The following material is provided as clarification for the subject of shutting down socket connections closing the sockets. It is important to distinguish the difference between shutting down a socket connection and closing a socket. Shutting down a socket connection involves an exchange of protocol messages between the two endpoints, hereafter referred to as a shutdown sequence. Two general classes of shutdown sequences are defined: graceful and abortive (also called "hard"). In a graceful shutdown sequence, any data that has been queued but not yet transmitted can be sent prior to the connection being closed. In an abortive shutdown, any unsent data is lost. The occurrence of a shutdown sequence (graceful or abortive) can also be used to provide an FD_CLOSE indication to the associated applications signifying that a shutdown is in progress.
Closing a socket, on the other hand, causes the socket handle to become deallocated so that the application can no longer reference or use the socket in any manner.
In Windows Sockets, both the shutdown function, and the WSASendDisconnect function can be used to initiate a shutdown sequence, while the closesocket function is used to deallocate socket handles and free up any associated resources. Some amount of confusion arises, however, from the fact that the closesocket function will implicitly cause a shutdown sequence to occur if it has not already happened. In fact, it has become a rather common programming practice to rely on this feature and use closesocket to both initiate the shutdown sequence and deallocate the socket handle.
To facilitate this usage, the sockets interface provides for controls by way of the socket option mechanism that allow the programmer to indicate whether the implicit shutdown sequence should be graceful or abortive, and also whether the closesocket function should linger (that is not complete immediately) to allow time for a graceful shutdown sequence to complete. These important distinctions and the ramifications of using closesocket in this manner have not been widely understood.
By establishing appropriate values for the socket options SO_LINGER and SO_DONTLINGER, the following types of behavior can be obtained with the closesocket function:
One technique that can be used to minimize the chance of problems occurring during connection teardown is to avoid relying on an implicit shutdown being initiated by closesocket. Instead, use one of the two explicit shutdown functions, shutdown or WSASendDisconnect. This in turn will cause an FD_CLOSE indication to be received by the peer application indicating that all pending data has been received. To illustrate this, the following table shows the functions that would be invoked by the client and server components of an application, where the client is responsible for initiating a graceful shutdown.
| Client Side | Server Side |
|---|---|
| (1) Invoke shutdown(s, SD_SEND) to signal end of session and that client has no more data to send. | |
| (2) Receive FD_CLOSE, indicating graceful shutdown in progress and that all data has been received. | |
| (3) Send any remaining response data. | |
| (5') Get FD_READ and invoke recv to get any response data sent by server | (4) Invoke shutdown(s, SD_SEND) to indicate server has no more data to send. |
| (5) Receive FD_CLOSE indication | (4') Invoke closesocket |
| (6) Invoke closesocket |
Note The timing sequence is maintained from step (1) to step (6) between the client and the server, except for step (4') and (5') which only has local timing significance in the sense that step (5) follows step (5') on the client side while step (4') follows step (4) on the server side, with no timing relationship with the remote party.