Re: Portability issues in shm_mq

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Sun, Mar 16, 2014 at 10:45 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Well, it will result in padding space when you maxalign the length word,
>> but I don't see why it wouldn't work; and it would certainly be no less
>> efficient than what's there today.

> Well, the problem is with this:

> /* Write the message length into the buffer. */
> if (!mqh->mqh_did_length_word)
> {
> res = shm_mq_send_bytes(mqh, sizeof(uint64), &nbytes, nowait,
> &bytes_written);

> If I change nbytes to be of type Size, and the second argument to
> sizeof(Size), then it's wrong whenever sizeof(Size) % MAXIMUM_ALIGNOF
> != 0.

Well, you need to maxalign the number of bytes physically inserted into
the queue. Doesn't shm_mq_send_bytes do that? Where do you do the
maxaligning of the message payload data, when the payload is odd-length?
I would have expected some logic like "copy N bytes but then advance
the pointer by maxalign(N)".

regards, tom lane