Re: Multithread select() bug

From: Eric Dumazet
Date: Tue May 11 2004 - 01:38:01 EST

Andre Ben Hamou wrote:

I see where you're coming from, in that there is a potential race condition as to the socket being connected as I reach the select call.

This is an important concern but it is, I think, orthogonal to the original problem as there are two possible socket states at the point at which select gets called (as far as I can see)...

1. The socket is in its connected state
2. The socket has already been closed by the parent thread

As I understand it, if 1 is true (which corresponds to my original post), then select should return the moment the socket gets closed and, if 2 is true (which I believe corresponds to your concern), then select should return immediately anyway as the socket would not block if read from.

Sorry to be a pest, but I'm trying to get this clear in my head. Is it possible I've over-estimated the thread-safety of the select and close calls?

As the safety of what you are trying to do cannot be guaranted, you should not even try to do that.
Then, apart from this safety showstopper, you should consider that what you assumed is simply wrong :
- Some Operating Systems do something special to signal blocked threads (blocked on a read()/write()/select()/poll()) if the underlying file handle is closed.
- linux does NOT. And this is done for several reasons. These reasons are known and were discussed on this list at several occasions.

So the rule is : close() syscall should be serialized, according to a synchronization of your choice, so that no other thread could be using the file descriptor at the same time.
If you want to 'send some signal to the blocked thread' in a portable way, you could setup a pipe to be able to send a special message on it. The 'worker thread' could add the read side of the pipe (pipefd[0]) to its select fd set, and will be notified of the Thread 2) willing to dismantle your socket, using a standard write(pipefd[1]). This will be portable : When the 'worker thread' returns from select() call, it can then close the socket fd himself.

By the way, prefer poll() syscall to select() one. Poll is much more scalable/efficient if you have only one or two fd to monitor.

See you


Andre Ben Hamou
Imperial College London

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