When Sockets Die
Probably the worst thing about using blocking sockets is what happens when the other side comes down hard (without doing a close). Your socket is likely to hang. SOCKSTREAM is a reliable protocol, and it will wait a long, long time before giving up on a connection. If you're using threads, the entire thread is essentially dead. There's not much you can do about it. As long as you aren't doing something dumb, like holding a lock while doing a blocking read, the thread isn't really consuming much in the way of resources. Do not try to kill the thread - part of the reason that threads are more efficient than processes is that they avoid the overhead associated with the automatic recycling of resources. In other words, if you do manage to kill the thread, your whole process is likely to be screwed up.
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Disconnecting
Strictly speaking, you're supposed to use shutdown on a socket before you close it.
The shutdown is an advisory to the socket at the other end.
Depending on the argument you pass it, it can mean "I'm not going to send anymore, but I'll still listen", or "I'm not listening, good riddance!".
Most socket libraries, however, are so used to programmers neglecting to use this piece of etiquette that normally a close is the same as shutdown(); close().
So in most situations, an explicit shutdown is not needed.
One way to use shutdown effectively is in an HTTP-like exchange. The client sends a request and then does a shutdown(1). This tells the server "This client is done sending, but can still receive."
The server can detect "EOF" by a receive of 0 bytes. It can assume it has the complete request.
The server sends a reply. If the send completes successfully then, indeed, the client was still receiving.
Python takes the automatic shutdown a step further, and says that when a socket is garbage collected, it will automatically do a close if it's needed. But relying on this is a very bad habit. If your socket just disappears without doing a close, the socket at the other end may hang indefinitely, thinking you're just being slow. Please close your sockets when you're done.
Next: Non-blocking Sockets
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