Closing a socket and reclaiming its resources is not always a straightforward operation. In certain situations, such as when a process exits, a soclose() call is expected to be successful. However, when a socket promising reliable delivery of data is closed with data still queued for transmission or awaiting acknowledgment of reception, the socket must attempt to transmit the data. If the socket discards the queued data to allow the soclose() call to complete successfully, it violates its promise to deliver data reliably. Discarding data can cause naive processes, which depend upon the implicit semantics of the soclose() call, to work unreliably in a network environment. However, if sockets block until all data has been transmitted successfully, in some communication domains a soclose() call may never complete.
The sockets layer compromises in an effort to address this problem and maintain the semantics of the soclose() call. In normal operation, closing a socket causes any queued but unaccepted connections to be discarded. If the socket is in a connected state, a disconnect is initiated. When the disconnect request completes, the network support notifies the sockets layer, and the socket resources are reclaimed. The network layer may then attempt to transmit any data queued in the socket's send buffer, although this is not guaranteed.
Alternatively, a socket may be marked explicitly to force the application program to linger when closing until pending data are flushed and the connection has shut down. This option is marked in the socket data structure using the setsockopt() call with the SO_LINGER option. The setsockopt() call, using the linger option, takes a linger structure. When an application program indicates that a socket is to linger, it also specifies a duration for the lingering period. If the lingering period expires before the disconnect is completed, the socket layer forcibly shuts down the socket, discarding any data still pending.
An example of deallocating the socket descriptor s using the soclose() call
is:
An Application Using the soclose() Call
... /* close the socket */ soclose(s); ...