The Windows Sockets WSARecv function receives data from a socket.

int WSARecv (


LPWSABUF lpBuffers,

DWORD dwBufferCount,

LPDWORD lpNumberOfBytesRecvd,

LPDWORD lpFlags,






[in] A descriptor identifying a connected socket.


[in/out] A pointer to an array of WSABUF structures. Each WSABUF structure contains a pointer to a buffer and the length of the buffer.


[in] The number of WSABUF structures in the lpBuffers array.


[out] A pointer to the number of bytes received by this call if the receive operation completes immediately.


[in/out] A pointer to flags.


[in] A pointer to a WSAOVERLAPPED structure (ignored for nonoverlapped sockets).


[in] A pointer to the completion routine called when the receive operation has been completed (ignored for nonoverlapped sockets).


This function provides functionality over and above the standard recv function in three important areas:

  1. It can be used in conjunction with overlapped sockets to perform overlapped receive operations.

  2. It allows multiple receive buffers to be specified making it applicable to the scatter/gather type of I/O.

  3. The lpFlags parameter is both an INPUT and an OUTPUT parameter, allowing applications to sense the output state of the MSG_PARTIAL flag bit. Note however, that the MSG_PARTIAL flag bit is not supported by all protocols.

WSARecv is used on connected sockets or bound connectionless sockets specified by the s parameter and is used to read incoming data.

For overlapped sockets WSARecv is used to post one or more buffers into which incoming data will be placed as it becomes available, after which the application-specified completion indication (invocation of the completion routine or setting of an event object) occurs. If the operation does not complete immediately, the final completion status is retrieved through the completion routine or WSAGetOverlappedResult.

If both lpOverlapped and lpCompletionRoutine are NULL, the socket in this function will be treated as a nonoverlapped socket.

For nonoverlapped sockets, the blocking semantics are identical to that of the standard recv function and the lpOverlapped and lpCompletionRoutine parameters are ignored. Any data which has already been received and buffered by the transport will be copied into the supplied user buffers. For the case of a blocking socket with no data currently having been received and buffered by the transport, the call will block until data is received.

The supplied buffers are filled in the order in which they appear in the array pointed to by lpBuffers, and the buffers are packed so that no holes are created.

The array of WSABUF structures pointed to by the lpBuffers parameter is transient. If this operation completes in an overlapped manner, it is the service provider's responsibility to capture these WSABUF structures before returning from this call. This enables applications to build stack-based WSABUF arrays.

For byte stream style sockets (for example, type SOCK_STREAM), incoming data is placed into the buffers until the buffers are filled, the connection is closed, or internally buffered data is exhausted. Regardless of whether or not the incoming data fills all the buffers, the completion indication occurs for overlapped sockets. For message-oriented sockets (for example, type SOCK_DGRAM), an incoming message is placed into the supplied buffers, up to the total size of the buffers supplied, and the completion indication occurs for overlapped sockets. If the message is larger than the buffers supplied, the buffers are filled with the first part of the message. If the MSG_PARTIAL feature is supported by the underlying service provider, the MSG_PARTIAL flag is set in lpFlags and subsequent receive operations will retrieve the rest of the message. If MSG_PARTIAL is not supported but the protocol is reliable, WSARecv generates the error WSAEMSGSIZE and a subsequent receive operation with a larger buffer can be used to retrieve the entire message. Otherwise, (that is, the protocol is unreliable and does not support MSG_PARTIAL), the excess data is lost, and WSARecv generates the error WSAEMSGSIZE.

For connection-oriented sockets, WSARecv can indicate the graceful termination of the virtual circuit in one of two ways, depending on whether the socket is a byte stream or message oriented. For byte streams, zero bytes having been read indicates graceful closure and that no more bytes will ever be read. For message-oriented sockets, where a zero byte message is often allowable, a return error code of WSAEDISCON is used to indicate graceful closure. In any case a return error code of WSAECONNRESET indicates an abortive close has occurred.

lpFlags may be used to influence the behavior of the function invocation beyond the options specified for the associated socket. That is, the semantics of this function are determined by the socket options and the lpFlags parameter. The latter is constructed by or-ing any of the following values:

Peek at the incoming data. The data is copied into the buffer but is not removed from the input queue. This flag is valid only for nonoverlapped sockets.
Process out-of-band data. (See section Out-Of-Band data for a discussion of this topic.)
This flag is for message-oriented sockets only. On output, indicates that the data supplied is a portion of the message transmitted by the sender. Remaining portions of the message will be supplied in subsequent receive operations. A subsequent receive operation with MSG_PARTIAL flag cleared indicates end of sender's message.
As an input parameter, this flag indicates that the receive operation should complete even if only part of a message has been received by the service provider.

For message-oriented sockets, the MSG_PARTIAL bit is set in the lpFlags parameter if a partial message is received. If a complete message is received, MSG_PARTIAL is cleared in lpFlags. In the case of delayed completion, the value pointed to by lpFlags is not updated. When completion has been indicated the application should call WSAGetOverlappedResult and examine the flags pointed to by the lpdwFlags parameter.

Overlapped socket I/O:

If an overlapped operation completes immediately, WSARecv returns a value of zero and the lpNumberOfBytesRecvd parameter is updated with the number of bytes received and the flag bits pointed by the lpFlags parameter are also updated. If the overlapped operation is successfully initiated and will complete later, WSARecv returns SOCKET_ERROR and indicates error code WSA_IO_PENDING. In this case, lpNumberOfBytesRecvd and lpFlags are not updated. When the overlapped operation completes the amount of data transferred is indicated either through the cbTransferred parameter in the completion routine (if specified), or through the lpcbTransfer parameter in WSAGetOverlappedResult. Flag values are obtained by examining the lpdwFlags parameter of WSAGetOverlappedResult.

This function may be called from within the completion routine of a previous WSARecv, WSARecvFrom, WSASend or WSASendTo function. For a given socket, I/O completion routines will not be nested. This permits time-sensitive data transmissions to occur entirely within a preemptive context.

The lpOverlapped parameter must be valid for the duration of the overlapped operation. If multiple I/O operations are simultaneously outstanding, each must reference a separate overlapped structure. The WSAOVERLAPPED structure has the following form:

typedef struct _WSAOVERLAPPED {

DWORD Internal; // reserved

DWORD InternalHigh; // reserved

DWORD Offset; // reserved

DWORD OffsetHigh; // reserved



If the lpCompletionRoutine parameter is NULL, the hEvent field of lpOverlapped is signaled when the overlapped operation completes if it contains a valid event object handle. An application can use WSAWaitForMultipleEvents or WSAGetOverlappedResult to wait or poll on the event object.

If lpCompletionRoutine is not NULL, the hEvent field is ignored and can be used by the application to pass context information to the completion routine.

The completion routine follows the same rules as stipulated for Win32 file I/O completion routines. The completion routine will not be invoked until the thread is in an alertable wait state such as can occur when the function WSAWaitForMultipleEvents with the fAlertable parameter set to TRUE is invoked.

The transport providers allow an application to invoke send and receive operations from within the context of the socket I/O completion routine, and guarantee that, for a given socket, I/O completion routines will not be nested. This permits time-sensitive data transmissions to occur entirely within a preemptive context.

The prototype of the completion routine is as follows:

  • oid CALLBACK CompletionROUTINE(
    IN DWORD dwFlags

CompletionRoutine is a placeholder for an application-defined or library-defined function name. dwError specifies the completion status for the overlapped operation as indicated by lpOverlapped. cbTransferred specifies the number of bytes received. dwFlags contains information that would have appeared in lpFlags if the receive operation had completed immediately. This function does not return a value.

Returning from this function allows invocation of another pending completion routine for this socket. When using WSAWaitForMultipleEvents, all waiting completion routines are called before the alertable thread's wait is satisfied with a return code of WSA_IO_COMPLETION. The completion routines may be called in any order, not necessarily in the same order the overlapped operations are completed. However, the posted buffers are guaranteed to be filled in the same order they are supplied.

Return Values

If no error occurs and the receive operation has completed immediately, WSARecv returns zero. Note that in this case, the completion routine will have already been scheduled, and to be called once the calling thread is in the alertable state. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code may be retrieved by calling WSAGetLastError. The error code WSA_IO_PENDING indicates that the overlapped operation has been successfully initiated and that completion will be indicated at a later time. Any other error code indicates that the overlapped operation was not successfully initiated and no completion indication will occur.

Error Codes

A successful WSAStartup must occur before using this function.
The network subsystem has failed.
The socket is not connected.
The (blocking) call was canceled through WSACancelBlockingCall.
A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
The connection has been broken due to the remote host resetting.
The descriptor is not a socket.
The lpBuffers argument is not totally contained in a valid part of the user address space.
MSG_OOB was specified, but the socket is not stream style such as type SOCK_STREAM, out-of-band data is not supported in the communication domain associated with this socket, or the socket is unidirectional and supports only send operations.
The socket has been shut down; it is not possible to WSARecv on a socket after shutdown has been invoked with how set to SD_RECEIVE or SD_BOTH.
Overlapped sockets: There are too many outstanding overlapped I/O requests. Nonoverlapped sockets: The socket is marked as nonblocking and the receive operation cannot be completed immediately.
The message was too large to fit into the specified buffer and (for unreliable protocols only) any trailing portion of the message that did not fit into the buffer has been discarded.
The socket has not been bound with bind, or the socket is not created with the overlapped flag.
The virtual circuit was terminated due to a time-out or other failure.
The virtual circuit was reset by the remote side.
Socket s is message oriented and the virtual circuit was gracefully closed by the remote side.
An overlapped operation was successfully initiated and completion will be indicated at a later time.
The overlapped operation has been canceled due to the closure of the socket.

See Also

, WSACreateEvent, WSAGetOverlappedResult, WSASocket, WSAWaitForMultipleEvents

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