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I have a simple C server that accepts connection to which I try connecting using telnet or netcat. Each time I receive a connection, I print out the descriptor and close the connection in a child process.

I run an instance of netcat, connect to the server, disconnect(Ctrl^C), and repeat this a few times. The values printed at the server side on the descriptors used are 4,5,6,7 .. and it goes on increasing.

I tried repeating this exercise after a period of time and the values still keep increasing. I'm concerned that my descriptions aren't closing(despite an explicit call to close).

Is there some signal I should be handling, setting the handler to close the connection?

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After a fork the child process has a copied set of the parent's file descriptors. So the proper procedure is, after the fork, to (1) close the parent's listening socket in the child and to (2) close the new connection socket inherited by the child in the parent.

Open file descriptors are reference counted by the kernel. So when the child inherits the connection socket the reference count is 2. After the parent closes the connection socket the count remains at 1 until the child is done and closes it. The reference count having then dropped to 0 the connection is then closed. (Some details omitted.)

The upshot is that after making this change you then see a lot of FDs equal to 4 in the parent because the same FD will continue to be opened/closed/reused even though multiple connections are being processed by the children.

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after fork both parent and child have a copy of the socket file descriptor;

you should close the socket in the parent process after fork. Now that does not close the connection, only when the child process closes the socket too, this then closes the connection.

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