Before voting to close, please read, I know that there are similar questions (:
Here’s my situation – I have an application, that is multithreaded. So, lets say I have 10 threads. All of them read from the same file descriptor (it’s actually a socket ). And in a very rare situation, when a critical error occurs, the socket should be shutdown by one of the threads. The thing is, that any of these thread can do this. If the closing of the socket has failed, _Exit( FAILURE ) is executed (I know, that this sounds like an awful design or problem in the code, but it actually isn’t, as this is caused by a non-opensource 3rd party lib, that has a bug).
And here’s the problem situation – it’s possible all of them to try to shutdown the socket in the same time. And one closes it, but the others cannot close it (shutdown returns -1, as the socket is already closed) and the bad _Exit( FAILURE ) is executed and that ruins everything.
Obviously, I need an additional check – if the socket is already closed (it’s possible all threads to have failed shutting down the socket for some reason, and then at least one must execute _Exit, that’s why checking the return code of shutdown is not enough).
Well, I found this question and it looks like that’s exactly what I’m trying to do.
But I know, that any kind of system calls take time (of course ) and it’s OS dependent when exactly the socket will be closed.
And here’s the question – how can I make difference if a socket is already closed or it cannot be closed for some reason? Will the fcntl ensure me, that if one thread has closed the socket and at the same time if other thread try to shutdown the socket, it will fail and then, if I make this check ( with fcntl ), this will work for me?
I also saw the other answers like: "you can use select or poll", but they are still system calls and I don’t know if they will be the better choice. I also don’t know how exactly to use them, but it’s not a big deal, I guess.
Thanks!
I can also check the errno set by shutdown, but what does "connected" mean? And what is the difference between "connected" and "not a valid descriptor"
ENOTCONN
The specified socket is not connected.
Also, what bothers me is, that the FD, I’m trying to close, could be invalid, as I take it from /proc/net/tcp mapped with proc/PID/fd and I don’t know if all files will look like the way, they look on my OS (the OS will be for sure RHEL4 or RHEL5, if that matters)
Doh! It’s damn long, but I can’t explain it shorter.
I assume you’re saying it’s possible for your application to reasonably continue after shutting down the socket?
It seems that a better approach would be to have a mediator thread that gets notified of socket close requests from any of the worker threads, notifies the other threads that the socket is now dead, and takes care of closing the socket itself. This eliminates the worries about the reason for failure because it’s all handled in a single thread.