(make.info)Automatic


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Automatic Variables
-------------------

   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:

`$@'
     The file name of the target of the rule.  If the target is an
     archive member, then `$@' is the name of the archive file.  In a
     pattern rule that has multiple targets (Note: Introduction to
     Pattern Rules.), `$@' is the name of whichever
     target caused the rule's commands to be run.

`$%'
     The target member name, when the target is an archive member.
     Note: Archives.  For example, if the target is `foo.a(bar.o)'
     then `$%' is `bar.o' and `$@' is `foo.a'.  `$%' is empty when the
     target is not an archive member.

`$<'
     The name of the first dependency.  If the target got its commands
     from an implicit rule, this will be the first dependency added by
     the implicit rule (Note: Implicit Rules.).

`$?'
     The names of all the dependencies that are newer than the target,
     with spaces between them.  For dependencies which are archive
     members, only the member named is used (Note: Archives.).

`$^'
     The names of all the dependencies, with spaces between them.  For
     dependencies which are archive members, only the member named is
     used (Note: Archives.).  A target has only one dependency on
     each other file it depends on, no matter how many times each file
     is listed as a dependency.  So if you list a dependency more than
     once for a target, the value of `$^' contains just one copy of the
     name.

`$*'
     The stem with which an implicit rule matches (Note: How Patterns
     Match.).  If the target is `dir/a.foo.b' and the
     target pattern is `a.%.b' then the stem is `dir/foo'.  The stem is
     useful for constructing names of related files.

     In a static pattern rule, the stem is part of the file name that
     matched the `%' in the target pattern.

     In an explicit rule, there is no stem; so `$*' cannot be determined
     in that way.  Instead, if the target name ends with a recognized
     suffix (Note: Old-Fashioned Suffix Rules.), `$*' is
     set to the target name minus the suffix.  For example, if the
     target name is `foo.c', then `$*' is set to `foo', since `.c' is a
     suffix.  GNU `make' does this bizarre thing only for compatibility
     with other implementations of `make'.  You should generally avoid
     using `$*' 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 `dir' and `notdir' can be used to get
a similar effect (Note: 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:

`$(@D)'
     The directory part of the file name of the target, with the
     trailing slash removed.  If the value of `$@' is `dir/foo.o' then
     `$(@D)' is `dir'.  This value is `.' if `$@' does not contain a
     slash.

`$(@F)'
     The file-within-directory part of the file name of the target.  If
     the value of `$@' is `dir/foo.o' then `$(@F)' is `foo.o'.  `$(@F)'
     is equivalent to `$(notdir $@)'.

`$(*D)'
`$(*F)'
     The directory part and the file-within-directory part of the stem;
     `dir' and `foo' in this example.

`$(%D)'
`$(%F)'
     The directory part and the file-within-directory part of the target
     archive member name.  This makes sense only for archive member
     targets of the form `ARCHIVE(MEMBER)' and is useful only when
     MEMBER may contain a directory name.  (Note: Archive Members as
     Targets.)

`$(<D)'
`$(<F)'
     The directory part and the file-within-directory part of the first
     dependency.

`$(^D)'
`$(^F)'
     Lists of the directory parts and the file-within-directory parts
     of all dependencies.

`$(?D)'
`$(?F)'
     Lists of the directory parts and the file-within-directory parts of
     all dependencies that are newer than the target.

   Note that we use a special stylistic convention when we talk about
these automatic variables; we write "the value of `$<'", rather than
"the variable `<'" as we would write for ordinary variables such as
`objects' and `CFLAGS'.  We think this convention looks more natural in
this special case.  Please do not assume it has a deep significance;
`$<' refers to the variable named `<' just as `$(CFLAGS)' refers to the
variable named `CFLAGS'.  You could just as well use `$(<)' in place of
`$<'.


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