Re: new compiler warnings

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Oct 18, 2011 at 1:01 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> The chunks are sent indivisibly, because they are less than the pipe
>> buffer size. Read the pipe man page. It's guaranteed that the write
>> will either succeed or fail as a whole, not write a partial message.
>> If we cared to retry a failure, there would be some point in checking
>> the return code.

> On MacOS X v10.6.8, I see no such guarantee in the pipe(2) man page.

Sorry, maybe write(2) is the place to look. The Single Unix Spec quoth
(at http://pubs.opengroup.org/onlinepubs/9699919799/)

Write requests to a pipe or FIFO shall be handled in the same
way as a regular file with the following exceptions:

There is no file offset associated with a pipe, hence each write
request shall append to the end of the pipe.

Write requests of {PIPE_BUF} bytes or less shall not be
interleaved with data from other processes doing writes on the
same pipe. Writes of greater than {PIPE_BUF} bytes may have data
interleaved, on arbitrary boundaries, with writes by other
processes, whether or not the O_NONBLOCK flag of the file status
flags is set.

If the O_NONBLOCK flag is clear, a write request may cause the
thread to block, but on normal completion it shall return nbyte.

Note the last in particular. Short writes are specifically disallowed
on pipes.

If this were not the case, the logging collector protocol would be
useless.

regards, tom lane