10.5.1 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 its prerequisite patterns must name files that
exist or can be made. These files become prerequisites of the target.
Thus, a rule of the form
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 section 10.6 Defining Last-Resort Default Rules.
Pattern rules may have more than one target. Unlike normal rules, this
does not act as many different rules with the same prerequisites and
commands. If a pattern rule has multiple targets,
make knows that
the rule's commands are responsible for making all of the targets. The
commands are executed only once to make all the targets. When searching
for a pattern rule to match a target, the target patterns of a rule other
than the one that matches the target in need of a rule are incidental:
make worries only about giving commands and prerequisites to the file
presently in question. However, when this file's commands are run, the
other targets are marked as having been updated themselves.
The order in which pattern rules appear in the makefile is important
since this is the order in which they are considered.
Of equally applicable
rules, only the first one found is used. 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.