if (*src == '%' && isxdigit (*(src + 1)) && isxdigit (*(src + 2)))
if (*src == '%' && isxdigit ((unsigned char) *(src + 1)) && isxdigit ((unsigned char) *(src + 2)))
static void rfc2231_decode_one (char *dest, char *src){  char *d;  for (d = dest; *src; src++)  {    if (*src == '%' && isxdigit (*(src + 1)) && isxdigit (*(src + 2)))    {      *d++ = (hexval (*(src + 1)) << 4) | (hexval (*(src + 2)));      src += 2;    }    else      *d++ = *src;  }    *d = '\0';}