On 27/04/13 02:48, Tom Lane wrote:
Robert Haas <robertmh...@gmail.com> writes:
On Thu, Apr 25, 2013 at 1:56 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
However, the documentation in libpq.sgml is a bit bogus too, because it
counsels trying the PQputCopyEnd() call again, which will not work
(since we already changed the asyncStatus).  We could make that say "a
zero result is informational, you might want to try PQflush() later".
The trouble with this, though, is that any existing callers that were
coded to the old spec would now be broken.
Changing the meaning of a 0 return code seems like a bad idea.
However, not ever returning 0 isn't great either: someone could be
forgiven for writing code that calls PQputCopyData/End() until they
get a 0 result, then waits for the socket to become write-OK before
Anybody who tried that would have already discovered that it doesn't
work, so that argument seems pretty hollow.

What I'm suggesting is that we fix the documentation to match what
the code actually does, ie 1 and -1 are the only return codes, but
in nonblock mode it may not actually have flushed all the data.
I do not see how that can break any code that works now.

An alternative possibility is to document the zero return case as
"can't happen now, but defined for possible future expansion", which
I rather suspect was the thought process last time this was looked at.
The trouble with that is that, if we ever did try to actually return
zero, we'd more than likely break some code that should have been
checking for the case and wasn't.

Anyway, in view of the lack of complaints from the field, I think
changing the behavior of this code would be much more likely to cause
problems than fix them.

                        regards, tom lane

The original usage of returns codes was as an offset for the Operating System to jump to in a list of addresses to execute, after program completion. Zero offset was to the first address for normal completion, then 4 for the next address... Addresses were stored in 4 bytes. Hence an historical tendency to define return codes in multiple of 4. This dates back to the Mainframe days, pre UNIX, even!

Personally I would prefer zero to indicate an error, on the basis that is the default value for memory, but historical usage makes that impractical! So we are stuck with zero being interpreted as the return code indicating 'Normal' completion - bash, and other Linux shells, are designed on this assumption.

I first came across this return code convention when I was programming Mainframes in the early 1970's.


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