Creating a Passive UDP Socket

int passiveUDP(const char *service)
{
    return passivesock(service, "udp", 0);
}

u_short portbase = 0;

int passivesock(const char *service, const char *transport, int qlen)
{
    struct servent  *pse;
    struct protoent *ppe;
    struct sockaddr_in sin;
    int     s, type;

    memset(&sin, 0, sizeof(sin));
    sin.sin_family = AF_INET;
    sin.sin_addr.s_addr = INADDR_ANY;

    /* Map service name to port number */
    if ( pse = getservbyname(service, transport) )
        sin.sin_port = htons(ntohs((u_short)pse->s_port)
                    + portbase);
    else if ( (sin.sin_port = htons((u_short)atoi(service))) == 0 )
        errexit("can't get \"%s\" service entry\n", service);

    /* Map protocol name to protocol number */
    if ( (ppe = getprotobyname(transport)) == 0)
        errexit("can't get \"%s\" protocol entry\n", transport);

    /* Use protocol to choose a socket type */
    if (strcmp(transport, "udp") == 0)
        type = SOCK_DGRAM;
    else
        type = SOCK_STREAM;

    /* Allocate a socket */
    s = socket(PF_INET, type, ppe->p_proto);
    if (s < 0)
        errexit("can't create socket: %s\n", strerror(errno));

    /* Bind the socket */
    if (bind(s, (struct sockaddr *)&sin, sizeof(sin)) < 0)
        errexit("can't bind to %s port: %s\n", service,
                strerror(errno));
    if (type == SOCK_STREAM && listen(s, qlen) < 0)
        errexit("can't listen on %s port: %s\n", service,
                strerror(errno));
    return s;
}