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5.2 Arguments to macros
Macros can have arguments. The nth argument is denoted by
$n in the expansion text, and is replaced by the nth actual
argument, when the macro is expanded. Replacement of arguments happens
before rescanning, regardless of how many nesting levels of quoting
appear in the expansion. Here is an example of a macro with
two arguments.
define(`exch', `$2, $1') ⇒ exch(`arg1', `arg2') ⇒arg2, arg1
This can be used, for example, if you like the arguments to
define to be reversed.
define(`exch', `$2, $1') ⇒ define(exch(``expansion text'', ``macro'')) ⇒ macro ⇒expansion text
See section On Quoting Arguments to macros, for an explanation of the double quotes.
(You should try and improve this example so that clients of exch
do not have to double quote; or see section Answers).
As a special case, the zeroth argument, $0, is always the name
of the macro being expanded.
define(`test', ``Macro name: $0'') ⇒ test ⇒Macro name: test
If you want quoted text to appear as part of the expansion text, remember that quotes can be nested in quoted strings. Thus, in
define(`foo', `This is macro `foo'.') ⇒ foo ⇒This is macro foo.
The ‘foo’ in the expansion text is not expanded, since it is a quoted string, and not a name.
GNU m4 allows the number following the ‘$’ to
consist of one or more digits, allowing macros to have any number of
arguments. The extension of accepting multiple digits is incompatible
with POSIX, and is different than traditional implementations
of m4, which only recognize one digit. Therefore, future
versions of GNU M4 will phase out this feature. To portably
access beyond the ninth argument, you can use the argn macro
documented later (see section Recursion in m4).
POSIX also states that ‘$’ followed immediately by
‘{’ in a macro definition is implementation-defined. This version
of M4 passes the literal characters ‘${’ through unchanged, but M4
2.0 will implement an optional feature similar to sh, where
‘${11}’ expands to the eleventh argument, to replace the current
recognition of ‘$11’. Meanwhile, if you want to guarantee that you
will get a literal ‘${’ in output when expanding a macro, even
when you upgrade to M4 2.0, you can use nested quoting to your
advantage:
define(`foo', `single quoted $`'{1} output')
⇒
define(`bar', ``double quoted $'`{2} output'')
⇒
foo(`a', `b')
⇒single quoted ${1} output
bar(`a', `b')
⇒double quoted ${2} output
To help you detect places in your M4 input files that might change in behavior due to the changed behavior of M4 2.0, you can use the ‘--warn-macro-sequence’ command-line option (see section Invoking m4) with the default regular expression. This will add a warning any time a macro definition includes ‘$’ followed by multiple digits, or by ‘{’. The warning is not enabled by default, because it triggers a number of warnings in Autoconf 2.61 (and Autoconf uses ‘-E’ to treat warnings as errors), and because it will still be possible to restore older behavior in M4 2.0.
$ m4 --warn-macro-sequence
define(`foo', `$001 ${1} $1')
error-->m4:stdin:1: Warning: definition of `foo' contains sequence `$001'
error-->m4:stdin:1: Warning: definition of `foo' contains sequence `${1}'
⇒
foo(`bar')
⇒bar ${1} bar
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