va_start

va_start

Begin Varying-Length Argument List Processing

Portability: ISO/ANSI C conforming

SYNOPSIS
DESCRIPTION
RETURN VALUE
CAUTION
EXAMPLE
RELATED FUNCTIONS
SEE ALSO

SYNOPSIS

#include <stdarg.h>

void va_start(va_list ap, arg_name);

DESCRIPTION

va_start initializes processing of a varying-length argument list. The first argument, ap , is a work area of type va_list , which is used by the expansions of the various <stdarg.h> macros. The second argument, arg_name , is the name of the parameter to the calling function after which the varying part of the parameter list begins.

This function is one of three macros used to advance through a list of arguments whose number and type are unknown when the function is compiled. The other two macros are

va_arg accesses an argument from a varying-length argument list.

va_end ends varying-length argument list processing.

These macros and the type va_list are defined in the header file <stdarg.h> .

The type va_list defines a buffer that is used as a work area during argument list processing. A routine that accepts a varying number of arguments must declare an auto variable of this type.

In general, a function that uses the <stdarg.h> facilities has this form:

#include <stdarg.h>

   /* The arguments in the list are the ones that must */
   /*  always be present.                              */
func(type  arg1, type  arg2)
{
   va_list ap;      /* Declare stdarg work area.       */

      /* Note that first varying-length argument       */
      /*  follows arg2 in the list.                    */
   va_start(ap, arg2);

   while (more_args_expected) {

         /* Get next argument value.                   */
      this_arg = va_arg(ap, type);
      process(this_arg);
   }
   va_end(ap);      /* finished argument processing    */
}

RETURN VALUE

va_start has no return value.

CAUTION

The results of va_start are unpredictable if the argument values are not appropriate.

EXAMPLE

See the example for va_arg .

RELATED FUNCTIONS

va_arg , va_end

SEE ALSO

Varying-Length Argument List Functions

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`va_arg`	accesses an argument from a varying-length argument list.
`va_end`	ends varying-length argument list processing.