STDARG(3) Library Functions Manual STDARG(3)

stdarg — variable argument lists

#include <stdarg.h>

void
va_start(va_list ap, last);

type
va_arg(va_list ap, type);

void
va_copy(va_list dest, va_list src);

void
va_end(va_list ap);

A function may be called with a varying number of arguments of varying types. The include file <stdarg.h> declares a type (va_list) and defines three macros for stepping through a list of arguments whose number and types are not known to the called function.

The called function must declare an object of type va_list which is used by the macros (), va_arg(), va_copy(), and va_end().

The () macro must be called first, and it initializes ap, which can be passed to va_arg() for each argument to be processed. Calling va_end() signals that there are no further arguments, and causes ap to be invalidated. Note that each call to va_start() must be matched by a call to va_end(), from within the same function.

The parameter last is the name of the last parameter before the variable argument list, i.e., the last parameter of which the calling function knows the type.

Because the address of this parameter is used in the () macro, it should not be declared as a register variable, or as a function or an array type.

The () macro expands to an expression that has the type and value of the next argument in the call. The parameter ap is the va_list ap initialized by va_start(). Each call to va_arg() modifies ap so that the next call returns the next argument. The parameter type is a type name specified so that the type of a pointer to an object that has the specified type can be obtained simply by adding a * to type.

If there is no next argument, or if type is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), random errors will occur.

The first use of the () macro after that of the va_start() macro returns the argument after last. Successive invocations return the values of the remaining arguments.

The () macro copies the state of the variable argument list, src, previously initialized by va_start(), to the variable argument list, dest, which must not have been previously initialized by va_start(), without an intervening call to va_end(). The state preserved in dest is equivalent to calling va_start() and va_arg() on dest in the same way as was used on src. The copied variable argument list can subsequently be passed to va_arg(), and must finally be passed to va_end() when through with it.

After a variable argument list is invalidated by (), it can be reinitialized with va_start() or made a copy of another variable argument list with va_copy().

The function takes a string of format characters and prints out the argument associated with each format character based on the type.

void foo(char *fmt, ...)
{
	va_list ap, ap2;
	int d;
	char c, *s;

	va_start(ap, fmt);
	va_copy(ap2, ap);
	while (*fmt)
		switch(*fmt++) {
		case 's':			/* string */
			s = va_arg(ap, char *);
			printf("string %s\n", s);
			break;
		case 'd':			/* int */
			d = va_arg(ap, int);
			printf("int %d\n", d);
			break;
		case 'c':			/* char */
			/* Note: char is promoted to int. */
			c = va_arg(ap, int);
			printf("char %c\n", c);
			break;
		}
	va_end(ap);
	...
	/* use ap2 to iterate over the arguments again */
	...
	va_end(ap2);
}

These macros are compatible with the historic macros they replace. A backward compatible version can be found in the include file <varargs.h>.

The va_start(), va_arg(), va_copy(), and va_end() macros conform to ISO/IEC 9899:1999 (“ISO C99”).

Unlike the varargs macros, the stdarg macros do not permit programmers to code a function with no fixed arguments. This problem generates work mainly when converting varargs code to stdarg code, but it also creates difficulties for variadic functions that wish to pass all of their arguments on to a function that takes a va_list argument, such as vfprintf(3).

October 25, 2002 Mac OS X 12