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by rdtsc 4328 days ago
Have you tried:

* closing the socket (it would be from another, monitoring thread in this case)

* setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO)

* Would that be different than blocking on select with a an infinite timeout. How do you cancel that? Or are you relying on other sockets getting constant stream of data to wake you up?

* do something with an ALARM signal
1 comments
mikeash 4328 days ago
Closing the socket results in a race condition. You might close the socket, and then another thread opens a new file or socket and happens to get the same fd as your socket used to hold. Now your reader reads from some random unaffiliated fd it shouldn't be touching, causing all sorts of havoc.

A timeout will work fine, but now you're polling, meaning you have an unpleasant tradeoff between efficiency and how long it takes for your thread to notice that it's dead.

Canceling select or any other multi-fd call is really easy. Create a pipe and add it to your fd set. Any time you want the thread to wake up (e.g. because you need to tell it that you're canceling something) you just write to the pipe.

Signals have a similar race condition as closing the socket. If the signal is delivered after you check for cancellation but before you enter the system call, you'll hang.

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rdtsc 4328 days ago
> Closing the socket results in a race condition.

That is true. To go more in-depth, you'd do shutdown first. But I think you have to be connected for that.

> Canceling select or any other multi-fd call is really easy. Create a pipe and add it to your fd set. Any time you want the thread to wake up (e.g. because you need to tell it that you're canceling something) you just write to the pipe.

That a good way, agree. But I would still use a select with 2 file descriptors per thread. One fd for the pipe and one for socket itself. Each thread handles its own request and processing as needed without having one global dispatch in the whole application. Pipe is exposed to the outside in case shutdown needs to be triggered (from another thread).

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mikeash 4328 days ago
2 fds per thread works well. Unfortunately, this means you hit select's performance problems twice over, since the performance scales with the maximum fd you pass it, not the number of fds. But as long as that's OK for what you're doing, it's a nice way to arrange things.
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