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Chapter 10: Using Implicit Rules 123

‘test’. The pattern rule prerequisites are turned into actual ﬁle names by substituting the

stem for the character ‘%’. Thus, if in the same example one of the prerequisites is written

as ‘%.c’, it expands to ‘test.c’.

When the target pattern does not contain a slash (and it usually does not), directory

names in the ﬁle names are removed from the ﬁle name before it is compared with the target

preﬁx and suﬃx. After the comparison of the ﬁle name to the target pattern, the directory

names, along with the slash that ends them, are added on to the prerequisite ﬁle names

generated from the pattern rule’s prerequisite patterns and the ﬁle name. The directories

are ignored only for the purpose of ﬁnding an implicit rule to use, not in the application

of that rule. Thus, ‘e%t’ matches the ﬁle name src/eat, with ‘src/a’ as the stem. When

prerequisites are turned into ﬁle names, the directories from the stem are added at the front,

while the rest of the stem is substituted for the ‘%’. The stem ‘src/a’ with a prerequisite

pattern ‘c%r’ gives the ﬁle name src/car.

A pattern rule can be used to build a given ﬁle only if there is a target pattern that

matches the ﬁle name, and all prerequisites in that rule either exist or can be built. The

rules you write take precedence over those that are built in. Note however, that a rule

whose prerequisites actually exist or are mentioned always takes priority over a rule with

prerequisites that must be made by chaining other implicit rules.

It is possible that more than one pattern rule will meet these criteria. In that case,

make will choose the rule with the shortest stem (that is, the pattern that matches most

speciﬁcally). If more than one pattern rule has the shortest stem, make will choose the ﬁrst

one found in the makeﬁle.

This algorithm results in more speciﬁc rules being preferred over more generic ones; for

example:

%.o: %.c

$(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@

%.o : %.f

$(COMPILE.F) $(OUTPUT_OPTION) $<

lib/%.o: lib/%.c

$(CC) -fPIC -c $(CFLAGS) $(CPPFLAGS) $< -o $@

Given these rules and asked to build bar.o where both bar.c and bar.f exist, make

will choose the ﬁrst rule and compile bar.c into bar.o. In the same situation where bar.c

does not exist, then make will choose the second rule and compile bar.f into bar.o.

If make is asked to build lib/bar.o and both lib/bar.c and lib/bar.f exist, then the

third rule will be chosen since the stem for this rule (‘bar’) is shorter than the stem for the

ﬁrst rule (‘lib/bar’). If lib/bar.c does not exist then the third rule is not eligible and

the second rule will be used, even though the stem is longer.

10.5.5 Match-Anything Pattern Rules

When a pattern rule’s target is just ‘%’, it matches any ﬁle name whatever. We call these

rules match-anything rules. They are very useful, but it can take a lot of time for make to

think about them, because it must consider every such rule for each ﬁle name listed either

as a target or as a prerequisite.

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