5.3 Special arguments to macros

There is a special notation for the number of actual arguments supplied, and for all the actual arguments.

The number of actual arguments in a macro call is denoted by $# in the expansion text. Thus, a macro to display the number of arguments given can be

     define(`nargs', `$#')
     =>
     nargs
     =>0
     nargs()
     =>1
     nargs(`arg1', `arg2', `arg3')
     =>3
     nargs(`commas can be quoted, like this')
     =>1
     nargs(arg1#inside comments, commas do not separate arguments
     still arg1)
     =>1
     nargs((unquoted parentheses, like this, group arguments))
     =>1

The notation $* can be used in the expansion text to denote all the actual arguments, unquoted, with commas in between. For example

     define(`echo', `$*')
     =>
     echo(arg1,    arg2, arg3 , arg4)
     =>arg1,arg2,arg3 ,arg4

Often each argument should be quoted, and the notation $@ handles that. It is just like $*, except that it quotes each argument. A simple example of that is:

     define(`echo', `$@')
     =>
     echo(arg1,    arg2, arg3 , arg4)
     =>arg1,arg2,arg3 ,arg4

Where did the quotes go? Of course, they were eaten, when the expanded text were reread by m4. To show the difference, try

     define(`echo1', `$*')
     =>
     define(`echo2', `$@')
     =>
     define(`foo', `This is macro `foo'.')
     =>
     echo1(foo)
     =>This is macro This is macro foo..
     echo1(`foo')
     =>This is macro foo.
     echo2(foo)
     =>This is macro foo.
     echo2(`foo')
     =>foo

See Trace, if you do not understand this. As another example of the difference, remember that comments encountered in arguments are passed untouched to the macro, and that quoting disables comments.

     define(`echo1', `$*')
     =>
     define(`echo2', `$@')
     =>
     define(`foo', `bar')
     =>
     echo1(#foo'foo
     foo)
     =>#foo'foo
     =>bar
     echo2(#foo'foo
     foo)
     =>#foobar
     =>bar'

A `$' sign in the expansion text, that is not followed by anything m4 understands, is simply copied to the macro expansion, as any other text is.

     define(`foo', `$$$ hello $$$')
     =>
     foo
     =>$$$ hello $$$

If you want a macro to expand to something like `$12', the judicious use of nested quoting can put a safe character between the $ and the next character, relying on the rescanning to remove the nested quote. This will prevent m4 from interpreting the $ sign as a reference to an argument.

     define(`foo', `no nested quote: $1')
     =>
     foo(`arg')
     =>no nested quote: arg
     define(`foo', `nested quote around $: `$'1')
     =>
     foo(`arg')
     =>nested quote around $: $1
     define(`foo', `nested empty quote after $: $`'1')
     =>
     foo(`arg')
     =>nested empty quote after $: $1
     define(`foo', `nested quote around next character: $`1'')
     =>
     foo(`arg')
     =>nested quote around next character: $1
     define(`foo', `nested quote around both: `$1'')
     =>
     foo(`arg')
     =>nested quote around both: arg