poll(2) doesn't empty the event queue

Question

Hi,

Im using linux as my programming platform. I am using poll(2) to know if my device is triggering an event.

The first call of poll is ok, it blocks and wait for the event to happen. But in the second poll function call, it will return but it capture the event. Below are my code

ret = poll( fds, 1, 2000); //2 secs timeout

if( fds[0].revents & POLLIN && ret > 0)
{
   printf("event occur\n");
}

It seems the queue/buffer is not empty, im just assuming.

What do you think is the problem?

Thanks.

Answer 1

if you have an POLLIN event, which means "There is data to read" - do you call some read() function on your fd before poll()'ing again ?

Answer 2

Obviously if you are polling incoming data you should consume available data (calling read()) or it will still be there and poll will return immediately. Symmetrically no operation is really necessary for POLLOUT, but you usually want to call the next write() as soon as possible. So as a rule of thumb POLLIN -> read, POLLOUT -> write.

You should also reset you pollfd struct before calling poll again.

fds[0].fd = sck;
fds[0].events = POLLIN;
fds[0].revents = 0;
ret = poll( fds, 1, 2000); //2 secs timeout

if( fds[0].revents & POLLIN && ret > 0)
{
   printf("event occur\n");
}

If you do not reset it each time, garbage from previous call can change poll behavior (well, not really, this is just a portability issue).

In production code you must also check for return value, as poll may have been interrupted for another reason than the expected event (like a signal). Then you usually want to call it again instead of reading data that is not available (as a reminder the poll return value is the number of events, 0 timeout, -1 some error, whose number is in errno).

Errors on files descriptors provided to poll may also occur. They will not make poll return an error but will set POLLERR, POLLHUP or POLLNVAL in the revent field of the pollfd structure for that file descriptor. If these events are set calling read will return some error code that you can check.

Answer 3

Its considered good practice to check for POLLHUP or POLLNVAL, prior to reading the file descriptor. However, I believe that read() would just fail if that was the case.

You are probably not:

• Reading the event FD at all before the next poll()
• Reading all of the available data

If you have initialized the struct pollfd array prior to calling poll(), there should not be any 'garbage' to speak of.

Still, its probably a good idea to also check and be sure there is something worth bothering read() to do prior to calling it.

Answer 4

poll gives you an event if there's data/events to read/errors/when you can write.

If you get an event saying "there's data to read" and you don't read anything - there will still be "data to read" the next time you call poll and you get another event.