GNU Info
(make.info)Automatic
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 prerequisites 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 prerequisite. If the target got its
commands from an implicit rule, this will be the first
prerequisite added by the implicit rule (Note: Implicit Rules).
`$?'
The names of all the prerequisites that are newer than the target,
with spaces between them. For prerequisites which are archive
members, only the member named is used (Note: Archives).
`$^'
The names of all the prerequisites, with spaces between them. For
prerequisites which are archive members, only the member named 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 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 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 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 $?
Of the variables listed above, four have values that are single file
names, and three have values that are lists of file names. These seven
have variants that get just the file's directory name or just the file
name within the directory. The variant variables' names are formed by
appending `D' or `F', respectively. These variants are semi-obsolete
in GNU `make' since the functions `dir' and `notdir' can be used to get
a similar effect (Note: Functions for File Names.
). Note, however, that the `F' variants all omit the
trailing slash which always appears in the output of the `dir'
function. Here is a table of the variants:
`$(@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)'
`$(<F)'
The directory part and the file-within-directory part of the first
prerequisite.
`$(^D)'
`$(^F)'
Lists of the directory parts and the file-within-directory parts
of all prerequisites.
`$(?D)'
`$(?F)'
Lists of the directory parts and the file-within-directory parts of
all prerequisites that are newer than the target.
Note that we use a special stylistic convention when we talk about
these automatic variables; we write "the value of `$<'", 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 `$(<)' in place of
`$<'.
automatically generated by info2www version 1.2.2.9