void flipbits32(char* a, int len ) {

  int blocks = len >> 2;

  if ( sizeof blocks == 4 ) { /* try 32bit blocks */
    for ( ; blocks-- ; a += 4 )  *( (unsigned int*) a ) ^= 0xffffffffU;
    /* TODO: add treatment for last 1..3 bytes if len is not a multiple of 4 */
  } else { /* fallback to 8bit */
    for ( ; len--   ; ++a    )   *a ^= 0xff;
  }

}