Variadic macro in the C preprocessor

A variadic macro is a feature of the C preprocessor whereby a macro may be declared to accept a varying number of arguments.

Variable-argument macros were introduced in the ISO/IEC 9899:1999 revision of the C Programming Language standard in 1999.

Declaration Syntax

The declaration syntax is similar to that of variadic functions: an ellipsis "..." is used to indicate that one or more (possibly empty) arguments must be passed. During macro expansion each occurrence of the special identifier __VA_ARGS__ in the macro replacement list is replaced by the passed arguments.

No means is provided to access individual arguments in the variable argument list, nor to find out how many were passed.

Support

The GNU Compiler Collection and Visual Studio 2005 support variable-argument macros. Previous versions of Visual Studio lack this feature.

Example

If a printf-like function dprintf() were desired, which would take the file and line number from which it was called as arguments, the following macro might be used:

void realdprintf (char const *file, int line, char const *fmt, ...); 
#define dprintf(...) realdprintf(__FILE__, __LINE__, __VA_ARGS__);

dprintf() could then be called as:

dprintf("Hello, world");

/* Expands to: 
 *     realdprintf(__FILE__, __LINE__, "Hello, world"); 
 */

or:

dprintf("%d + %d = %d", 2, 2, 5);

/* Expands to: 
 *     realdprintf(__FILE__, __LINE__, "%d + %d = %d", 2, 2, 5);
 */

See also

Variadic function