va_arg

Defined in header <stdarg.h>
T va_arg( va_list ap, T );

The va_arg macro expands to an expression of type T that corresponds to the next parameter from the va_list ap.

Prior to calling va_arg, ap must be initialized by a call to either va_start or va_copy, with no intervening call to va_end. Each invocation of the va_arg macro modifies ap to point to the next variable argument.

If va_arg is called when there are no more arguments in ap, or if the type of the next argument in ap (after promotions) is not compatible with T, the behavior is undefined, unless:

• one type is a signed integer type, the other type is the corresponding unsigned integer type, and the value is representable in both types; or
• one type is pointer to void and the other is a pointer to a character type.

Parameters

ap	-	an instance of the va_list type
T	-	the type of the next parameter in ap

Expanded value

the next variable parameter in ap

Example

Run this code

#include <stdio.h>
#include <stdarg.h>
#include <math.h>
 
double stddev(int count, ...) 
{
    double sum = 0;
    double sum_sq = 0;
    va_list args;
    va_start(args, count);
    for (int i = 0; i < count; ++i) {
        double num = va_arg(args, double);
        sum += num;
        sum_sq += num*num;
    }
    va_end(args);
    return sqrt(sum_sq/count - (sum/count)*(sum/count));
}
 
int main(void) 
{
    printf("%f\n", stddev(4, 25.0, 27.3, 26.9, 25.7));
}

Output:

0.920258

See also

va_copy (C99)	makes a copy of the variadic function arguments (function macro)
va_end	ends traversal of the variadic function arguments (function macro)
va_list	holds the information needed by va_start, va_arg, va_end, and va_copy (typedef)
va_start	enables access to variadic function arguments (function macro)
C++ documentation for va_arg