Get the wrong answer——about uncertain number of variables

Question

When I ran the program below in the Linux system, I can't get the expected answer "9". But I can get it in the windows system.

Why does this happen?

#include <stdio.h>

int sum(int num, ...){
    int* p = &num + 1;
    int res = 0;
    while(num--){
        res += *p++;
    }
    return res;
}


int main(){
    printf("%d\n", sum(3,2,3,4));
    return 0;

}

I debug it in GDB, shown as below: enter image description here

My System configuration is below.

Linux version 5.11.0-41-generic (buildd@lgw01-amd64-005) (gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34)

Answer 1

You need to use macros defined in the header <stdarg.h>.

Here you are.

#include <stdio.h>
#include <stdarg.h>

long long int sum( size_t n, ... ) 
{
    long long int result = 0;

    va_list ap;

    va_start( ap, n );

    while (n--)
    {
        result += va_arg( ap, int );
    }

    va_end( ap );

    return result;
}

int main( void )
{
    printf( "%lld\n", sum( 9, 1, 2, 3, 4, 5, 6, 7, 8, 9 ) );
}

The program output is

45