Programming with sockets

Closing sockets

Once a socket is no longer of interest, it may be discarded by applying a close to the descriptor,

   close(s);

If data is associated with a socket that promises reliable delivery (for example, a stream socket) when a close takes place, the system will continue to attempt to transfer the data. However, if the data is still undelivered after a fairly long period of time, it will be discarded. If a user has no use for pending data, a shutdown(3sock) may be performed on the socket before closing it. This call is of the form:

   shutdown(s, how);

where how is 0 if the user is no longer interested in reading data, 1 if no more data will be sent, and 2 if no data is to be sent or received.

The following two code samples illustrate how to initiate and accept an Internet domain stream connection.


   #include <sys/types.h>
   #include <sys/socket.h>
   #include <netinet/in.h>
   #include <netdb.h>
   #include <stdio.h>
   
   #define DATA "Half a league, half a league . . ."
   

   /*
    * This program creates a socket and initiates a connection with the socket
    * given in the command line.  One message is sent over the connection and
    * then the socket is closed, ending the connection.  The form of the command
    * line is: streamwrite hostname portnumber
    */
   

   main(argc, argv)
   	int argc;
   	char *argv[];
   {
   	int sock;
   	struct sockaddr_in server;
   	struct hostent *hp, *gethostbyname();
   	char buf[1024];
   

   	/* Create socket. */
   	sock = socket(AF_INET, SOCK_STREAM, 0);
   	if (sock < 0) {
   		perror("opening stream socket");
   		exit(1);
   	}
   	/* Connect socket using name specified by command line. */
   	server.sin_len = sizeof(server);
   	server.sin_family = AF_INET;
   	hp = gethostbyname(argv[1]);
   	if (hp == 0) {
   		fprintf(stderr, "%s: unknown host\n", argv[1]);
   		exit(2);
   	}
   	memcpy((char *)&server.sin_addr, (char *)hp->h_addr,
   	  hp->h_length);
   	server.sin_port = htons(atoi(argv[2]));
   

   	if (connect(sock,
   	  (struct sockaddr *)&server, sizeof(server)) < 0) {
   		perror("connecting stream socket");
   		exit(1);
   	}
   	if (write(sock, DATA, sizeof(DATA)) < 0)
   		perror("writing on stream socket");
   	close(sock);
   	exit(0);
   }


   #include <sys/types.h>
   #include <sys/socket.h>
   #include <netinet/in.h>
   #include <netdb.h>
   #include <stdio.h>
   #define TRUE 1
   
   /*
    * This program creates a socket and then begins an infinite loop.  Each time
    * through the loop it accepts a connection and prints out messages from it.
    * When the connection breaks, or a termination message comes through, the
    * program accepts a new connection.
    */
   

   main()
   {
   	int sock, length;
   	struct sockaddr_in server;
   	int msgsock;
   	char buf[1024];
   	int rval;
   

   	/* Create socket. */
   	sock = socket(AF_INET, SOCK_STREAM, 0);
   	if (sock < 0) {
   		perror("opening stream socket");
   		exit(1);
   	}
   	/* Name socket using wildcards. */
   	server.sin_len = sizeof(server);
   	server.sin_family = AF_INET;
   	server.sin_addr.s_addr = INADDR_ANY;
   	server.sin_port = 0;
   	if (bind(sock, (struct sockaddr *)&server, sizeof(server)) < 0) {
   		perror("binding stream socket");
   		exit(1);
   	}
   	/* Find out assigned port number and print it out. */
   	length = sizeof(server);
   	if (getsockname(sock, (struct sockaddr *)&server,
   	  &length) < 0) {
   		perror("getting socket name");
   		exit(1);
   	}
   	printf("Socket port #%d\n", ntohs(server.sin_port));
   

   	/* Start accepting connections. */
   	listen(sock, 5);
   	do {
   		msgsock = accept(sock, (struct sockaddr *)0, (int *)0);
   		if (msgsock == -1)
   			perror("accept");
   		else do {
   			memset(buf, 0, sizeof(buf));
   			if ((rval = read(msgsock, buf, 1024)) < 0)
   				perror("reading stream message");
   			if (rval == 0)
   				printf("Ending connection\n");
   			else
   				printf("-->%s\n", buf);
   		} while (rval != 0);
   		close(msgsock);
   	} while (TRUE);
   	/*
   	 * Since this program has an infinite loop, the socket "sock" is
   	 * never explicitly closed.  However, all sockets will be closed
   	 * automatically when a process is killed or terminates normally.
   	 */
   	 exit(0);
   }