closesocket function (winsock.h)

Article
08/19/2022

The closesocket function closes an existing socket.

Syntax

int closesocket(
  [in] SOCKET s
);

Parameters

[in] s

A descriptor identifying the socket to close.

Return value

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.

Error code	Meaning
WSANOTINITIALISED	A successful WSAStartup call must occur before using this function.
WSAENETDOWN	The network subsystem has failed.
WSAENOTSOCK	The descriptor is not a socket.
WSAEINPROGRESS	A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
WSAEINTR	The (blocking) Windows Socket 1.1 call was canceled through WSACancelBlockingCall.
WSAEWOULDBLOCK	The socket is marked as nonblocking, but the l_onoff member of the linger structure is set to nonzero and the l_linger member of the linger structure is set to a nonzero timeout value.

Remarks

The closesocket function closes a socket. Use it to release the socket descriptor passed in the s parameter. Note that the socket descriptor passed in the s parameter may immediately be reused by the system as soon as closesocket function is issued. As a result, it is not reliable to expect further references to the socket descriptor passed in the s parameter to fail with the error WSAENOTSOCK. A Winsock client must never issue closesocket on s concurrently with another Winsock function call.

Any pending overlapped send and receive operations ( WSASend/ WSASendTo/ WSARecv/ WSARecvFrom with an overlapped socket) issued by any thread in this process are also canceled. Any event, completion routine, or completion port action specified for these overlapped operations is performed. The pending overlapped operations fail with the error status WSA_OPERATION_ABORTED.

An application should not assume that any outstanding I/O operations on a socket will all be guaranteed to completed when closesocket returns. The closesocket function will initiate cancellation on the outstanding I/O operations, but that does not mean that an application will receive I/O completion for these I/O operations by the time the closesocket function returns. Thus, an application should not cleanup any resources (WSAOVERLAPPED structures, for example) referenced by the outstanding I/O requests until the I/O requests are indeed completed.

An application should always have a matching call to closesocket for each successful call to socket to return any socket resources to the system.

The linger structure maintains information about a specific socket that specifies how that socket should behave when data is queued to be sent and the closesocket function is called on the socket.

The l_onoff member of the linger structure determines whether a socket should remain open for a specified amount of time after a closesocket function call to enable queued data to be sent. This member can be modified in two ways:

Call the setsockopt function with the optname parameter set to SO_DONTLINGER. The optval parameter determines how the l_onoff member is modified.
Call the setsockopt function with the optname parameter set to SO_LINGER. The optval parameter specifies how both the l_onoff and l_linger members are modified.

The l_linger member of the linger structure determines the amount of time, in seconds, a socket should remain open. This member is only applicable if the l_onoff member of the linger structure is nonzero.

The default parameters for a socket are the l_onoff member of the linger structure is zero, indicating that the socket should not remain open. The default value for the l_linger member of the linger structure is zero, but this value is ignored when the l_onoff member is set to zero.

To enable a socket to remain open, an application should set the l_onoff member to a nonzero value and set the l_linger member to the desired timeout in seconds. To disable a socket from remaining open, an application only needs to set the l_onoff member of the linger structure to zero.

If an application calls the setsockopt function with the optname parameter set to SO_DONTLINGER to set the l_onoff member to a nonzero value, the value for the l_linger member is not specified. In this case, the timeout used is implementation dependent. If a previous timeout has been established for a socket (by previously calling the setsockopt function with the optname parameter set to SO_LINGER), this timeout value should be reinstated by the service provider.

The semantics of the closesocket function are affected by the socket options that set members of linger structure.

l_onoff	l_linger	Type of close	Wait for close?
zero	Do not care	Graceful close	No
nonzero	zero	Hard	No
nonzero	nonzero	Graceful if all data is sent within timeout value specified in the l_linger member. Hard if all data could not be sent within timeout value specified in the l_linger member.	Yes

If the l_onoff member of the LINGER structure is zero on a stream socket, the closesocket call will return immediately and does not receive WSAEWOULDBLOCK whether the socket is blocking or nonblocking. However, any data queued for transmission will be sent, if possible, before the underlying socket is closed. This is also called a graceful disconnect or close. In this case, the Windows Sockets provider cannot release the socket and other resources for an arbitrary period, thus affecting applications that expect to use all available sockets. This is the default behavior for a socket.

If the l_onoff member of the linger structure is nonzero and l_linger member 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. On Windows, any recv call on the remote side of the circuit will fail with WSAECONNRESET.

If the l_onoff member of the linger structure is set to nonzero and l_linger member is set to a nonzero timeout on a blocking socket, the closesocket call blocks until the remaining data has been sent or until the timeout expires. This is called a graceful disconnect or close if all of the data is sent within timeout value specified in the l_linger member. If the timeout expires before all data has been sent, the Windows Sockets implementation terminates the connection before closesocket returns and this is called a hard or abortive close.

Setting the l_onoff member of the linger structure to nonzero and the l_linger member with a nonzero timeout 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.

If the l_onoff member of the linger structure is nonzero and the l_linger member is a nonzero timeout interval on a blocking socket, the result of the closesocket function can't be used to determine whether all data has been sent to the peer. If the data is sent before the timeout specified in the l_linger member expires or if the connection was aborted, the closesocket function won't return an error code (the return value from the closesocket function is zero).

The closesocket call will only block until all data has been delivered to the peer or the timeout expires. If the connection is reset because the timeout expires, then the socket will not go into TIME_WAIT state. If all data is sent within the timeout period, then the socket can go into TIME_WAIT state.

If the l_onoff member of the linger structure is nonzero and the l_linger member is a zero timeout interval on a blocking socket, then a call to closesocket will reset the connection. The socket will not go to the TIME_WAIT state.

The getsockopt function can be called with the optname parameter set to SO_LINGER to retrieve the current value of the linger structure associated with a socket.

Note 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, an FD_CLOSE network event is not posted after closesocket is called.

Here is a summary of closesocket behavior:

If the l_onoff member of the LINGER structure is zero (the default for a socket), closesocket returns immediately and the connection is gracefully closed in the background.
If the l_onoff member of the linger structure is set to nonzero and the l_linger member is set to zero (no timeout) closesocket returns immediately and the connection is reset or terminated.
If the l_onoff member of the linger structure is set to nonzero and the l_linger member is set to a nonzero timeout:– For a blocking socket, closesocket blocks until all data is sent or the timeout expires.
– For a nonblocking socket, closesocket returns immediately indicating failure.

For additional information please see Graceful Shutdown, Linger Options, and Socket Closure for more information.

Note When issuing a blocking Winsock call such as closesocket, Winsock may need to wait for a network event before the call can complete. Winsock performs an alertable wait in this situation, which can be interrupted by an asynchronous procedure call (APC) scheduled on the same thread. Issuing another blocking Winsock call inside an APC that interrupted an ongoing blocking Winsock call on the same thread will lead to undefined behavior, and must never be attempted by Winsock clients.

Notes for IrDA Sockets

Keep the following in mind:

The Af_irda.h header file must be explicitly included.
The standard linger options are supported.
Although IrDA does not provide a graceful close, IrDA will defer closing until receive queues are purged. Thus, an application can send data and immediately call the socket function, and be confident that the receiver will copy the data before receiving an FD_CLOSE message.

Notes for ATM

The following are important issues associated with connection teardown when using Asynchronous Transfer Mode (ATM) and Windows Sockets 2:

Using the closesocket or shutdown functions with SD_SEND or SD_BOTH results in a RELEASE signal being sent out on the control channel. Due to ATM's use of separate signal and data channels, it is possible that a RELEASE signal could reach the remote end before the last of the data reaches its destination, resulting in a loss of that data. One possible solutions is programming a sufficient delay between the last data sent and the closesocket or shutdown function calls for an ATM socket.
Half close is not supported by ATM.
Both abortive and graceful disconnects result in a RELEASE signal being sent out with the same cause field. In either case, received data at the remote end of the socket is still delivered to the application. See Graceful Shutdown, Linger Options, and Socket Closure for more information.

Windows Phone 8: This function is supported for Windows Phone Store apps on Windows Phone 8 and later.

Windows 8.1 and Windows Server 2012 R2: This function is supported for Windows Store apps on Windows 8.1, Windows Server 2012 R2, and later.

Requirements

Requirement	Value
Minimum supported client	Windows 8.1, Windows Vista [desktop apps \| UWP apps]
Minimum supported server	Windows Server 2003 [desktop apps \| UWP apps]
Target Platform	Windows
Header	winsock.h (include Winsock2.h)
Library	Ws2_32.lib
DLL	Ws2_32.dll