TCP Echo Server

We show the concurrent server program.

1 #include      "unp.h"

2 int
3 main(int argc, char **argv)
4 {
5     int     listenfd, connfd;
6     pid_t   childpid;
7     socklen_t clilen;
8     struct sockaddr_in cliaddr, servaddr;

9     listenfd = Socket (AF_INET, SOCK_STREAM, 0);

10     bzero(&servaddr, sizeof(servaddr));
11     servaddr.sin_family = AF_INET;
12     servaddr.sin_addr.s_addr = htonl (INADDR_ANY);
13     servaddr.sin_port = htons (SERV_PORT);

14     Bind(listenfd, (SA *) &servaddr, sizeof(servaddr));

15     Listen(listenfd, LISTENQ);

16     for ( ; ; )  {
17         clilen = sizeof(cliaddr);
18         connfd = Accept(listenfd, (SA *) &cliaddr, &clilen);

19         if ( (childpid = Fork()) == 0) { /* child process */
20             Close(listenfd);    /* close listening socket */
21             str_echo(connfd);   /* process the request */
22             exit (0);
23         }
24         Close(connfd);          /* parent closes connected socket */
25     }
26 }

pid_t Fork(void)
{
    pid_t pid;

    if ((pid = fork()) == -1)
        err_sys("fork error");
    return(pid);
}

Create socket, bind server's well-known port

9–15 A TCP socket is created. An Internet socket address structure is filled in with the wildcard address (INADDR_ANY) and the server's well-known port (SERV_PORT, which is defined as 9877 in our unp.h header). Binding the wildcard address tells the system that we will accept a connection destined for any local interface, in case the system is multihomed.  It should be greater than 1023 (we do not need a reserved port), greater than 5000 (to avoid conflict with the ephemeral ports allocated by many Berkeley-derived implementations), less than 49152 (to avoid conflict with the "correct" range of ephemeral ports), and it should not conflict with any registered port. The socket is converted into a listening socket by listen.

Wait for client connection to complete

17–18 The server blocks in the call to accept, waiting for a client connection to complete.

Concurrent server

19–24 For each client, fork spawns a child, and the child handles the new client. The child closes the listening socket and the parent closes the connected socket. The child then calls str_echo to handle the client.