Macros with a Variable Number of Arguments

5.14 Macros with a Variable Number of Arguments.

In the ISO C standard of 1999, a macro can be declared to accept a variable number of

arguments much as a function can. The syntax for defining the macro is similar to that of

a function. Here is an example:

#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)

Here ‘...’ is a variable argument. In the invocation of such a macro, it represents the

zero or more tokens until the closing parenthesis that ends the invocation, including any

commas. This set of tokens replaces the identifier __VA_ARGS__ in the macro body wherever

it appears. See the CPP manual for more information.

GCC has long supported variadic macros, and used a different syntax that allowed you

to give a name to the variable arguments just like any other argument. Here is an example:

#define debug(format, args...) fprintf (stderr, format, args)

This is in all ways equivalent to the ISO C example above, but arguably more readable

and descriptive.

GNU CPP has two further variadic macro extensions, and permits them to be used with

either of the above forms of macro definition.

In standard C, you are not allowed to leave the variable argument out entirely; but you

are allowed to pass an empty argument. For example, this invocation is invalid in ISO C,

because there is no comma after the string:

debug ("A message")

GNU CPP permits you to completely omit the variable arguments in this way. In the

above examples, the compiler would complain, though since the expansion of the macro still

has the extra comma after the format string.

To help solve this problem, CPP behaves specially for variable arguments used with the

token paste operator, ‘##’. If instead you write

#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)

and if the variable arguments are omitted or empty, the ‘##’ operator causes the preprocessor

to remove the comma before it. If you do provide some variable arguments in

your macro invocation, GNU CPP does not complain about the paste operation and instead

places the variable arguments after the comma. Just like any other pasted macro argument,

these arguments are not macro expanded.