#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>

/*
 * In the included file <netinet/in.h> a sockaddr_in is defined as follows:
 * struct sockaddr_in {
 *    short    sin_family;
 *    u_short  sin_port;
 *    struct in_addr sin_addr;
 *    char     sin_zero[8];
 * };
 *
 * This program creates a datagram socket, binds a name to it, then reads
 * from the socket.
 */

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

     /* Create socket from which to read. */
     sock = socket(AF_INET, SOCK_DGRAM, 0);
     if (sock < 0) {
          perror("opening datagram socket");
          exit(1);
     }

     /* Create name with wildcards. */
     name.sin_family = AF_INET;
     name.sin_addr.s_addr = INADDR_ANY;
     name.sin_port = 0;
     if (bind(sock, &name, sizeof(name))) {
          perror("binding datagram socket");
          exit(1);
     }

     /* Find assigned port value and print it out. */
     length = sizeof(name);
     if (getsockname(sock, &name, &length)) {
          perror("getting socket name");
          exit(1);
     }
     printf("Socket has port #%d\n", ntohs(name.sin_port));

     /* Read from the socket */
     if (read(sock, buf, 1024) < 0)
          perror("receiving datagram packet");
     printf("-->%s\n", buf);
     close(sock);
}