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DESCRIPTION

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

The va_start() macro initializes ap for subsequent use by va_arg(), va_copy(), and va_end(), and must be called first.

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 va_start() macro, it should not be declared as a register variable, or as a function or an array type.

The va_arg() 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() or va_copy(). 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 va_arg() macro after that of the va_start() macro returns the argument after last. Successive invocations return the values of the remaining arguments.

The va_copy() macro copies a variable argument list, previously initialized by va_start(), from src to dest. The state is preserved such that it is equivalent to calling va_start() with the same second argument used with src, and calling va_arg() the same number of times as called with src.

The va_end() macro cleans up any state associated with the variable argument list ap.

Each invocation of va_start() or va_copy() must be paired with a corresponding invocation of va_end() in the same function.

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

	va_start(ap, fmt);
	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);
}