6.6 shutdown Function
The normal way to terminate a network connection
is to call the close function. But, there are two
limitations with close that can be avoided with
shutdown:
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close
decrements the descriptor's reference count and closes the socket
only if the count reaches 0. We talked about this in Section
4.8. With shutdown, we can initiate TCP's normal
connection termination sequence (the four segments beginning with a
FIN in Figure 2.5),
regardless of the reference count.
-
close
terminates both directions of data transfer, reading and writing.
Since a TCP connection is full-duplex, there are times when we want
to tell the other end that we have finished sending, even though
that end might have more data to send us. This is the scenario we
encountered in the previous section with batch input to our
str_cli function. Figure 6.12 shows the typical function calls in
this scenario.
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#include <sys/socket.h>
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int shutdown(int sockfd, int howto);
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Returns: 0 if OK, 鈥? on error
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The action of the function depends on the value
of the howto argument.
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SHUT_RD
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The read half of the
connection is closed鈥?/span> No more data can be received on the
socket and any data currently in the socket receive buffer is
discarded. The process can no longer issue any of the read
functions on the socket. Any data received after this call for a
TCP socket is acknowledged and then silently discarded.
By default, everything written to a routing
socket (Chapter
18) loops back as possible input to all routing sockets on the
host. Some programs call shutdown with a second argument
of SHUT_RD to prevent the loopback copy. An alternative
way to prevent this loopback copy is to clear the
SO_USELOOPBACK socket option.
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SHUT_WR
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The write half of the
connection is closed鈥?/span> In the case of TCP, this is called a
half-close (Section 18.5 of
TCPv1). Any data currently in the socket send buffer will be sent,
followed by TCP's normal connection termination sequence. As we
mentioned earlier, this closing of the write half is done
regardless of whether or not the socket descriptor's reference
count is currently greater than 0. The process can no longer issue
any of the write functions on the socket.
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SHUT_RDWR
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The read half and the
write half of the connection are both closed鈥?/span> This is
equivalent to calling shutdown twice: first with
SHUT_RD and then with SHUT_WR.
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Figure 7.12 will
summarize the different possibilities available to the process by
calling shutdown and close. The operation of
close depends on the value of the SO_LINGER
socket option.
The three SHUT_xxx names are defined by the POSIX
specification. Typical values for the howto argument that you will encounter will be
0 (close the read half), 1 (close the write half), and 2 (close the
read half and the write half).
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