Server socket finishes when client closes connection

Question

I'm trying to create a server socket with C++ in order to accept one client connection at a time. The program successfully creates the server socket and waits for incoming connections but when a connection is closed by the client the program would loop endlessly. Otherwise if the connection is interrupted it would keep waiting for new connections as expected. Any idea why this is happening? Thanks

This is my C++ server code:

   int listenfd, connfd, n;
   struct sockaddr_in servaddr, cliaddr;
   socklen_t clilen;
   pid_t     childpid;
   char mesg[1000];

   listenfd = socket(AF_INET, SOCK_STREAM, 0);

   bzero(&servaddr, sizeof(servaddr));
   servaddr.sin_family = AF_INET;
   servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
   servaddr.sin_port = htons(32000);
   bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr));

   listen(listenfd, 1024);

    while (true) {
        clilen = sizeof(cliaddr);
        connfd = accept(listenfd, (struct sockaddr *)&cliaddr, &clilen);

        if ((childpid = fork()) == 0) {
            close (listenfd);

            while (true) {
                n = recvfrom(connfd, mesg, 1000, 0, (struct sockaddr *)&cliaddr, &clilen);
                sendto(connfd, mesg, n, 0, (struct sockaddr *)&cliaddr, sizeof(cliaddr));

                mesg[n] = 0;
                printf("%d: %s \n", n, mesg);

                if (n <= 0) break;
            }
            close(connfd);
        }
    }

For some reason when the client closes the connection the program would keep printing -1: even with the if-break clause..

If 'n' is -1 you should call perror(), or use strerror[] in an error message, and you should *not* use it as an array index before testing it. — user207421, Aug 05 '14 at 03:52

Serge Ballesta · Accepted Answer · 2014-08-04T16:24:41.413

2

You never close connfd in parent process (when childpid != 0), and you do not properly terminate child process that will try to loop. Your if block should look like :

if ((childpid = fork()) == 0) {
    ...
    close(connfd);
    exit(0);
}
else {
    close(connfd);
}

But as you say you want to accept one connection at a time, you can simply not fork.

And as seen in other answers :

do not use mesg[n] without testing n >= 0
recvfrom and sendto are overkill for TCP simply use recv and send (or even read and write)

edited Aug 04 '14 at 16:24

answered Aug 04 '14 at 16:19

Serge Ballesta

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thanks @Serge, removing the unnecesary fork, testing `n` before using it as index for `mesg` and using `recv` and `send` did solve the issue – Joaquín L. Robles Aug 04 '14 at 16:29

score 1 · Answer 2 · answered Aug 04 '14 at 16:01

1

mesg[n] = 0;

This breaks when n<0, ie. socket closed

answered Aug 04 '14 at 16:01

Non-maskable Interrupt

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Even if I remove that line the program would still loop printing "-1: ", seems never to reach the break clause.. – Joaquín L. Robles Aug 04 '14 at 16:07
1

Are you sure the repeated -1 is not from "new fork"? I suggest you printf after entered fork. By the way, using fork for socket server is ancient and terrible for modern situation, try epoll instead. – Non-maskable Interrupt Aug 04 '14 at 16:09
thanks @Calvin, as Serge suggested my scenario didn't need a fork (but I'll take your suggestion for the next time) – Joaquín L. Robles Aug 04 '14 at 16:31

score 1 · Answer 3 · edited May 23 '17 at 12:11

The problem is your "n" and recvfrom. You are having a TCP client so the recvfrom won't return the correct value.

try to have a look on : How to send and receive data socket TCP (C/C++)

Edit 1 :

Take note that you do the binding not connect() http://www.beej.us/guide/bgnet/output/html/multipage/recvman.html
means there is an error in recieving data, errno will be set accordingly, please try to check the error flag.

score 0 · Answer 4 · answered Aug 04 '14 at 16:09

0

you've written a TCP server, but you use recvfrom and sendto which are specific for connection-less protocols (UDP).

try with recv and send. maybe that might help.

answered Aug 04 '14 at 16:09

macfij

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recvfrom is valid for tcp socket according to man page (although I use recv() too) – Non-maskable Interrupt Aug 04 '14 at 16:13

Server socket finishes when client closes connection

4 Answers4