4.9 close Function
The normal Unix close function is also
used to close a socket and terminate a TCP connection.
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#include <unistd.h>
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int close (int sockfd);
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Returns: 0 if OK, -1 on error
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The default action of close with a TCP
socket is to mark the socket as closed and return to the process
immediately. The socket descriptor is no longer usable by the
process: It cannot be used as an argument to read or
write. But, TCP will try to send any data that is already
queued to be sent to the other end, and after this occurs, the
normal TCP connection termination sequence takes place (Section
2.6).
In Section 7.5, we
will describe the SO_LINGER socket option, which lets us
change this default action with a TCP socket. In that section, we
will also describe what a TCP application must do to be guaranteed
that the peer application has received any outstanding data.
Descriptor Reference Counts
At the end of Section 4.8, we
mentioned that when the parent process in our concurrent server
closes the connected socket, this just decrements the
reference count for the descriptor. Since the reference count was
still greater than 0, this call to close did not initiate
TCP's four-packet connection termination sequence. This is the
behavior we want with our concurrent server with the connected
socket that is shared between the parent and child.
If we really want to send a FIN on a TCP
connection, the shutdown function can be used (Section
6.6) instead of close. We will describe the motivation
for this in Section 6.5.
We must also be aware of what happens in our
concurrent server if the parent does not call close for
each connected socket returned by accept. First, the
parent will eventually run out of descriptors, as there is usually
a limit to the number of descriptors that any process can have open
at any time. But more importantly, none of the client connections
will be terminated. When the child closes the connected socket, its
reference count will go from 2 to 1 and it will remain at 1 since
the parent never closes the connected socket. This will
prevent TCP's connection termination sequence from occurring, and
the connection will remain open.
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