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10.5.2 Pattern Rule Examples

Here are some examples of pattern rules actually predefined in make. First, the rule that compiles ‘.c’ files into ‘.o’ files:

%.o : %.c
        $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@

defines a rule that can make any file ‘x.o’ from ‘x.c’. The recipe uses the automatic variables ‘$@’ and ‘$<’ to substitute the names of the target file and the source file in each case where the rule applies (see section Automatic Variables).

Here is a second built-in rule:

% :: RCS/%,v
        $(CO) $(COFLAGS) $<

defines a rule that can make any file ‘x’ whatsoever from a corresponding file ‘x,v’ in the sub-directory ‘RCS’. Since the target is ‘%’, this rule will apply to any file whatever, provided the appropriate prerequisite file exists. The double colon makes the rule terminal, which means that its prerequisite may not be an intermediate file (see section Match-Anything Pattern Rules).

This pattern rule has two targets:

%.tab.c %.tab.h: %.y
        bison -d $<

This tells make that the recipe ‘bison -d x.y’ will make both ‘x.tab.c’ and ‘x.tab.h’. If the file ‘foo’ depends on the files ‘parse.tab.o’ and ‘scan.o’ and the file ‘scan.o’ depends on the file ‘parse.tab.h’, when ‘parse.y’ is changed, the recipe ‘bison -d parse.y’ will be executed only once, and the prerequisites of both ‘parse.tab.o’ and ‘scan.o’ will be satisfied. (Presumably the file ‘parse.tab.o’ will be recompiled from ‘parse.tab.c’ and the file ‘scan.o’ from ‘scan.c’, while ‘foo’ is linked from ‘parse.tab.o’, ‘scan.o’, and its other prerequisites, and it will execute happily ever after.)


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