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va_start

Defined in header <stdarg.h> 
void va_start( va_list ap, parmN );
 (until C23) 
void va_start( va_list ap, ... );
 (since C23)

The va_start macro enables access to the variable arguments following the named argument parmN(until C23).

va_start shall 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.

 (until C23)

Only the first argument passed to va_start is evaluated. Any additional arguments are neither expanded nor used in any way.

 (since C23)

Parameters

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

Expanded value

(none)

Example

#include <stdio.h>
#include <stdarg.h>
 
int add_nums_C99(int count, ...)
{
    int result = 0;
    va_list args;
    va_start(args, count); // count can be omitted since C23
 
    for (int i = 0; i < count; ++i) {
        result += va_arg(args, int);
    }
 
    va_end(args);
    return result;
}
 
#if __STDC_VERSION__ > 201710L
// Same as above, valid since C23
int add_nums_C23(...)
{
    int result = 0;
    va_list args;
    va_start(args);
 
    int count = va_arg(args, int);
    for (int i = 0; i < count; ++i) {
        result += va_arg(args, int);
    }
 
    va_end(args);
    return result;
}
#endif
 
int main(void)
{
    printf("%d\n", add_nums_C99(4, 25, 25, 50, 50));
#if __STDC_VERSION__ > 201710L
    printf("%d\n", add_nums_C23(4, 25, 25, 50, 50));
#endif
}

Possible output:

150
150

References

  • C17 standard (ISO/IEC 9899:2018):
    • 7.16.1.4 The va_start macro (p: 198-199)
  • 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)
C++ documentation for va_start

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