The recv function receives data from a connected socket or a bound connectionless socket.

Syntax

int recv(
  __in   SOCKET s,
  __out  char *buf,
  __in   int len,
  __in   int flags
);

Parameters

s [in]

The descriptor that identifies a connected socket.

buf [out]

A pointer to the buffer to receive the incoming data.

len [in]

The length, in bytes, of the buffer pointed to by the buf parameter.

flags [in]

A set of flags that influences the behavior of this function. See remarks below. See the Remarks section for details on the possible value for this parameter.

Return Value

If no error occurs, recv returns the number of bytes received and the buffer pointed to by the buf parameter will contain this data received. If the connection has been gracefully closed, the return value is zero.

Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError.

Remarks

The recv function is used to read incoming data on connection-oriented sockets, or connectionless sockets. When using a connection-oriented protocol, the sockets must be connected before calling recv. When using a connectionless protocol, the sockets must be bound before calling recv.

The local address of the socket must be known. For server applications, use an explicit bind function or an implicit accept or WSAAccept function. Explicit binding is discouraged for client applications. For client applications, the socket can become bound implicitly to a local address using connect, WSAConnect, sendto, WSASendTo, or WSAJoinLeaf.

For connected or connectionless sockets, the recv function restricts the addresses from which received messages are accepted. The function only returns messages from the remote address specified in the connection. Messages from other addresses are (silently) discarded.

For connection-oriented sockets (type SOCK_STREAM for example), calling recv will return as much data as is currently available—up to the size of the buffer specified. If the socket has been configured for in-line reception of OOB data (socket option SO_OOBINLINE) and OOB data is yet unread, only OOB data will be returned. The application can use the ioctlsocket or WSAIoctlSIOCATMARK command to determine whether any more OOB data remains to be read.

For connectionless sockets (type SOCK_DGRAM or other message-oriented sockets), data is extracted from the first enqueued datagram (message) from the destination address specified by the connect function.

If the datagram or message is larger than the buffer specified, the buffer is filled with the first part of the datagram, and recv generates the error WSAEMSGSIZE. For unreliable protocols (for example, UDP) the excess data is lost; for reliable protocols, the data is retained by the service provider until it is successfully read by calling recv with a large enough buffer.

If no incoming data is available at the socket, the recv call blocks and waits for data to arrive according to the blocking rules defined for WSARecv with the MSG_PARTIAL flag not set unless the socket is nonblocking. In this case, a value of SOCKET_ERROR is returned with the error code set to WSAEWOULDBLOCK. The select, WSAAsyncSelect, or WSAEventSelect functions can be used to determine when more data arrives.

If the socket is connection oriented and the remote side has shut down the connection gracefully, and all data has been received, a recv will complete immediately with zero bytes received. If the connection has been reset, a recv will fail with the error WSAECONNRESET.

Note  When issuing a blocking Winsock call, such as send, recv, select, accept, or connect function calls, 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, and thereby create unspecified results. Do not issue blocking Winsock function calls without being certain that the current thread is not waiting inside another blocking Winsock function call; doing so results in unpredictable behavior.

The flags parameter can be used to influence the behavior of the function invocation beyond the options specified for the associated socket. The semantics of this function are determined by the socket options and the flags parameter. The possible value of flags parameter is constructed by using the bitwise OR operator with any of the following values.

Example Code

The following code example shows the use of the recv function.

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

#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "27015"

int __cdecl main() {

    //----------------------
    // Declare and initialize variables.
    WSADATA wsaData;
    int iResult;

    SOCKET ConnectSocket;
    struct sockaddr_in clientService; 

    char *sendbuf = "this is a test";
    char recvbuf[DEFAULT_BUFLEN];
    int recvbuflen = DEFAULT_BUFLEN;
  
    //----------------------
    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
    if (iResult != NO_ERROR) {
      printf("WSAStartup failed: %d\n", iResult);
      return 1;
    }

    //----------------------
    // Create a SOCKET for connecting to server
    ConnectSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (ConnectSocket == INVALID_SOCKET) {
        printf("Error at socket(): %ld\n", WSAGetLastError() );
        WSACleanup();
        return 1;
    }

    //----------------------
    // The sockaddr_in structure specifies the address family,
    // IP address, and port of the server to be connected to.
    clientService.sin_family = AF_INET;
    clientService.sin_addr.s_addr = inet_addr( "127.0.0.1" );
    clientService.sin_port = htons( 27015 );

    //----------------------
    // Connect to server.
    iResult = connect( ConnectSocket, (SOCKADDR*) &clientService, sizeof(clientService) );
    if ( iResult == SOCKET_ERROR) {
        closesocket (ConnectSocket);
        printf("Unable to connect to server: %ld\n", WSAGetLastError());
        WSACleanup();
        return 1;
    }

    // Send an initial buffer
    iResult = send( ConnectSocket, sendbuf, (int)strlen(sendbuf), 0 );
    if (iResult == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    printf("Bytes Sent: %ld\n", iResult);

    // shutdown the connection since no more data will be sent
    iResult = shutdown(ConnectSocket, SD_SEND);
    if (iResult == SOCKET_ERROR) {
        printf("shutdown failed: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    // Receive until the peer closes the connection
    do {

        iResult = recv(ConnectSocket, recvbuf, recvbuflen, 0);
        if ( iResult > 0 )
            printf("Bytes received: %d\n", iResult);
        else if ( iResult == 0 )
            printf("Connection closed\n");
        else
            printf("recv failed: %d\n", WSAGetLastError());

    } while( iResult > 0 );

    // cleanup
    closesocket(ConnectSocket);
    WSACleanup();

    return 0;
}

Example Code

For more information, and another example of the recv function, see Getting Started With Winsock.

Requirements

See Also

Winsock Reference

Winsock Functions

recvfrom

select

send

socket

WSAAsyncSelect

WSARecv

WSARecvEx

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Build date: 11/12/2009