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);
The called function must declare an object of type va_list which is used by the macros va_start(), va_arg(), and va_end().
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.
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.
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 historic setup is:
#include <varargs.h>
void
foo(va_alist)
va_dcl
{
va_list ap;
va_start(ap);
while (...) {
...
x = va_arg(ap, type);
...
}
va_end(ap);
}
On some systems,
va_end
contains a closing '}' matching a '{' in
va_start,
so that both macros must occur in the same function, and in a way
that allows this.
#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);
}