4.2 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. It simply exchanges the order of the 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 Quoting Arguments, 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. see Answers)

GNU m4 allows the number following the `$' to consist of one or more digits, allowing macros to have any number of arguments. This is not so in UNIX implementations of m4, which only recognize one digit.

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.