Re: Consensus on MT-, AS- and AC-Safety docs.

["Carlos O'Donell" <carlos at redhat dot com>]

On 12/03/2013 10:39 AM, Andreas Schwab wrote:
> "Carlos O'Donell" <carlos@redhat.com> writes:
> 
>> * Was it ever POSIX's intent to allow a signal handler to
>>   modify the errno of the interrupted code sequence or was
>>   that simply a consequence of being a signal handler and
>>   modifying global state?
> 
> <http://pubs.opengroup.org/onlinepubs/9699919799/functions/sigaction.htm>:
> 
>     Note in particular that even the "safe" functions may modify errno; the
>     signal-catching function, if not executing as an independent thread,
>     should save and restore its value in order to avoid the possibility that
>     delivery of a signal in between an error return from a function that
>     sets errno and the subsequent examination of errno could result in the
>     signal-catching function changing the value of errno.

It isn't clear from this statement, which I've read before, if it was
ever intended that such error changing behaviour be allowed.

Did POSIX purposely allow signal handlers to change errno because they
saw a use case that required it or was it simply a performance choice
to allow signal handlers to avoid the penalty of saving and restoring
the thread-local value?

I surmise that performance was the reason not to mandate that errno
be saved and restored, but I'd like to get the Austin Groups opinion
on this.

Cheers,
Carlos.