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

On 12/03/2013 11:17 AM, OndÅej BÃlka wrote:
> On Tue, Dec 03, 2013 at 10:49:54AM -0500, Carlos O'Donell wrote:
>> On 12/03/2013 10:39 AM, Andreas Schwab wrote:
>>> "Carlos O'Donell" <> 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?
>>> <>:
>>>     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?
> A text above is recomendation of saving errno. How that implies that
> returning errno by signal handler is allowed? 

A signal handler is allowed to modify errno and return. Is that something
anyone would ever want to do? If we wrap signal handlers to save and
restore errno to avoid this common programming error will it have any
unintended consequences other than slowing things down?


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