在一个接收子线程里面接收下位机上送的数据，如下：
while (ConnectFlag)
{
if (pDlg->RecvFlag)
{
int err = recvfrom(pDlg->socketclient,recvbuf,1000,0,(SOCKADDR*)&addrsv,&lenp);
}
}
流程：上位机先下发命令给下位机配置参数，下位机一旦收到相应的信息（不是前句话讲的参数命令）就开始上送数据。问题：如果下位机停止上送数据接近一分钟，再次上送数据，recvfrom却接收不到数据，从PC端的截包工具已经看到下位机上送了数据且数据格式内容正确，这个时候只有再下发一次命令后，下位机一旦收到相应信息，recvfrom又能收到数据，至少说明接收子线程没有退出。注明：下发命令其实主要就是sendto函数，“截包工具已经看到下位机上送了数据”说明配置参数命令其实已经在下位机了，不明白为什么要再次下发命令一下又才能recvform。
我把 recvfrom改成非阻塞模式，也是同样的现象。
------解决思路----------------------
recvfrom
The Windows Sockets recvfrom function receives a datagram and stores the source address.
int recvfrom (
SOCKET s,
char FAR* buf,
int len,
int flags,
struct sockaddr FAR* from,
int FAR* fromlen
);
Parameters
s
[in] A descriptor identifying a bound socket.
buf
[out] A buffer for the incoming data.
len
[in] The length of buf.
flags
[in] An indicator specifying the way in which the call is made.
from
[out] An optional pointer to a buffer that will hold the source address upon return.
fromlen
[in/out] An optional pointer to the size of the from buffer.
Remarks
The recvfrom function reads incoming data on both connected and unconnected sockets and captures the address from which the data was sent. The socket must not be connected. The local address of the socket 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 stream oriented sockets such as those of type SOCK_STREAM, a call to recvfrom returns as much information as is currently available—up to the size of the buffer supplied. If the socket has been configured for in-line reception of out-of-band data (socket option SO_OOBINLINE) and out-of-band data is yet unread, only out-of-band data will be returned. The application can use the ioctlsocket or WSAIoctl SIOCATMARK command to determine whether any more out-of-band data remains to be read. The from and fromlen parameters 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 recvfrom generates the error WSAEMSGSIZE. For unreliable protocols (for example, UDP) the excess data is lost.
If the from paramter 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 SOCKADDR structure. The value pointed to by fromlen is initialized to the size of this structure and is modified, on return, to indicate the actual size of the address stored in the SOCKADDR structure.
If no incoming data is available at the socket, the recvfrom function 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 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 recvfrom will complete immediately with zero bytes received. If the connection has been reset recvfrom 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 latter is constructed by or-ing the following values:
Value Meaning
MSG_PEEK Peek at the incoming data. The data is copied into the buffer but is not removed from the input queue, and the function returns the number of bytes currently pending to receive.
MSG_OOB Process out-of-band data. (See section DECnet Out-Of-band data for a discussion of this topic.)
Return Values
If no error occurs, recvfrom returns the number of bytes 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.
Error Codes
WSANOTINITIALISED A successful WSAStartup must occur before using this function.
WSAENETDOWN The network subsystem has failed.
WSAEFAULT The buf or from parameters are not part of the user address space, or the fromlen parameter is too small to accommodate the peer address.
WSAEINTR The (blocking) call was canceled through WSACancelBlockingCall.
WSAEINPROGRESS A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
WSAEINVAL 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-style sockets only) len was zero or negative.
WSAEISCONN The socket is connected. This function is not permitted with a connected socket, whether the socket is connection-oriented or connectionless.
WSAENETRESET The connection has been broken due to the "keep-alive" activity detecting a failure while the operation was in progress.
WSAENOTSOCK The descriptor is not a socket.
WSAEOPNOTSUPP 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.
WSAESHUTDOWN The socket has been shut down; it is not possible to recvfrom on a socket after shutdown has been invoked with how set to SD_RECEIVE or SD_BOTH.
WSAEWOULDBLOCK The socket is marked as nonblocking and the recvfrom operation would block.
WSAEMSGSIZE The message was too large to fit into the specified buffer and was truncated.
WSAETIMEDOUT The connection has been dropped, because of a network failure or because the system on the other end went down without notice.
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. On a UDP datagram socket this error would indicate that a previous send operation resulted in an ICMP "Port Unreachable" message.
QuickInfo
Windows NT: Yes
Windows: Yes
Windows CE: Use version 1.0 and later.
Header: Declared in winsock2.h.
Import Library: Link with ws2_32.lib.
See Also
recv, send, socket, WSAAsyncSelect, WSAEventSelect