The called function must declare an object of type va_list which is used by the macros Fn va_start , Fn va_arg , and Fn va_end .
The Fn va_start macro initializes Fa ap for subsequent use by Fn va_arg and Fn va_end , and must be called first.
The parameter Fa 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 Fn va_start macro, it should not be declared as a register variable, or as a function or an array type.
The Fn va_start macro returns no value.
The Fn va_arg macro expands to an expression that has the type and value of the next argument in the call. The parameter Fa ap is the va_list Fa ap initialized by Fn va_start . Each call to Fn va_arg modifies Fa ap so that the next call returns the next argument. The parameter Fa 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 Fa type .
If there is no next argument, or if Fa 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 Fn va_arg macro after that of the Fn va_start macro returns the argument after Fa last . Successive invocations return the values of the remaining arguments.
The Fn va_end macro handles a normal return from the function whose variable argument list was initialized by Fn va_start .
The Fn va_end macro returns no value.
void foo(char *fmt, ...)
{
va_list ap;
int d;
char c, *p, *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 */
c = va_arg(ap, char);
printf("char %c\n", c);
break;
}
va_end(ap);
}