Why this program is not compile in dev c++ compiler?

Question

#include <stdio.h>

void * square (const void *num);

int main() {
int x, sq_int;
x = 6;
sq_int = square(&x);
printf("%d squared is %d\n", x, sq_int);

return 0;
}

void* square (const void *num) {
static int result;

result = (*(int *)num) * (*(int *)num);
return(result);
}