How to know if the client has terminated in sockets

Question

Suppose, I have a connected socket after writing this code..

if ((sd = accept(socket_d, (struct sockaddr *)&client_addr, &alen)) < 0)
{
    perror(\"a         


        

Answer 1

If the client program exits, then the OS on the client will close its end of the socket. When you call recv() it will return 0, or -1 with errno ECONNRESET if a TCP RST has been received (e.g. because you attempted to send data after the client had closed). If the whole client machine goes down, or the network becomes disconnected, then in that case you may not receive anything if the server is not trying to send anything; if that is important to detect, you can either send some data periodically, or set the SO_KEEPALIVE socket option using setsockopt() to force it to send a packet with no data after long periods (hours) of inactivity. When no acknowledgment is received, recv() will then return -1 with errno ETIMEDOUT or another error if more specific information is available.

In addition, if you attempt to send data on a socket that has been disconnected, by default the SIGPIPE signal will terminate your program. This can be avoided by setting the SIGPIPE signal action to SIG_IGN (ignore), or by using send() with the MSG_NOSIGNAL flag on systems that support it (Linux).

Answer 2

After accepting the connection, your recv() on the socket will return 0 or -1 in special cases.

Excerpt from recv(3) man page:

Upon successful completion, recv() shall return the length of the message in bytes. If no messages are available to be received and the peer has performed an orderly shutdown, recv() shall return 0. Otherwise, -1 shall be returned and errno set to indicate the error.

So, if your client exited gracefully, you will get 0 from recv() at some point. If the connection was somehow lost, you may also get -1 and checking for appropriate errno would tell you if the connection was lost of some other error occured. See more details at recv(3) man page.

Edit:

I see that you are using read(). Still, the same rules as with recv() apply.

Your server can also fail when trying to write() to your clients. If your client disconnects write() will return -1 and the errno would probably be set to EPIPE. Also, SIGPIPE signal will be send to you process and kill him if you do not block/ignore this signal. And you don't as I see and this is why your server terminates when client presses Ctrl-C. Ctrl-C terminates client, therefore closes client socket and makes your server's write() fail.

See mark4o's answer for nice detailed explanation of what else might go wrong.