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 command 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 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 subdirectory 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 Match-Anything Pattern Rules).

This pattern rule has two targets:

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

This tells make that the command ‘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 command ‘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.)