Suppose you are writing a pattern rule to compile a `.c' file into a `.o' file: how do you write the `cc' command so that it operates on the right source file name? You cannot write the name in the command, because the name is different each time the implicit rule is applied.
What you do is use a special feature of
make, the automatic
variables. These variables have values computed afresh for each rule that
is executed, based on the target and dependencies of the rule. In this
example, you would use `$@' for the object file name and `$<'
for the source file name.
Here is a table of automatic variables:
maketo Update Archive Files. For example, if the target is `foo.a(bar.o)' then `$%' is `bar.o' and `$@' is `foo.a'. `$%' is empty when the target is not an archive member.
maketo Update Archive Files).
maketo Update Archive Files). A target has only one dependency on each other file it depends on, no matter how many times each file is listed as a dependency. So if you list a dependency more than once for a target, the value of
$^contains just one copy of the name.
makedoes this bizarre thing only for compatibility with other implementations of
make. You should generally avoid using `$*' except in implicit rules or static pattern rules. If the target name in an explicit rule does not end with a recognized suffix, `$*' is set to the empty string for that rule.
`$?' is useful even in explicit rules when you wish to operate on only the dependencies that have changed. For example, suppose that an archive named `lib' is supposed to contain copies of several object files. This rule copies just the changed object files into the archive:
lib: foo.o bar.o lose.o win.o ar r lib $?
Of the variables listed above, four have values that are single file
names, and two have values that are lists of file names. These six have
variants that get just the file's directory name or just the file name
within the directory. The variant variables' names are formed by
appending `D' or `F', respectively. These variants are
semi-obsolete in GNU
make since the functions
notdir can be used to get a similar effect (see section Functions for File Names). Note, however, that the
`F' variants all omit the trailing slash which always appears in
the output of the
dir function. Here is a table of the variants:
Note that we use a special stylistic convention when we talk about these
automatic variables; we write "the value of `$<'", rather than
<" as we would write for ordinary variables
CFLAGS. We think this convention
looks more natural in this special case. Please do not assume it has a
deep significance; `$<' refers to the variable named
as `$(CFLAGS)' refers to the variable named
You could just as well use `$(<)' in place of `$<'.
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