Let's say I'm running a simple server and have accept()ed a connection from a client.
What is the best way to tell when the client has disconnected? Normally, a client is supposed to send a close command, but what if it disconnects manually or loses network connection altogether? How can the server detect or handle this?
To expand on this a bit more:
If you are running a server you either need to use TCP_KEEPALIVE to monitor the client connections, or do something similar yourself, or have knowledge about the data/protocol that you are running over the connection.
Basically, if the connection gets killed (i.e. not properly closed) then the server won't notice until it tries to write something to the client, which is what the keepalive achieves for you. Alternatively, if you know the protocol better, you could just disconnect on an inactivity timeout anyway.
If you're using overlapped (i.e. asynchronous) I/O with completion routines or completion ports, you will be notified immediately (assuming you have an outstanding read) when the client side closes the connection.
The return value of receive will be -1 if connection is lost else it will be size of buffer.
void ReceiveStream(void *threadid)
{
while(true)
{
while(ch==0)
{
char buffer[1024];
int newData;
newData = recv(thisSocket, buffer, sizeof(buffer), 0);
if(newData>=0)
{
std::cout << buffer << std::endl;
}
else
{
std::cout << "Client disconnected" << std::endl;
if (thisSocket)
{
#ifdef WIN32
closesocket(thisSocket);
WSACleanup();
#endif
#ifdef LINUX
close(thisSocket);
#endif
}
break;
}
}
ch = 1;
StartSocket();
}
}
select (with the read mask set) will return with the handle signalled, but when you use ioctl* to check the number of bytes pending to be read, it will be zero. This is a sign that the socket has been disconnected.
This is a great discussion on the various methods of checking that the client has disconnected: Stephen Cleary, Detection of Half-Open (Dropped) Connections.
* for Windows use ioctlsocket.