File: make.info,  Node: Automatic Variables,  Next: Pattern Match,  Prev: Pattern Examples,  Up: Pattern Rules

10.5.3 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 recipe,
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 prerequisites of the rule.  In
this example, you would use '$@' for the object file name and '$<' for
the source file name.

   It's very important that you recognize the limited scope in which
automatic variable values are available: they only have values within
the recipe.  In particular, you cannot use them anywhere within the
target list of a rule; they have no value there and will expand to the
empty string.  Also, they cannot be accessed directly within the
prerequisite list of a rule.  A common mistake is attempting to use '$@'
within the prerequisites list; this will not work.  However, there is a
special feature of GNU 'make', secondary expansion (*note Secondary
Expansion::), which will allow automatic variable values to be used in
prerequisite lists.

   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: Pattern Intro.), '$@' is the name of whichever
     target caused the rule's recipe 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 prerequisite.  If the target got its recipe
     from an implicit rule, this will be the first prerequisite added by
     the implicit rule (*note Implicit Rules::).

'$?'
     The names of all the prerequisites that are newer than the target,
     with spaces between them.  If the target does not exist, all
     prerequisites will be included.  For prerequisites which are
     archive members, only the named member is used (*note Archives::).

     '$?' is useful even in explicit rules when you wish to operate on
     only the prerequisites 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 $?

'$^'
     The names of all the prerequisites, with spaces between them.  For
     prerequisites which are archive members, only the named member is
     used (*note Archives::).  A target has only one prerequisite on
     each other file it depends on, no matter how many times each file
     is listed as a prerequisite.  So if you list a prerequisite more
     than once for a target, the value of '$^' contains just one copy of
     the name.  This list does *not* contain any of the order-only
     prerequisites; for those see the '$|' variable, below.

'$+'
     This is like '$^', but prerequisites listed more than once are
     duplicated in the order they were listed in the makefile.  This is
     primarily useful for use in linking commands where it is meaningful
     to repeat library file names in a particular order.

'$|'
     The names of all the order-only prerequisites, with spaces between
     them.

'$*'
     The stem with which an implicit rule matches (*note How Patterns
     Match: Pattern 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 '$*' cannot be determined
     in that way.  Instead, if the target name ends with a recognized
     suffix (*note Old-Fashioned Suffix Rules: Suffix Rules.), '$*' 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.

   Of the variables listed above, four have values that are single file
names, and three have values that are lists of file names.  These seven
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.  The functions 'dir' and 'notdir'
can be used to obtain a similar effect (*note Functions for File Names:
File Name Functions.).  Note, however, that the 'D' 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: Archive Members.)

'$(