va_arg

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 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.

If va_arg is called when there are no more arguments in ap , the behavior is undefined.

Parameters

Expanded value

the next variable parameter in ap

Example

#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));
}

0.920258

References