#include <string.h>
  
  int memcmp(const void * s1, const void * s2, size_t n) {
    char * source1 = (char *)s1;
    char * source2 = (char *)s2;
    while (n--) {
      if (*source1 != *source2) {
        return *source1 - *source2;
      }
      source1++;
      source2++;
    }
    return 0;
  }