If no error occurs, this function returns the number of bytes received. If the connection has been gracefully closed, the return value is zero. If an error occurs, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError.

The following table shows a list of possible error codes.

This function reads incoming data on both connected and unconnected sockets and captures the address from which the data was sent. The local address of the socket must be known. For server applications, this is usually done explicitly through bind (Windows Sockets). 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, or WSASendTo.

For stream-oriented sockets such as those of type SOCK_STREAM, a call to recvfromreturns as much information as is currently available — up to the size of the buffer supplied. If the socket has been configured for inline reception of out of band (OOB) data (socket option SO_OOBINLINE) and OOB data is yet unread, only OOB data will be returned. The application can use the ioctlsocketor WSAIoctlfunction SIOCATMARK command to determine whether any more OOB data remains to be read. The fromand fromlenparameters are ignored for connection-oriented sockets.

For message-oriented sockets, data is extracted from the first enqueued message, up to the size of the buffer supplied. If the datagram or message is larger than the buffer supplied, the buffer is filled with the first part of the datagram and recvfromgenerates the error WSAEMSGSIZE. For unreliable protocols (for example, UDP) the excess data is lost.

If the fromparameter is nonzero and the socket is not connection-oriented, (type SOCK_DGRAM, for example), the network address of the peer that sent the data is copied to the corresponding sockaddrstructure. The value pointed to by fromlenis initialized to the size of this structure and is modified, on return, to indicate the actual size of the address stored in the sockaddrstructure.

If no incoming data is available at the socket, the recvfromfunction blocks and waits for data to arrive according to the blocking rules defined for WSARecvwith 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 selector WSAEventSelectfunction 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, the call to recvfromwill complete immediately with zero bytes received. If the connection has been reset, recvfromwill fail with the error WSAECONNRESET.

The flagsparameter can be used to influence the behavior of the function invocation beyond the options specified for the associated socket. The semantics of the recvfromfunction are determined by the socket options and the flagsparameter. The following table shows the values can be used to construct the flags parameter. The bitwise OR operator is applied to these values.