Re: Portability issues in shm_mq

On Mon, Mar 17, 2014 at 12:03 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Sun, Mar 16, 2014 at 10:45 PM, Tom Lane <tgl@sss.pgh.pa.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)".

Oh, yeah.  Duh.  Clearly my brain isn't working today.  Hmm, so maybe
this will be fairly simple... will try it out.

-- 
Robert Haas
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