I have a UDP client C++ code - based on WSA sockets - that works well. The code was originally written in VS6 and I recently recompiled it in VS2010 for 64bit environment, with only little adjustments.
Now, the sendto() fails to send something, if there is no Sleep(..) or any equivalent delay after the sendto() and before closesocket(). "Fails" means, that sendto() returns the proper amount of data, but I see no message on the network (I used wireshark to check this).
This is my code:
void CTest::SendHello()
{
SOCKET sSocket;
sSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr(m_strDstIpAddr);
addr.sin_port = htons(m_nTxPort);
int nMsgLen = 8;
char pTxBuffer[8];
*((DWORD*) &pTxBuffer[ 0]) = 0x11223344;
*((DWORD*) &pTxBuffer[ 4]) = 0;
int nSent = sendto(sSocket, pTxBuffer, nMsgLen, 0, (struct sockaddr *) &addr, sizeof(addr));
Sleep(10); // <- this seems to be necessary
if (nSent != nMsgLen)
{
CString s = "error sending HELO\n";
AfxMessageBox(s);
}
closesocket(sSocket);
}
Without the Sleep(), the code does not send anything, yet it returns no errors. With the Sleep() it works. Also, this is happens in release version, when compiled for debug, the code also works without the Sleep().
It seems, as if the closesocket() shuts the socket down, before the message is finally sent, but I thought sendto() is a synchronous function. I tried using SO_LINGER, but this is not applicable for SOCK_DGRAM sockets.
Since the code is inside a DLL, I can't create the socket in ctor and delete it in the dtor, because SendHello() might be called from different thread contexts, and I like to avoid to make the code too complicated.
thanks for any help
With UDP there is no ordering of data between the sender and receiver and data sent using UDP datagram sockets isn't guaranteed to arrive. All sleep is doing in your case is practically providing enough time for the data to arrive at the other end. If you want confirmation of receipt and error checking, then you can either code a scheme for UDP or use TCP. In fact, you can turn off the server completely and your client will happily fire UDP packets out without error even though there is nobody listening.
to insure a connection, look at connect(). Nothing prevents using connect with UDP and you can then use send() recv().
Related
Sorry for improper description of my question.
What my program do is that connect a server, send some data and close connection. I simplified my code as below:
WSAStartup(MAKEWORD(2, 2), &wsaData);
SOCKET s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
connect(s, (const sockaddr*)&dstAddr, sizeof(dstAddr));
send(s, (const char*)pBuffer, fileLen, 0);
shutdown(s, SD_SEND);
closesocket(s);
WSACleanup();
Only partial data was received by server before found a RST causing communication shutdown.
I wrote a simulate server program to accept connection and receive data, but the simulator could get all data. Because I couldn't access server's source code, I didn't know if something made wrong in it. Is there a way I can avoid this error by adding some code in client, or can I prove that there is something wrong in server program?
Setting socket's linger option can fix the bug. But I need to give a magic number for the value of linger time.
linger l;
l.l_onoff = 1;
l.l_linger = 30;
setsockopt(socket, SOL_SOCKET, SO_LINGER, (const char*)&l, sizeof(l));
WSASend returns before sending data to device actually
Correct.
I created a non-blocking socket and tried to send data to server.
WSASocket(AF_INET, SOCK_STREAM, IPPROTO_TCP, NULL, 0, WSA_FLAG_OVERLAPPED)
No you didn't. You created an overlapped I/O socket.
After executed, returnValue was SOCKET_ERROR and WSAGetLastError() returned WSA_IO_PENDING. Then I called WSAWaitForMultipleEvents to wait for event being set. After it returned WSA_WAIT_EVENT_0, I called WSAGetOverlappedResult to get actual sent data length and it is the same value with I sent.
So all the data got transferred into the socket send buffer.
I called WSASocket first, then WSASend/WSAWaitForMultipleEvents/WSAGetOverlappedResult several times to send a bunch of data, and closesocket at the end.
So at the end of that process all the data and the close had been transferred to the socket send buffer.
But server couldn't receive all data, I used Wireshark to view tcp packets and found that client sent RST before all packet were sent out.
That could be for a number of reasons none of which is determinable without seeing some code.
If I slept 1 minute before calling closesocket, then server would receive all data.
Again this would depend on what else had happened in your code.
It seemed like that WSASend/WSAWaitForMultipleEvents/WSAGetOverlappedResult returned before sending data to server actually.
Correct.
The data were saved in buffer and waiting for being sent out.
Correct.
When I called closesocket, communication was shut down.
Correct.
They didn't work as my expectation.
So your expectation was wrong.
What did I go wrong? This problem only occurred in specific PCs, the application run well in others.
Impossible to answer without seeing some code. The usual reason for issuing an RST is that the peer had written data to a connection that you had already closed: in other words, an application protocol error; but there are other possibilities.
I am very new to networking and have an issue with sending messages during a while loop.
To my knowledge I should do something along the lines of this:
Create Socket()
Connect()
While
Do logic
Send()
End while
Close Socket()
However it sends once and returns -1 there after.
The code will only work when I create the socket in the loop.
While
Create Socket()
Connect()
Do logic
Send()
Close Socket()
End while
Here is a section of the code I am using but doesn't work:
//init winsock
WSAStartup(MAKEWORD(2, 0), &wsaData);
//open socket
sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
//connect
memset(&serveraddr, 0, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
serveraddr.sin_addr.s_addr = inet_addr(ipaddress);
serveraddr.sin_port = htons((unsigned short) port);
connect(sock, (struct sockaddr *) &serveraddr, sizeof(serveraddr));
while(true) {
if (send(sock, request.c_str(), request.length(), 0)< 0 /*!= request.length()*/) {
OutputDebugString(TEXT("Failed to send."));
} else {
OutputDebugString(TEXT("Activity sent."));
}
Sleep(30000);
}
//disconnect
closesocket(sock);
//cleanup
WSACleanup();
The function CheckForLastError() returns:10053
WSAECONNABORTED
Software caused connection abort.
An established connection was aborted by the software in your host computer, possibly due to a data transmission time-out or protocol error
Thanks
I have been looking for a solution to this problem too. I am having the same problem with my server. When trying to send a response from inside the loop, the client seems never to receive it.
As I understand the problem, according to user207421's suggestions, when you establish a connection between a client and a server, the protocol should have enough information to let the client know when the server has finished sending the response. If you see this example, you have a minimum HTTP server that responds to requests. In this case, you can use a browser or an application like Postman. And if you see the response message, you will see a header called Connection. Setting its value to close tells the client which one is the last message from the server for that request. The message is being sent, but the client keeps waiting, maybe because there is no closing element the client can recognize. I was also missing the Content-Length header. My HTTP response message was wrong, and the client was lost.
This diagram shows what needs to be outside the loop and what needs to be inside.
To understand how and why your program fails,you have to understand the functions you use.
Some of them are blocking functions and some are them not. Some of them need previous calles of other functions and some of them don't.
Now from what i understand we are talking about a client here,not a server.
The client has only non blocking functions in this case. That means that whenever you call a function,it will be executed without waiting.
So send() will send data the second it is called and the stream will go on to the next line of code.
If the information to be sent was not yet ready...you will have a problem,since nothing will be sent.
To solve it you could use some sort of a delay. The problem with delays is that they are Blocking functions meaning your stream will stop once it hits the delay. To solve it you can create a thread and lock it untill the information is ready to be sent.
But that would do the job for one send(). You will send the info and thats that.
If you want to hold the communication and send repeatedly info,you will need to create a while loop. once you have a while loop you dont have to worry about anything. That is because you can verify that the information is ready with a stream control and you can use send over and over again before terminating the connection.
Now the question is what is happening on the server side of things?
"ipaddress" should hold the ip of the server. The server might reject your request to connect.Or worst he might accept your request but he is listening with diffrent settings in relation to your client.Meaning that maybe the server is not reciving (does not have recv() function)information and you are trying to send info... that might resault in errors/crashes and what not.
I have to receive an UDP broadcast (in Ubuntu if that makes any difference). Using Wireshark, I can see the packet being sent from the server machine, and I can see it being received by my client machine, but my program is completely oblivious. This is what I have:
sockaddr_in si_me, si_other;
int s;
assert((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))!=-1);
int port=6000;
int broadcast=1;
setsockopt(s, SOL_SOCKET, SO_BROADCAST,
&broadcast, sizeof broadcast);
memset(&si_me, 0, sizeof(si_me));
si_me.sin_family = AF_INET;
si_me.sin_port = htons(port);
si_me.sin_addr.s_addr = INADDR_ANY;
assert(::bind(s, (sockaddr *)&si_me, sizeof(sockaddr))!=-1);
while(1)
{
char buf[10000];
unsigned slen=sizeof(sockaddr);
recvfrom(s, buf, sizeof(buf)-1, 0, (sockaddr *)&si_other, &slen);
printf("recv: %s\n", buf);
}
It is compiled in debug mode, the asserts aren't being erased during compilation, and my program just blocks on recvfrom.
Is there any other hoop I have to jump through to receive an untargeted UDP broadcast?
Edit: just a bit more info, I have the two computers connected on a dedicated switch, no outside interference. I also have a second network card on my client computer that connects to the company network, which also works.
I can ping both the outside (Internet working) and my server machine (plus I can see the actual packets in Wireshark), but you never know what might cause this problem.
As it turns out my code is perfectly fine, as I thought it would be. There was a problem with the network setup itself.
For posterity, I had set up two static IP'd computers on their own hub, instead of using the built in DHCP server on the server machine to allocate the IP address for the other computer. Pretty localized for my problem but you never know..
To send and receive broadcast
Be sure that netmask is correct. in windows mask for broadcast packets does not matters, but not in linux.
bind socket to INADDR_ANY
setsockopt to BROADCAST
call sendto with sendaddr.sin_addr.s_addr = inet_addr("your_interface_broadcast_address").
or - call sento several times for each interface with its broadcast ip address.
call recvfrom. any time before calling recvfrom, set up length parameter.
I am trying to make a c++ program work which is written by somebody else. I am having hard time understanding it. I am not even %100 sure that we can use poll() with a UDP socket but the code I am refactoring, is using poll() to read from udp socket as follows:
fd.fd = m_bsocket;
fd.events = POLLIN;
iPollResult = poll(&fd, 1, iTimeout);
if(iPollResult > 0)
{
int iReceivedByteCount = recv(m_bsocket, p_pBuffer, p_iBufferSize, 0);
if(iReceivedByteCount > 0)
{
*p_pReadSize = iReceivedByteCount;
}
else
{
eReturnValue = UDP_READ_ERROR;
}
}
return eReturnValue;
I tried sending udp packets to this program using command line:
echo "123" | nc -u 127.0.0.1 25
It looks like poll() always times out and returns 0, therefore I can not read anything.
I also wrote a small c# program that sends udp datagram, but I can not receive the message. I am wondering what I am doing wrong...
While UDP sockets can be used to connect to another host, they are mostly used "connectionless". Reading your question and comments it makes no sense that you have a connected socket. Instead it should be connectionless as suggested by WouterH in his comment.
int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
struct sockaddr_in sin = { 0 };
sin.sin_family = AF_INET;
sin.sin_port = htons(25);
sin.sin_addr.s_addr = INADDR_ANY;
bind(sockfd, (struct sockaddr *) &sin, sizeof(sin));
// Make socket non-blocking if needed
With the above code, whenever someone sends UDP packets to port 25 on any address of your host, your socket will intercept it. Use e.g. poll or select to know when data is available.
You don't need to call connect() as UDP is connectionless. You need to bind() the socket to the IP of the interface you are listening on or 0.0.0.0 (INADDR_ANY) for all interfaces. And when sending to a destination, use sendto().
For completeness: if you call connect() on a UDP socket, you are just setting a default destination for the send() function (then you can use send instead of sendto).
If you want to receive data, you always have to bind() the socket to the interface, or all interfaces. Beware that you will have to verify the source address from the messages you are receiving. So you might want to filter the sender by using recvfrom() and checking the source address.
I am hooking WSASend, and WSARecv in C++ using the same method I've used to hook the client's WSASend and WSARecv functions. In the client I am able to get the IP, Port, and Socket from the SOCKET structure passed by WSASend/WSARecv; however, for the server when I try to use getpeername or getsockname() they both return the error 10057 (Socket not connected)...
I'm fairly sure that the hook is correct on the server, since it prints the bytes successfully, and I'm also sure the socket SHOULD be valid seeing how client and server establish a successful connection.
Is there a way to resolve this problem by any other alternative methods? I've been looking around the internet to find a solution, but I haven't seen anyone with the same problem.
I've tried this:
sockaddr *address = new sockaddr;
int peer_len;
getpeername(s, address, &peer_len);
int err = WSAGetLastError();
if(err==0)
{
char *Str = inet_ntoa(((sockaddr_in*)address)->sin_addr);
printf("[%s", Str);
printf(":%d]",ntohs(((sockaddr_in*)address)->sin_port));
}
else
{
printf("Error %i\n",err);
}
(Using both getpeername and getsockname)Both result in the same socket not connected error.
I'm planning on using the packets the C++ dll gets and forward the information to the C# dll since it'll be easier to manage on that (for me anyways), but I'd need to distinguish each packet with it's socket id.
You can only do that on the connected socket, i.e. the one returned from the accept() call, not on the listening "server" socket.