va_start

< c‎ | variadic
 
C
 
Variadic functions
 
Defined in header <stdarg.h>
void va_start( va_list ap, parmN );

The va_start macro enables access to the variable arguments following the named argument parmN.

va_start should be invoked with an instance to a valid va_list object ap before any calls to va_arg.

If parmN is declared with register storage class specifier, with an array type, with a function type, or with a type not compatible with the type that results from default argument promotions, the behavior is undefined.

Parameters

ap - an instance of the va_list type
parmN - the named parameter preceding the first variable parameter

Expanded value

(none)

Example

Run this code
#include <stdio.h>
#include <stdarg.h>
 
int add_nums(int count, ...) 
{
    int result = 0;
    va_list args;
    va_start(args, count);
    for (int i = 0; i < count; ++i) {
        result += va_arg(args, int);
    }
    va_end(args);
    return result;
}
 
int main(void) 
{
    printf("%d\n", add_nums(4, 25, 25, 50, 50));
}

Possible output: 

150

References

  • C11 standard (ISO/IEC 9899:2011):
  • 7.16.1.4 The va_start macro (p: 271-272)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.15.1.4 The va_start macro (p: 251-252)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 4.8.1.1 The va_start macro

See also

 accesses the next variadic function argument
(function macro)
(C99)
 makes a copy of the variadic function arguments
(function macro)
ends traversal of the variadic function arguments
(function macro)
holds the information needed by va_start, va_arg, va_end, and va_copy
(typedef)