strlen(s) {
  l=0;

  while (*s++)
    l++;

  return l;
}

strcpy(s, d) {
  i=0;

  while (s[i]) {
    d[i] = s[i];

    i++;
  }
}

memcpy(s, d, n) {
  for (i=0;i<n;i++)
    d[i] = s[i];
}