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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 dependency 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 dependency patterns must name files that exist or can be made.
These files become dependencies of the target.
Thus, a rule of the form
%.o : %.c ; COMMAND...
specifies how to make a file `N.o', with another file `N.c' as its
dependency, provided that `N.c' exists or can be made.
There may also be dependencies that do not use `%'; such a dependency
attaches to every file made by this pattern rule. These unvarying
dependencies are useful occasionally.
A pattern rule need not have any dependencies that contain `%', or
in fact any dependencies at all. Such a rule is effectively a general
wildcard. It provides a way to make any file that matches the target
pattern. Note: Last Resort.
Pattern rules may have more than one target. Unlike normal rules,
this does not act as many different rules with the same dependencies 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 dependencies
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 dependencies actually exist or are mentioned always takes
priority over a rule with dependencies that must be made by chaining
other implicit rules.
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