The Windows Sockets closesocket
function closes a socket.
int closesocket (
[in] A descriptor identifying a socket.
This function closes a socket. More precisely, it releases the socket
, so that further references to s
will fail with the error WSAENOTSOCK. If this is the last reference to an
underlying socket, the associated naming information and queued data are
discarded. Any pending blocking, asynchronous calls issued by any thread in this process
are canceled without posting any notification messages, or signaling any event
objects. Any pending overlapped send and receive operations (WSASend
with an overlapped socket) issued by any thread in this process are also
canceled without setting the event object or invoking the completion routine, if
specified. In this case, the pending overlapped operations fail with the error
status WSA_OPERATION_ABORTED. An application should always have a matching call
for each successful call to socket
to return socket resources to the system.
The semantics of closesocket
are affected by the socket options SO_LINGER and SO_DONTLINGER as follows
(Note: by default SO_DONTLINGER is enabled. That is, SO_LINGER is disabled):
||Type of close
||Wait for close?
If SO_LINGER is set (that is, the l_onoff
field of the linger structure is nonzero; see Multipoint and Multicast Semantics
) with a zero time-out interval (l_linger
is zero), closesocket
is not blocked even if queued data has not yet been sent or acknowledged.
This is called a "hard" or "abortive" close, because the socket's virtual circuit
is reset immediately, and any unsent data is lost. Any recv
call on the remote side of the circuit will fail with WSAECONNRESET.
If SO_LINGER is set with a nonzero time-out interval on a blocking socket, the closesocket
call blocks on a blocking socket until the remaining data has been sent or
until the time-out expires. This is called a graceful disconnect. If the time-out
expires before all data has been sent, the Windows Sockets implementation
terminates the connection before closesocket
Enabling SO_LINGER with a nonzero time-out interval on a nonblocking socket is
not recommended. In this case, the call to closesocket
will fail with an error of WSAEWOULDBLOCK if the close operation cannot be
completed immediately. If closesocket
fails with WSAEWOULDBLOCK the socket handle is still valid, and a disconnect
is not initiated. The application must call closesocket
again to close the socket, although closesocket
can continue to fail unless the application disables SO_DONTLINGER, enables
SO_LINGER with a zero time-out, or calls shutdown
to initiate closure.
If SO_DONTLINGER is set on a stream socket (that is, the l_onoff
field of the linger structure is zero; see Multipoint and Multicast Semantics
) the closesocket
call will return immediately. However, any data queued for transmission will
be sent if possible before the underlying socket is closed. This is also called
a graceful disconnect. Note that in this case, the Windows Sockets provider
cannot release the socket and other resources for an arbitrary period, which can
affect applications which expect to use all available sockets. This is the
To assure that all data is sent and received on a connection, an application
should call shutdown
before calling closesocket
(see Graceful shutdown, linger options and socket closure
for more information). Also note, FD_CLOSE will not be posted after closesocket
Here is a summary of closesocket
- if SO_DONTLINGER enabled (the default setting) it always returns immediately connection is gracefully closed "in the background"
- if SO_LINGER enabled with a zero time-out: it always returns immediately -
connection is reset/terminated
- if SO_LINGER enabled with nonzero time-out:
with blocking socket it blocks until all data sent or time-out expires
with nonblocking socket it returns immediately indicating failure
For additional information please see Graceful shutdown, linger options and socket closure
for more information.
If no error occurs, closesocket
returns zero. Otherwise, a value of SOCKET_ERROR is returned, and a specific
error code can be retrieved by calling WSAGetLastError
||A successful WSAStartup must occur before using this function.
||The network subsystem has failed.
||The descriptor is not a socket.
||A blocking Windows Sockets 1.1 call is in progress, or the service provider is
still processing a callback function.
||The (blocking) call was canceled through WSACancelBlockingCall.
||The socket is marked as nonblocking and SO_LINGER is set to a nonzero time-out
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