datainet.c

#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;
    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);
}