I wrote a short TCP client application to test a SW broker using blocking sockets. It works well and I am happy with it. Now I need to modify it and I have a problem reading not solecited messages (messages coming from the server not initiated by the client) because recv() blocks the program flow until there is no data to read.
Good solution for me may be to find a way to check if there are available data to read before calling recv() or that recv() may exit if there are no data to read within a certain period.
So I tried with this function:
static bool isDataAvailable(int socket)
{
fd_set sready;
struct timeval nowait;
FD_ZERO(&sready);
FD_SET((unsigned int) socket, &sready);
memset((char *) &nowait, 0, sizeof(nowait));
bool res = select(socket + 1, &sready, NULL, NULL, &nowait);
if(FD_ISSET(socket, &sready))
res = true;
else
res = false;
return res;
}
//------------------------------------------------------------------------
but the result is that after the call all my application works as I selected to work using non blocking sockets. I normally use it on Linux and it works. I found some info around the web about select() and seems to me it would work also under Windows. But... something is going wrong.
Any help and suggestion will be appreciated.
Thanks,
Massimo
Hi I am figuring out a way to listen to a socket and connect to a different socket(on same ip but different port number) simultaneously in the same program.when I listen to a socket then it keeps blocking until it receives some message so I am not able to listen and connect simultaneously.
I actually need to simulate exchange of LSP packets between different routers. So I am writing a program to simulate a router so as to run it n(number of routers)times.
Could anyone please share on how to proceed ??
If I understood your problem correctly, one of these might help.
Multi-thread or Multi-process
Basically, when you receive a client you can process a client separately in a separate thread or in a new process. You will be able to receive incoming connections and connect to new clients from other sources while processing the ones who are already connected.
Pseudo Code:
main() {
while(1) {
accept client
/*
After the fork or creation of the new thread, the loop goes back to
accepting clients while connected clients are being processed.
*/
fork or create new thread passing and client socket to it
}
}
processClient() {
do whatever you need to do...
}
Select
Select is another good way of doing non-blocking sockets. Select basically waits for data to come (ie. data, new client requests) to the server and processes them one-by-one. The server will not block on accept as it will wait until it receives something before processing it.
Psuedo Code:
main() {
while(1) {
wait on select
if new client {
accept it
}
for client in clients {
if client has data {
process it
}
}
}
}
ePoll (if you're in Linux)
ePoll is similar to Select only it can handle WAY more clients and it's a lot sexier.
Here's a repo that has each of those. My code isn't perfect here as it was a project that I did while in school.
https://github.com/koralarts/ServerBenchmarking
First, a little background to explain the motivation: I'm working on a very simple select()-based TCP "mirror proxy", that allows two firewalled clients to talk to each other indirectly. Both clients connect to this server, and as soon as both clients are connected, any TCP bytes sent to the server by client A is forwarded to client B, and vice-versa.
This more or less works, with one slight gotcha: if client A connects to the server and starts sending data before client B has connected, the server doesn't have anywhere to put the data. I don't want to buffer it up in RAM, since that could end up using a lot of RAM; and I don't want to just drop the data either, as client B might need it. So I go for the third option, which is to not select()-for-read-ready on client A's socket until client B has also connected. That way client A just blocks until everything is ready to go.
That more or less works too, but the side effect of not selecting-for-read-ready on client A's socket is that if client A decides to close his TCP connection to the server, the server doesn't get notified about that fact -- at least, not until client B comes along and the server finally selects-for-read-ready on client A's socket, reads any pending data, and then gets the socket-closed notification (i.e. recv() returning 0).
I'd prefer it if the server had some way of knowing (in a timely manner) when client A closed his TCP connection. Is there a way to know this? Polling would be acceptable in this case (e.g. I could have select() wake up once a minute and call IsSocketStillConnected(sock) on all sockets, if such a function existed).
If you want to check if the socket is actually closed instead of data, you can add the MSG_PEEK flag on recv() to see if data arrived or if you get 0 or an error.
/* handle readable on A */
if (B_is_not_connected) {
char c;
ssize_t x = recv(A_sock, &c, 1, MSG_PEEK);
if (x > 0) {
/* ...have data, leave it in socket buffer until B connects */
} else if (x == 0) {
/* ...handle FIN from A */
} else {
/* ...handle errors */
}
}
Even if A closes after sending some data, your proxy probably wants to forward that data to B first before forwarding the FIN to B, so there is no point in knowing that A has sent FIN on the connection sooner than after having read all the data it has sent.
A TCP connection isn't considered closed until after both sides send FIN. However, if A has forcibly shutdown its endpoint, you will not know that until after you attempt to send data on it, and receive an EPIPE (assuming you have suppressed SIGPIPE).
After reading your mirror proxy application a bit more, since this is a firewall traversal application, it seems that you actually need a small control protocol to allow to you verify that these peers are actually allowed to talk to each other. If you have a control protocol, then you have many solutions available to you, but the one I would advocate would be to have one of the connections describe itself as the server, and the other connection describe itself as the client. Then, you can reset the connection the client if there is no server present to take its connection. You can let servers wait for a client connection up to some timeout. A server should not initiate any data, and if it does without a connected client, you can reset the server connection. This eliminates the issue of buffering data for a dead connection.
It appears the answer to my question is "no, not unless you are willing and able to modify your TCP stack to get access to the necessary private socket-state information".
Since I'm not able to do that, my solution was to redesign the proxy server to always read data from all clients, and throw away any data that arrives from a client whose partner hasn't connected yet. This is non-optimal, since it means that the TCP streams going through the proxy no longer have the stream-like property of reliable in-order delivery that TCP-using programs expect, but it will suffice for my purpose.
For me the solution was to poll the socket status.
On Windows 10, the following code seemed to work (but equivalent implementations seem to exist for other systems):
WSAPOLLFD polledSocket;
polledSocket.fd = socketItf;
polledSocket.events = POLLRDNORM | POLLWRNORM;
if (WSAPoll(&polledSocket, 1, 0) > 0)
{
if (polledSocket.revents &= (POLLERR | POLLHUP))
{
// socket closed
return FALSE;
}
}
I don't see the problem as you see it. Let's say A connects to the server sends some data and close, it does not need any message back. Server won't read its data until B connects, once it does server read socket A and send the data to B. The first read will return the data A had sent and the second return either 0 or -1 in either case the socket is closed, server close B. Let's suppose A send a big chunk of data, the A's send() method will block until server starts reading and consumes the buffer.
I would use a function with a select which returns 0, 1, 2, 11, 22 or -1,
where;
0=No data in either socket (timeout)
1=A has data to read
2=B has data to read
11=A socket has an error (disconnected)
22=B socket has an error (disconnected)
-1: One/both socket is/are not valid
int WhichSocket(int sd1, int sd2, int seconds, int microsecs) {
fd_set sfds, efds;
struct timeval timeout={0, 0};
int bigger;
int ret;
FD_ZERO(&sfds);
FD_SET(sd1, &sfds);
FD_SET(sd2, &sfds);
FD_SET(sd1, &efds);
FD_SET(sd2, &efds);
timeout.tv_sec=seconds;
timeout.tv_usec=microsecs;
if (sd1 > sd2) bigger=sd1;
else bigger=sd2;
// bigger is necessary to be Berkeley compatible, Microsoft ignore this param.
ret = select(bigger+1, &sfds, NULL, &efds, &timeout);
if (ret > 0) {
if (FD_ISSET(sd1, &sfds)) return(1); // sd1 has data
if (FD_ISSET(sd2, &sfds)) return(2); // sd2 has data
if (FD_ISSET(sd1, &efds)) return(11); // sd1 has an error
if (FD_ISSET(sd2, &efds)) return(22); // sd2 has an error
}
else if (ret < 0) return -1; // one of the socket is not valid
return(0); // timeout
}
Since Linux 2.6.17, you can poll/epoll for POLLRDHUP/EPOLLRDHUP. See epoll_ctl(2):
EPOLLRDHUP (since Linux 2.6.17)
Stream socket peer closed connection, or shut down writing half of connection. (This flag is especially useful for writing simple code to detect peer shutdown when using Edge Triggered monitoring.)
If your proxy must be a general purpose proxy for any protocol, then you should handle also those clients which sends data and immediately calls close after the send (one way data transfer only).
So if client A sends a data and closes the connection before the connection is opened to B, don't worry, just forward the data to B normally (when connection to B is opened).
There is no need to implement special handling for this scenario.
Your proxy will detect the closed connection when:
read returns zero after connection to B is opened and all pending data from A is read. or
your programs try to send data (from B) to A.
You could check if the socket is still connected by trying to write to the file descriptor for each socket. Then if the return value of the write is -1 or if errno = EPIPE, you know that socket has been closed.for example
int isSockStillConnected(int *fileDescriptors, int numFDs){
int i,n;
for (i=0;i<numFDs;i++){
n = write(fileDescriptors+i,"heartbeat",9);
if (n < 0) return -1;
if (errno == EPIPE) return -1;
}
//made it here, must be okay
return 0;
}
I am working on newtwork event based socket application.
When client has sent some data and there is something to be read on the socket, FD_READ network event is generated.
Now according to my understanding, when server wants to write over the socket, there must be an event generated i.e. FD_WRITE. But how this message will be generated?
When there is something available to be read, FD_READ is automatically generated but what about FD_WRITE when server wants to write something?
Anyone who can help me with this confusion please?
Following is the code snippet:
WSAEVENT hEvent = WSACreateEvent();
WSANETWORKEVENTS events;
WSAEventSelect(newSocketIdentifier, hEvent, FD_READ | FD_WRITE);
while(1)
{ //while(1) starts
waitRet = WSAWaitForMultipleEvents(1, &hEvent, FALSE, WSA_INFINITE, FALSE);
//WSAResetEvent(hEvent);
if(WSAEnumNetworkEvents(newSocketIdentifier,hEvent,&events) == SOCKET_ERROR)
{
//Failure
}
else
{ //else event occurred starts
if(events.lNetworkEvents & FD_READ)
{
//recvfrom()
}
if(events.lNetworkEvents & FD_WRITE)
{
//sendto()
}
}
}
FD_WRITE means you can write to the socket right now. If the send buffers fill up (you're sending data faster than it can be sent on the network), eventually you won't be able to write anymore until you wait a bit.
Once you make a write that fails due to the buffers being full, this message will be sent to you to let you know you can retry that send.
It's also sent when you first open up the socket to let you know it's there and you can start writing.
http://msdn.microsoft.com/en-us/library/windows/desktop/ms741576(v=vs.85).aspx
The FD_WRITE network event is handled slightly differently. An
FD_WRITE network event is recorded when a socket is first connected
with a call to the connect, ConnectEx, WSAConnect, WSAConnectByList,
or WSAConnectByName function or when a socket is accepted with accept,
AcceptEx, or WSAAccept function and then after a send fails with
WSAEWOULDBLOCK and buffer space becomes available. Therefore, an
application can assume that sends are possible starting from the first
FD_WRITE network event setting and lasting until a send returns
WSAEWOULDBLOCK. After such a failure the application will find out
that sends are again possible when an FD_WRITE network event is
recorded and the associated event object is set.
So, ideally you're probably keeping a flag as to whether it's OK to write, right now. It starts off as true, but eventually, you get a WSAEWOULDBLOCK when calling sendto, and you set it to false. Once you receive FD_WRITE, you set the flag back to true and resume sending packets.
For an application in C, i need to response more than one clients.
I setup the connection with a code like,
bind(...);
listen(...);
while(1){
accept(...);//accept a client
recv(...);//receive something
send(...);//send something to client
bzero(buf);//clear buffer
}
This works great when i have only one client. Other clients also can connect to the server but although they command something, server does not response back to clients who connected after the first client. How can i solve this problem?
Write a server using asynchronous, nonblocking connections.
Instead of a single set of data about a client, you need to create a struct. Each instance of the struct holds the data for each client.
The code looks vaguely like:
socket(...)
fcntl(...) to mark O_NONBLOCK
bind(...)
listen(...)
create poll entry for server socket.
while(1) {
poll(...)
if( fds[server_slot].revents & POLLIN ) {
accept(...)
fcntl(...) mark O_NONBLOCK
create poll and data array entries.
}
if( fds[i].revents & POLLIN ) {
recv(...) into data[i]
if connection i closed then clean up.
}
if( fds[i].revents & POLLOUT ) {
send(...) pending info for data[i]
}
}
If any of your calls return the error EAGAIN instead of success then don't panic. You just try again later. Be prepared for EAGAIN even if poll claims the socket is ready: it's good practice and more robust.
i need to response more than one clients.
Use Threading.
Basically you want your main thread to only do the accept part, and then handle the rest to another thread of execution (which can be either a thread or a process).
Whenever your main thread returns from "accept", give the socket descriptor to another thread, and call accept again (this can be done with fork, with pthread_create, or by maintaining a thread pool and using synchronization, for instance condition variables, to indicate that a new client has been accepted).
While the main thread will handle possible new clients incoming, the other threads will deal with the recv/send.