stdarg, va_start, va_arg, va_end, va_copy - variable argument lists

#include <stdarg.h>

void va_start(va_list ap, last);
type va_arg(va_list ap, type);
void va_end(va_list ap);
void va_copy(va_list dest, va_list src);

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_start(), va_arg(), and va_end().

va_start()

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

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

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

va_arg()

The va_arg() macro expands to an expression that has the type and value of the next argument in the call. The argument 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 argument 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.

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.

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.

If ap is passed to a function that uses va_arg(ap,type), then the value of ap is undefined after the return of that function.

va_end()

Each invocation of va_start() must be matched by a corresponding invocation of va_end() in the same function. After the call va_end(ap) the variable ap is undefined. Multiple traversals of the list, each bracketed by va_start() and va_end() are possible. va_end() may be a macro or a function.

va_copy()

The va_copy() macro copies the (previously initialized) variable argument list src to dest. The behavior is as if va_start() were applied to dest with the same last argument, followed by the same number of va_arg() invocations that was used to reach the current state of src.

An obvious implementation would have a va_list be a pointer to the stack frame of the variadic function. In such a setup (by far the most common) there seems nothing against an assignment

va_list aq = ap;

Unfortunately, there are also systems that make it an array of pointers (of length 1), and there one needs

va_list aq;
*aq = *ap;

Finally, on systems where arguments are passed in registers, it may be necessary for va_start() to allocate memory, store the arguments there, and also an indication of which argument is next, so that va_arg() can step through the list. Now va_end() can free the allocated memory again. To accommodate this situation, C99 adds a macro va_copy(), so that the above assignment can be replaced by

va_list aq;
va_copy(aq, ap);
...
va_end(aq);

Each invocation of va_copy() must be matched by a corresponding invocation of va_end() in the same function. Some systems that do not supply va_copy() have __va_copy instead, since that was the name used in the draft proposal.

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

#include <stdio.h>
#include <stdarg.h>

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 */
            /* need a cast here since va_arg only
               takes fully promoted types */
            c = (char) va_arg(ap, int);
            printf("char %c\n", c);
            break;
        }
    va_end(ap);
}