Re: Problems with poll()/write() on non-blocking sockets
Re: Problems with poll()/write() on non-blocking sockets
- Subject: Re: Problems with poll()/write() on non-blocking sockets
- From: Eric Ogren <email@hidden>
- Date: Wed, 26 May 2010 09:51:23 -0400
(reforwarding to list since original bounced)
Sent from my iPhone
On May 26, 2010, at 9:41 AM, Eric Ogren <email@hidden> wrote:
Hi Jakub - you may want to investigate the snd_lowat socket option.
The Darwin implementation of poll() ignores this option so it may
report a socket writable even when the actual write() call will
fail. setting the lowat to 1 fixed this issue for me.
Eric
Sent from my iPhone
On May 26, 2010, at 9:11 AM, "Jakub Bednar" <email@hidden>
wrote:
Hello everyone,
during stress testing of my network application I have found some
strange behavior of Mac OS X sockets.
I have a communication thread, that calls a poll() checking whether
data can be written to a non-blocking
socket. Upon exit from this function I check for POLLERR or POLLHUP
and then perform a write() to the socket.
If the write returns -1 and errno is set to EINTR or EAGAIN, I go
back to poll. Otherwise I close the socket as there
was an error.
This works great most of the time, but during stress testing I
found out a strange behavior (e.g. after 8 hours of
video streaming). The poll() unblocks the socket and call to write
() returns -1 with errno EAGAIN immediately.
I go back to poll() and the same happens again leading to busy loop.
I found one post telling that it is correct for poll() to unblock a
socket and that the socket can become non-writable
before I manage to call write(), but this is happing all the time
for my socket and never recovers. Should I
consider EAGAIN to be an error and close the connection? Wouldn't
this cause another problems? I can't see
another way out of this issue and I don't even understand why this
is happening. I'm not asking for nor handling
OUT-OF-BAND data flags in poll() but I don't think it could cause
the problem.
Another thing is, that Shark's System Trace keeps telling me that
the syscall write returned 0x0 and not -1.
Maybe a bug in C library?
Can anyone please help me out of this or explain why is this
happening?
Thanks a lot,
Jakub _______________________________________________
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