The recv function receives data from a connected socket or a bound connectionless socket.
int recv( SOCKET s, char *buf, int len, int flags );
The descriptor that identifies a connected socket.
A pointer to the buffer to receive the incoming data.
The length, in bytes, of the buffer pointed to by the buf parameter.
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.
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.
|A successful WSAStartup call must occur before using this function.|
|The network subsystem has failed.|
|The buf parameter is not completely contained in a valid part of the user address space.|
|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.|
|For a connection-oriented socket, this error indicates that the connection has been broken due to keep-alive activity that detected a failure while the operation was in progress. For a datagram socket, this error indicates that the time to live has expired.|
|The descriptor is not a socket.|
|MSG_OOB was specified, but the socket is not stream-style such as type SOCK_STREAM, OOB 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 receive on a socket after shutdown has been invoked with how set to SD_RECEIVE or SD_BOTH.|
|The socket is marked as nonblocking and the receive operation would block.|
|The message was too large to fit into the specified buffer and was truncated.|
|The socket has not been bound with bind, or an unknown flag was specified, or MSG_OOB was specified for a socket with SO_OOBINLINE enabled or (for byte stream sockets only) len was zero or negative.|
|The virtual circuit was terminated due to a time-out or other failure. The application should close the socket as it is no longer usable.|
|The connection has been dropped because of a network failure or because the peer system failed to respond.|
|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. On a UDP-datagram socket, this error would indicate that a previous send operation resulted in an ICMP "Port Unreachable" message.|
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.
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.
|MSG_PEEK||Peeks at the incoming data. The data is copied into the buffer, but is not removed from the input queue. The function subsequently returns the amount of data that can be read in a single call to the recv (or recvfrom) function, which may not be the same as the total amount of data queued on the socket. The amount of data that can actually be read in a single call to the recv (or recvfrom) function is limited to the data size written in the send or sendto function call.|
|MSG_OOB||Processes Out Of Band (OOB) data.|
|MSG_WAITALL||The receive request will complete only when one of the following events occurs:
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.
|Minimum supported client||Windows 8.1, Windows Vista [desktop apps | UWP apps]|
|Minimum supported server||Windows Server 2003 [desktop apps | UWP apps]|
|Header||winsock.h (include Winsock2.h)|