the compilation and linking

with a single cc command. The optimized rule is used in preference to the step-by-step

chain because it comes earlier in the ordering of rules.

10.5 Deﬁning and Redeﬁning Pattern Rules

You deﬁne an implicit rule by writing a pattern rule. A pattern rule looks like an ordinary

rule, except that its target contains the character ‘%’ (exactly one of them). The target is

considered a pattern for matching ﬁle names; the ‘%’ can match any nonempty substring,

while other characters match only themselves. The prerequisites likewise use ‘%’ to show

how their names relate to the target name.

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