CODEWITHSUNDEEP
cunix
zer0stimulus
zer0stimulus

Reputation: 23666

Unix: What happens when a read file descriptor closes while calling select()

Say that I call select() on a FD_SET containing a bunch of read file descriptors. What happens if during the select() call, one of the file descriptor closes? Assuming that some sort of error occurs, then is it my responsibility to find and remove the closed file descriptor from the set?

Upvotes: 9

Views: 3839

Answers (3)

R.. GitHub STOP HELPING ICE
R.. GitHub STOP HELPING ICE

Reputation: 215517

I don't believe this is specified anywhere; some systems may immediately return from select while others may continue blocking. Note that the only way this can happen is in a multi-threaded process (otherwise, the close cannot happen during select; even if it happened from a signal handler, select would have already been interrupted by the signal). As such, this situation arising probably indicates you have bigger issues to worry about. If one of the file descriptors you're polling can be closed during select, the bigger issue is that the same file descriptor might be reassigned to a newly opened file (e.g. one opened in another unrelated thread) immediately after the close, and the thread that's polling might then wrongly perform IO on the new file "belonging to" a different thread.

If you have a data object that consists of a set of file descriptors that will be polled with select in a multithreaded program, you almost surely need to be using some sort of synchronization primitive to control access to that set, and adding or removing file descriptors should require a lock that's mutually exclusive with the possibility that select (or any IO on the members) is in progress.

Of course in a multi-threaded program, it may be better not to use select at all and instead let blocking IO in multiple threads achieve the desired result without complicated locking logic.

Upvotes: 4

Karoly Horvath
Karoly Horvath

Reputation: 96286

Even if you've read all the sent data, a closed socket is always regarded as ready to read. Select will unblock, signaling that socket to be available.

Upvotes: 1

Eugen Rieck
Eugen Rieck

Reputation: 65324

The select() system call takes three fd_set parameters: Send, Receive, Exception. To check, if an error occurs on a reading file descriptor include it in the read (receive) and in the error (exceprion) set - seeing it in the exception set on return from select() means, an exception has occurred on that socket, giving you the chance to find out what.

In general network sockets with any sort of exception will no longer be fit to send and receive.

Upvotes: 1

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