10 Using Implicit Rules

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.

Thus, a pattern rule %.o : %.c says how to make any file stem.o from another file stem.c.

Note that expansion using % in pattern rules occurs after any variable or function expansions, which take place when the makefile is read. See How to Use Variables, and Functions for Transforming Text.

5.1 Introduction to Pattern Rules

A pattern rule contains the character % (exactly one of them) in the target; otherwise, it looks exactly like an ordinary rule. The target is a pattern for matching file names; the % matches any nonempty substring, while other characters match only themselves.

For example, %.c as a pattern matches any file name that ends in .c. s.%.c as a pattern matches any file name that starts with s., ends in .c and is at least five characters long. (There must be at least one character to match the %.) The substring that the % matches is called the stem.

% in a prerequisite of a pattern rule stands for the same stem that was matched by the % in the target. In order for the pattern rule to apply, its target pattern must match the file name under consideration and all of its prerequisites (after pattern substitution) must name files that exist or can be made. These files become prerequisites of the target.

Thus, a rule of the form

%.o : %.c ; recipe…

specifies how to make a file n.o, with another file n.c as its prerequisite, provided that n.c exists or can be made.

There may also be prerequisites that do not use %; such a prerequisite attaches to every file made by this pattern rule. These unvarying prerequisites are useful occasionally.

A pattern rule need not have any prerequisites that contain %, or in fact any prerequisites at all. Such a rule is effectively a general wildcard. It provides a way to make any file that matches the target pattern. See Last Resort.

More than one pattern rule may match a target. In this case make will choose the “best fit” rule. See How Patterns Match.

Pattern rules may have more than one target; however, every target must contain a % character. Pattern rules are always treated as grouped targets (see Multiple Targets in a Rule) regardless of whether they use the : or &: separator.

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 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 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.)