MBOX_ALIGN_BYTES
if (space >= MBOX_ALIGN_BYTES)
space -= MBOX_ALIGN_BYTES;
space_at_end = mbox_consumer - mbox_producer - MBOX_ALIGN_BYTES;
space_at_end -= MBOX_ALIGN_BYTES;
space_at_start = mbox_consumer - MBOX_ALIGN_BYTES;
if (value % MBOX_ALIGN_BYTES) {
value += (MBOX_ALIGN_BYTES - (value % MBOX_ALIGN_BYTES));