Il 06/04/2011 12:04, Cristiano Belloni ha scritto:
Hi to all,
I wrote a custom shared memory source. it inherits from
FramedSource.
The shared memory is synchronized via Linux semaphores (simple
producer-consumer algorithm), but since I didn't want to subclass
TaskScheduler, I still use a "dummy" file descriptor-based
communication with live555. In pseudocode:
~~~Client (without live555):
wait on semaphore_empty (blocking)
copy frame in shared memory
write one byte in a dedicated FIFO (this should wake up live555'
TaskScheduler select())
post on semaphore_fill
~~~Server (with live555, in
SharedMemSource::incomingPacketHandler1())
[turnOnBackgroundReadHandling is called in doGetNextFrame]
wait on semaphore_fill (blocking)
read one byte from the dedicated FIFO (to flush the FIFO buffer)
copy frame from shared memory
post on semaphore_empty
This works. Altought the blocking wait on semaphore_fill might
make you wonder, the client wakes up my source with the write() in
the dedicated FIFO and immediately posts on semaphore_fill, so the
server almost never waits, and if it does, it doesn't block for a
really small time.
The problem is that, after a while (1 or 2 hours usually), the
client does its cycle and the server never wakes up. It *doesn't*
get stuck on the wait, I checked: it simply never wakes up, as if
the client write() was lost (but it *always* succeed on the client
side) or the select() didn't wake up even if the write succeeded.
I would like to emphasize this: the server *never* gets stuck
forever on its wait. When it gets stuck, the client is one frame
ahead of the server, incomingPacketHandler1() simply is never
called anymore and the wait is not even reached.
At this point, I have two questions:
1) In your knowledge, can the select() not wake up even if a
write() on the other side succeeded? If it can, how is it
possible? Note that the system is an embedded ARM processor, and
it could get quite busy while acquiring and streaming video.
2) First thing I do in SharedMemSource::incomingPacketHandler1()
is to check for isCurrentlyAwaitingData(). If it's false, I simply
return before doing all the cycle, and this happens quite often.
What's the meaning of isCurrentlyAwaitingData()? I mean, if the
select() in TaskScheduler returned, some data must be present on
the file/fifo/socket. How is it possible that the select() did
return but still there's no data available? I'm getting really
confused on this.
Thanks and regards,
Cristiano Belloni.
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Update: I put some logs in BasicTaskScheduler to see what happens.
one before the select():
printf ("[SYNCHROBUG] About to do the select, timeout %d.%d\n",
tv_timeToDelay.tv_sec, tv_timeToDelay.tv_usec);
int selectResult = select(fMaxNumSockets, &readSet,
&writeSet, &exceptionSet, &tv_timeToDelay);
if (selectResult < 0) {
[...]
two after the select(), (one catches an EINTR or EAGAIN error value
should they happen):
#else
if (errno != EINTR && errno != EAGAIN) {
#endif
// Unexpected error - treat this as fatal:
#if !defined(_WIN32_WCE)
perror("BasicTaskScheduler::SingleStep(): select() fails");
#endif
internalError();
}
}
if (errno == EINTR || errno == EAGAIN) {
perror ("[SYNCHROBUG] error is");
}
printf ("[SYNCHROBUG] Select done, getting sockets\n");
[...]
two after the first and second pass of readable socket check:
int resultConditionSet = 0;
if (FD_ISSET(sock, &readSet) && FD_ISSET(sock,
&fReadSet)/*sanity check*/) {
printf ("[SYNCHROBUG] Socket %d found readable on first
pass\n", sock);
resultConditionSet |= SOCKET_READABLE;
}
[...]
if (FD_ISSET(sock, &readSet) && FD_ISSET(sock,
&fReadSet)/*sanity check*/) {
printf ("[SYNCHROBUG] Socket %d found readable on second
pass\n", sock);
resultConditionSet |= SOCKET_READABLE;
}
And one to check if we found some readable/writable/excepting socket
at all:
if (handler == NULL) {
fLastHandledSocketNum = -1;//because we didn't call a handler
printf ("[SYNCHROBUG] No socket found at all\n");
}
at first everything is ok:
[SYNCHROBUG] About to do the select, timeout 0.0
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] No socket found at all
[SYNCHROBUG] About to do the select, timeout 0.0
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] About to do the select, timeout 0.0
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 5 found readable on first pass
[SYNCHROBUG] About to do the select, timeout 0.0
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 5 found readable on second pass
[SYNCHROBUG] About to do the select, timeout 0.0
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 5 found readable on second pass
[SYNCHROBUG] About to do the select, timeout 0.0
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 5 found readable on second pass
[SYNCHROBUG] About to do the select, timeout 0.0
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 5 found readable on second pass
[SYNCHROBUG] About to do the select, timeout 0.0
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 5 found readable on second pass
(socket 5 must be the FIFO, I guess)
But then, select keeps randomly return errno=11, aka EAGAIN or
"Resource temporarily unavailable":
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 5 found readable on second pass
[SYNCHROBUG] About to do the select, timeout 1.480311
[SYNCHROBUG] error is: Resource temporarily unavailable
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 6 found readable on first pass
[SYNCHROBUG] About to do the select, timeout 1.479309
[SYNCHROBUG] error is: Resource temporarily unavailable
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 5 found readable on second pass
[SYNCHROBUG] About to do the select, timeout 1.478362
[SYNCHROBUG] error is: Resource temporarily unavailable
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 6 found readable on first pass
[SYNCHROBUG] About to do the select, timeout 1.477358
[SYNCHROBUG] error is: Resource temporarily unavailable
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 5 found readable on second pass
[SYNCHROBUG] About to do the select, timeout 1.476431
[SYNCHROBUG] error is: Resource temporarily unavailable
[SYNCHROBUG] Select done, getting sockets
[SYNCHROBUG] Socket 6 found readable on first pass
Now, I don't even know the reason why a select() could return EAGAIN
(a lot of people say it souldn't at all, and even my "man 3 select"
agrees:
http://stackoverflow.com/questions/4193043/select-on-a-pipe-in-blocking-mode-returns-eagain
), but I see this case is handled in your code and ignored, just
like the EINTR case:
if (errno != EINTR && errno != EAGAIN) {
#endif
// Unexpected error - treat this as fatal:
#if !defined(_WIN32_WCE)
perror("BasicTaskScheduler::SingleStep(): select() fails");
#endif
internalError();
}
[if errno is EINTR or EAGAIN, then the scheduler goes on inspecting
the select()'s returned sets].
Could that be the origin of my problems?
As obviously you can't try my executables on my hardware, please
tell me what else could I log. BTW the rtsp/rtp client in this
picture is openRTSP. Here's an ascii schema :)
client program generating frames ----FIFO---> rtsp server based
on live555 ----RTSP/RTP/TCP----> openRTSP
Thank you and best regards,
Cristiano Belloni.
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