poll(2) and select(2) should document spurious EAGAIN for portable programs

poll(2) and select(2) should document spurious EAGAIN for portable programs

[Josh Triplett]

On some systems (reported by Jeremy Sequoia as occurring on Darwin),
poll(2) and select(2) can spuriously return EAGAIN if they fail to
allocate kernel-internal resources, rather than ENOMEM as Linux does.
The spec allows this for poll(2):
http://pubs.opengroup.org/onlinepubs/9699919799/functions/poll.html says
poll may return EAGAIN if "The allocation of internal data structures
failed but a subsequent request may succeed."  The spec for select(2)
at http://pubs.opengroup.org/onlinepubs/9699919799/functions/select.html
doesn't specify EAGAIN, but apparently Darwin can return it in that case
too.

I'd suggest the following text for poll(2)'s NOTES section:

Some other UNIX systems can return EAGAIN if they fail to allocate
kernel-internal resources, rather than ENOMEM as Linux does.  The Single
Unix Specification allows this behavior for poll(2).  Portable programs
may wish to check for EAGAIN and loop, just as with EINTR.


And the following text for select(2)'s NOTES section:

Some other UNIX systems can return EAGAIN if they fail to allocate
kernel-internal resources, rather than ENOMEM as Linux does.  The Single
Unix Specification allows this behavior for poll(2), but not for
select(2); nonetheless, some systems do return EAGAIN from select(2).
Portable programs may wish to check for EAGAIN and loop, just as with
EINTR.


- Josh Triplett
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Re: poll(2) and select(2) should document spurious EAGAIN for portable programs

[Michael Kerrisk (man-pages)]

Hi Josh,


On 23 August 2015 at 09:32, Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org> wrote:
> On some systems (reported by Jeremy Sequoia as occurring on Darwin),
> poll(2) and select(2) can spuriously return EAGAIN if they fail to
> allocate kernel-internal resources, rather than ENOMEM as Linux does.
> The spec allows this for poll(2):
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/poll.html says
> poll may return EAGAIN if "The allocation of internal data structures
> failed but a subsequent request may succeed."  The spec for select(2)
> at http://pubs.opengroup.org/onlinepubs/9699919799/functions/select.html
> doesn't specify EAGAIN, but apparently Darwin can return it in that case
> too.
>
> I'd suggest the following text for poll(2)'s NOTES section:
>
> Some other UNIX systems can return EAGAIN if they fail to allocate
> kernel-internal resources, rather than ENOMEM as Linux does.  The Single
> Unix Specification allows this behavior for poll(2).  Portable programs
> may wish to check for EAGAIN and loop, just as with EINTR.
>
>
> And the following text for select(2)'s NOTES section:
>
> Some other UNIX systems can return EAGAIN if they fail to allocate
> kernel-internal resources, rather than ENOMEM as Linux does.  The Single
> Unix Specification allows this behavior for poll(2), but not for
> select(2); nonetheless, some systems do return EAGAIN from select(2).
> Portable programs may wish to check for EAGAIN and loop, just as with
> EINTR.

I've added text to both pages similar to what you proposed above.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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