Something

char *urlencode(char *in)
{
  char *out, *p, *q;

  out = (char *)malloc(3 * strlen(in) + 1);
  p = in;
  q = out;

  while (*p != '\0') {
    if (isalnum(*p) || (*p == '_') || (*p == '-')) {
      *q = *p;
      ++q;
    } else {
      sprintf(q, "%%%02X", *p);
      q = &q[3];
    }
    ++p;
  }
  *q = '\0';
  return out;
}