How do you keep a socket connection open indefinitely in C? - c

Question

I'm trying to implement a C socket server in Linux using the code from Beej's sockets guide, which is here:
http://beej.us/guide/bgnet/examples/server.c
This works, and I've written a Windows client in C# to communicate with it. Once the client connects, I have it send a byte array to the server, the server reads it, then sends back a byte array. This works.
However, after this, if I have the client try to send another byte array, I get a Windows popup saying "An established connection was aborted by the software in your host machine." Then I have to re-connect with the client again. I want to keep the connection open indefinitely, until the client sends a disconnect command, but despite reading through Beej's guide, I just don't seem to get it. I'm not even trying to implement the disconnect command at present, I'm just trying to keep the connection open until I close the server.
I've tried removing the close() calls in Beej's code:
while(1) { // main accept() loop
sin_size = sizeof their_addr;
new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
if (new_fd == -1) {
perror("accept");
continue;
}
inet_ntop(their_addr.ss_family,
get_in_addr((struct sockaddr *)&their_addr),
s, sizeof s);
printf("server: got connection from %s\n", s);
if (!fork()) { // this is the child process
close(sockfd); // child doesn't need the listener
ProcessRequest(new_fd); // this is not Beej's code, I've replaced his code here (which was a simple string send()) with a function call that does a read() call, processes some data, then sends back a byte array to the client using send().
close(new_fd);
exit(0);
}
close(new_fd); // parent doesn't need this
}
But that just gets me an infinite loop of "socket accept: bad file descriptor" (I tried removing both the close(new_fd) lines, together and apart, and the close(sockfd) as well.
Can anyone more versed with C socket programming give me a hint where I should be looking? Thank you.

Answer 1

The reason for the accept() problem is that sockfd isn't valid. You must have closed it somewhere. NB if you get such an error you shouldn't just keep retrying as though it hadn't happened.
The reason for the client problem is that you're only processing one request in ProcessRequest(), as its name suggests, and as you describe in your comment. Use a loop, reading requests until recv() returns zero or an error occurs.

Answer 2

Cause
The reason client faces error is because of close(new_fd) either by the server-parent or server-child.
Solution
At any point of time, a server may get two kind of events:
Connection request from a new client
Data from an existing client
The server have to honor both of them. There are two (major) ways to handle this.
Solution Approach 1
Design the server as a concurrent server. In Beej's guide it is
7.2. select()—Synchronous I/O Multiplexing
http://beej.us/guide/bgnet/output/html/singlepage/bgnet.html#select
Since OP's approach is not this one, we do not explore it further.
Solution Approach 2
At server, fork() a process per client. This is the approach OP has taken and we explore here. Essentially, it is fine tuning the ProcessRequest() function in OP's code. Here is a sketch.
void ProcessRequest( int new_fd ) {
char buffer[ N ];
for( ; ; ) { // infinite loop until client disconnects or some error
int const recvLen = recv( new_fd, buffer, sizeof buffer, 0 );
if( recvLen == 0 ) { break; } // client disconnected
else if( recvLen == -1 ) { perror( "recv" ); break; }
int const sendLen = send( new_fd, buffer, recvLen, 0 );
if( sendLen == -1 ) { perror( "send" ); break; }
// TODO if( sendLen < recvLen ) then send() in loop
}
}
Note
I am sorry for having the half-baked solution four few hours. While I was editing the answer, I lost connectivity to stackoverflow.com which lasted for couple of hours.