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WSARecvFrom function

The WSARecvFrom function receives a datagram and stores the source address.

Syntax


int WSARecvFrom(
  _In_     SOCKET s,
  _Inout_  LPWSABUF lpBuffers,
  _In_     DWORD dwBufferCount,
  _Out_    LPDWORD lpNumberOfBytesRecvd,
  _Inout_  LPDWORD lpFlags,
  _Out_    struct sockaddr *lpFrom,
  _Inout_  LPINT lpFromlen,
  _In_     LPWSAOVERLAPPED lpOverlapped,
  _In_     LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
);

Parameters

s [in]

A descriptor identifying a socket.

lpBuffers [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.

dwBufferCount [in]

The number of WSABUF structures in the lpBuffers array.

lpNumberOfBytesRecvd [out]

A pointer to the number of bytes received by this call if the WSARecvFrom operation completes immediately.

Use NULL for this parameter if the lpOverlapped parameter is not NULL to avoid potentially erroneous results. This parameter can be NULL only if the lpOverlapped parameter is not NULL.

lpFlags [in, out]

A pointer to flags used to modify the behavior of the WSARecvFrom function call. See remarks below.

lpFrom [out]

An optional pointer to a buffer that will hold the source address upon the completion of the overlapped operation.

lpFromlen [in, out]

A pointer to the size, in bytes, of the "from" buffer required only if lpFrom is specified.

lpOverlapped [in]

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

lpCompletionRoutine [in]

A pointer to the completion routine called when the WSARecvFrom operation has been completed (ignored for nonoverlapped sockets).

Return value

If no error occurs and the receive operation has completed immediately, WSARecvFrom returns zero. In this case, the completion routine will have already been scheduled 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 can 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 codeMeaning
WSAECONNRESET

The virtual circuit was reset by the remote side executing a hard or abortive close. The application should close the socket as it is no longer usable. For a UPD datagram socket, this error would indicate that a previous send operation resulted in an ICMP "Port Unreachable" message.

WSAEFAULT

The lpBuffers, lpFlags, lpFrom, lpNumberOfBytesRecvd, lpFromlen, lpOverlapped, or lpCompletionRoutine parameter is not totally contained in a valid part of the user address space: the lpFrom buffer was too small to accommodate the peer address.

WSAEINPROGRESS

A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.

WSAEINTR

A blocking Windows Socket 1.1 call was canceled through WSACancelBlockingCall.

WSAEINVAL

The socket has not been bound (with bind, for example).

WSAEMSGSIZE

The message was too large for the specified buffer and (for unreliable protocols only) any trailing portion of the message that did not fit into the buffer has been discarded.

WSAENETDOWN

The network subsystem has failed.

WSAENETRESET

For a datagram socket, this error indicates that the time to live has expired.

WSAENOTCONN

The socket is not connected (connection-oriented sockets only).

WSAEWOULDBLOCK

Windows NT:  

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.

WSANOTINITIALISED

A successful WSAStartup call must occur before using this function.

WSA_IO_PENDING

An overlapped operation was successfully initiated and completion will be indicated later.

WSA_OPERATION_ABORTED

The overlapped operation has been canceled due to the closure of the socket.

 

Remarks

The WSARecvFrom function provides functionality over and above the standard recvfrom function in three important areas:

  • It can be used in conjunction with overlapped sockets to perform overlapped receive operations.
  • It allows multiple receive buffers to be specified making it applicable to the scatter/gather type of I/O.
  • The lpFlags parameter is both an input and an output parameter, allowing applications to sense the output state of the MSG_PARTIAL flag bit. Be aware that the MSG_PARTIAL flag bit is not supported by all protocols.

The WSARecvFrom function is used primarily on a connectionless socket specified by s. The socket's local address must be known. For server applications, this is usually done explicitly through bind. Explicit binding is discouraged for client applications. For client applications using this function the socket can become bound implicitly to a local address through sendto, WSASendTo, or WSAJoinLeaf.

For overlapped sockets, this function is used to post one or more buffers into which incoming data will be placed as it becomes available on a (possibly connected) socket, 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. Also, the values indicated by lpFrom and lpFromlen are not updated until completion is itself indicated. Applications must not use or disturb these values until they have been updated, therefore the application must not use automatic (that is, stack-based) variables for these parameters.

Note  All I/O initiated by a given thread is canceled when that thread exits. For overlapped sockets, pending asynchronous operations can fail if the thread is closed before the operations complete. See ExitThread for more information.

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 WSARecv function and the lpOverlapped and lpCompletionRoutine parameters are ignored. Any data that has already been received and buffered by the transport will be copied into the 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 buffers are filled in the order in which they appear in the array indicated by lpBuffers, and the buffers are packed so that no holes are created.

If this function is completed in an overlapped manner, it is the Winsock service provider's responsibility to capture the WSABUF structures before returning from this call. This enables applications to build stack-based WSABUF arrays pointed to by the lpBuffers parameter.

For connectionless socket types, the address from which the data originated is copied to the buffer indicated by lpFrom. The value pointed to by lpFromlen is initialized to the size of this buffer, and is modified on completion to indicate the actual size of the address stored there. As stated previously for overlapped sockets, the lpFrom and lpFromlen parameters are not updated until after the overlapped I/O has completed. The memory pointed to by these parameters must, therefore, remain available to the service provider and cannot be allocated on the application stack frame. The lpFrom and lpFromlen parameters are ignored for connection-oriented sockets.

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.
  • The 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, an incoming message is placed into the buffers up to the total size of the buffers, and the completion indication occurs for overlapped sockets. If the message is larger than the buffers, 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 operation(s) will retrieve the rest of the message. If MSG_PARTIAL is not supported, but the protocol is reliable, WSARecvFrom 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 WSARecvFrom generates the error WSAEMSGSIZE.

The lpFlags parameter can 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 using the bitwise OR operator with any of any of the values listed in the following table.

ValueMeaning
MSG_PEEKPreviews 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.
MSG_OOBProcesses OOB data.
MSG_PARTIALThis flag is for message-oriented sockets only. On output, this flag indicates that the data is a portion of the message transmitted by the sender. Remaining portions of the message will be transmitted in subsequent receive operations. A subsequent receive operation with MSG_PARTIAL flag cleared indicates the end of the 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.

Note  When issuing a blocking Winsock call such as WSARecvFrom with the lpOverlapped parameter set to NULL, 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.

Overlapped Socket I/O

If an overlapped operation completes immediately, WSARecvFrom 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, WSARecvFrom returns SOCKET_ERROR and indicates error code WSA_IO_PENDING. In this case, lpNumberOfBytesRecvd and lpFlags is 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 either through the dwFlags parameter of the completion routine, or by examining the lpdwFlags parameter of WSAGetOverlappedResult.

The WSARecvFrom function can 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 WSAOVERLAPPED structure.

If the lpCompletionRoutine parameter is NULL, the hEvent parameter 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 parameter is ignored and can be used by the application to pass context information to the completion routine. A caller that passes a non-NULL lpCompletionRoutine and later calls WSAGetOverlappedResult for the same overlapped I/O request may not set the fWait parameter for that invocation of WSAGetOverlappedResult to TRUE. In this case the usage of the hEvent parameter is undefined, and attempting to wait on the hEvent parameter would produce unpredictable results.

The completion routine follows the same rules as stipulated for Windows 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.



void CALLBACK CompletionROUTINE(
  IN DWORD dwError, 
  IN DWORD cbTransferred, 
  IN LPWSAOVERLAPPED lpOverlapped, 
  IN DWORD dwFlags
);


The CompletionRoutine is a placeholder for an application-defined or library-defined function name. The dwError specifies the completion status for the overlapped operation as indicated by lpOverlapped. The cbTransferred specifies the number of bytes received. The dwFlags parameter 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 can 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 specified.

Example Code

The following example demonstrates the use of the WSARecvFrom function.


#ifndef UNICODE
#define UNICODE
#endif

#define WIN32_LEAN_AND_MEAN

#include <winsock2.h>
#include <Ws2tcpip.h>
#include <stdio.h>

int __cdecl main()
{

    WSADATA wsaData;
    WSABUF DataBuf;
    WSAOVERLAPPED Overlapped;

    SOCKET RecvSocket = INVALID_SOCKET;
    struct sockaddr_in RecvAddr;
    struct sockaddr_in SenderAddr;

    int SenderAddrSize = sizeof (SenderAddr);
    u_short Port = 27015;

    char RecvBuf[1024];
    int BufLen = 1024;
    DWORD BytesRecv = 0;
    DWORD Flags = 0;

    int err = 0;
    int rc;
    int retval = 0;
    
    //-----------------------------------------------
    // Initialize Winsock
    rc = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (rc != 0) {
        /* Could not find a usable Winsock DLL */
        wprintf(L"WSAStartup failed with error: %ld\n", rc);
        return 1;
    }

    // Make sure the Overlapped struct is zeroed out
    SecureZeroMemory((PVOID) &Overlapped, sizeof(WSAOVERLAPPED) );

    // Create an event handle and setup the overlapped structure.
    Overlapped.hEvent = WSACreateEvent();
    if (Overlapped.hEvent == NULL) {
        wprintf(L"WSACreateEvent failed with error: %d\n", WSAGetLastError());
        WSACleanup();
        return 1;
    }
    //-----------------------------------------------
    // Create a receiver socket to receive datagrams
    RecvSocket = WSASocket(AF_INET,
                           SOCK_DGRAM,
                           IPPROTO_UDP, NULL, 0, WSA_FLAG_OVERLAPPED);

    if (RecvSocket == INVALID_SOCKET) {
        /* Could not open a socket */
        wprintf(L"WSASocket failed with error: %ld\n", WSAGetLastError());
        WSACloseEvent(Overlapped.hEvent);
        WSACleanup();
        return 1;
    }
    //-----------------------------------------------
    // Bind the socket to any address and the specified port.
    RecvAddr.sin_family = AF_INET;
    RecvAddr.sin_port = htons(Port);
    RecvAddr.sin_addr.s_addr = htonl(INADDR_ANY);

    rc = bind(RecvSocket, (SOCKADDR *) & RecvAddr, sizeof (RecvAddr));
    if (rc != 0) {
        /* Bind to the socket failed */
        wprintf(L"bind failed with error: %ld\n", WSAGetLastError());
        WSACloseEvent(Overlapped.hEvent);
        closesocket(RecvSocket);
        WSACleanup();
        return 1;
    }

    //-----------------------------------------------
    // Call the recvfrom function to receive datagrams
    // on the bound socket.
    DataBuf.len = BufLen;
    DataBuf.buf = RecvBuf;
    wprintf(L"Listening for incoming datagrams on port=%d\n", Port);
    rc = WSARecvFrom(RecvSocket,
                      &DataBuf,
                      1,
                      &BytesRecv,
                      &Flags,
                      (SOCKADDR *) & SenderAddr,
                      &SenderAddrSize, &Overlapped, NULL);

    if (rc != 0) {
        err = WSAGetLastError();
        if (err != WSA_IO_PENDING) {
            wprintf(L"WSARecvFrom failed with error: %ld\n", err);
            WSACloseEvent(Overlapped.hEvent);
            closesocket(RecvSocket);
            WSACleanup();
            return 1;
        }
        else {
            rc = WSAWaitForMultipleEvents(1, &Overlapped.hEvent, TRUE, INFINITE, TRUE);
            if (rc == WSA_WAIT_FAILED) {
                wprintf(L"WSAWaitForMultipleEvents failed with error: %d\n", WSAGetLastError());
                retval = 1;
            }

            rc = WSAGetOverlappedResult(RecvSocket, &Overlapped, &BytesRecv,
                                FALSE, &Flags);
            if (rc == FALSE) {
                wprintf(L"WSArecvFrom failed with error: %d\n", WSAGetLastError());
                retval = 1;
            }
            else
                wprintf(L"Number of received bytes = %d\n", BytesRecv);
                
            wprintf(L"Finished receiving. Closing socket.\n");
        }
        
    }
    //---------------------------------------------
    // When the application is finished receiving, close the socket.

    WSACloseEvent(Overlapped.hEvent);
    closesocket(RecvSocket);
    wprintf(L"Exiting.\n");

    //---------------------------------------------
    // Clean up and quit.
    WSACleanup();
    return (retval);
}


Windows Phone 8: This API is supported.

Requirements

Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]

Header

Winsock2.h

Library

Ws2_32.lib

DLL

Ws2_32.dll

See also

sendto
Winsock Reference
Winsock Functions
WSABUF
WSACloseEvent
WSACreateEvent
WSAGetOverlappedResult
WSAOVERLAPPED
WSASend
WSASendTo
WSASocket
WSAWaitForMultipleEvents

 

 

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