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va_start
Initializes a variable to the beginning of an argument list.
Syntax
#include <stdarg.h>
void va_start (va_list ap, parmN);
Parameters
ap A pointer to a double, as defined by type va_list in
<varargs.h>.
parmN The identifier of the rightmost parameter in the variable
parameter list in the function definition. This is the
identifier just before the horizontal ellipsis.
Return Values
None.
Description
The va_start macro initializes ap (of type va_list) for subsequent use by
va_arg and va_end. It must be invoked before va_arg can be used.
The macros va_start, va_arg, and va_end determine the arguments of a
function that can be called with a variable number of arguments. The
variable number of arguments are indicated by the ellipsis in the
function header.
NOTE The header <varargs.h> also contains this macro. However,
<varargs.h> is not defined by the ANSI C standard.
Examples
The following program uses <stdarg.h>:
#include <stdarg.h>
#include <stdio.h>
enum arglisttype {NO_VAR_LIST, VAR_LIST_PRESENT};
enum argtype {END_OF_LIST, CHAR, DOUB, INT, PINT};
int func (int a1, enum arglisttype a2, ...)
{
va_list ap;
enum argtype ptype;
int i, *p;
char c;
double d;
printf ("arg count = %d\n", a1);
if (a2 == VAR_LIST_PRESENT) {
/* Initialize the varargs mechanism */
va_start(ap, a2); /* pass a2 as an anchor */
/* pick up all the arguments */
do {
/* get the type of the argument */
ptype = va_arg (ap, enum argtype);
/* retrieve the argument based on the type */
switch (ptype) {
case CHAR: c = va_arg (ap, char);
printf ("char = %c\n", c);
break;
case DOUB: d = va_arg (ap, double);
printf ("double = %f\n", d);
break;
case PINT: p = va_arg (ap, int *);
printf ("pointer = %x\n", p);
break;
case INT : i = va_arg (ap, int);
printf ("int = %d\n", i);
break;
case END_OF_LIST :
break;
default: printf ("bad argument type %d\n", ptype);
ptype = END_OF_LIST; /* to break loop */
break;
} /* switch */
} while (ptype != END_OF_LIST);
/* Clean up */
va_end (ap);
} /* if */
}
main()
{
int x = 99;
func (1, NO_VAR_LIST);
func (2, VAR_LIST_PRESENT, DOUB, 3.0, PINT, &x, END_OF_LIST);
}
See Also
va_arg, va_end, ANSI C 4.8.1.1