UDP socket port allocation error

Question

I am creating a winsock UDP program. The code I'm using is shown below.

I always get a port assignment error.

I can’t understand why the port is always allocated, is zero. If anyone can help me with this ....

void UDPecho(const char *, const char *);

void errexit(const char *, ...);

#define LINELEN  128
#define WSVERS  MAKEWORD(2, 0)

void main(int argc, char *argv[])
{
    char *host = "localhost";
    char *service = "echo";
    WSADATA wsadata;
    switch (argc) {
    case 1:
        host = "localhost";
        break;

    case 3:
        service = argv[2];
        /* FALL THROUGH */

    case 2:
        host = argv[1];
        break;

    default:
        fprintf(stderr, "usage: UDPecho [host [port]]\n");
        exit(1);

    }

    if (WSAStartup(WSVERS, &wsadata))
        errexit("WSAStartup failed\n");

    UDPecho(host, service);

    WSACleanup();

    exit(0);
}

void UDPecho(const char *host, const char *service)
{
    char buf[LINELEN+1];
    SOCKET s;
    int nchars;
    struct hostent *phe;
    struct servent *pse;
    struct protoent *ppe;
    struct sockaddr_in sin, my_sin;
    int type, status, client_port, size;
    char *transport = "udp";

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

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

    /* Map host name to IP address, allowing for dotted decimal */
    if ( phe = gethostbyname(host) )
        memcpy(&sin.sin_addr, phe->h_addr, phe->h_length);
    else if ( (sin.sin_addr.s_addr = inet_addr(host)) == INADDR_NONE)
        errexit("can't get \"%s\" host entry\n", host);

    printf("Our target server is at address %s\n", inet_ntoa(sin.sin_addr));
    printf("The size of an FD set is %d\n", sizeof(FD_SET));

    /* 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 == INVALID_SOCKET)
        errexit("can't create socket: %d\n", GetLastError());

    size = sizeof(sin);
    memset(&my_sin, 0, sizeof(sin));
    getsockname (s, (struct sockaddr *) &my_sin, &size);
    client_port = ntohs(my_sin.sin_port);

    if (client_port != 0)
        printf ("We are using port %2d\n", client_port);
    else {
        printf("No port assigned yet\n");
    }
}


void errexit(const char *format, ...)
{
    va_list args;
    va_start(args, format);
    vfprintf(stderr, format, args);
    va_end(args);
    WSACleanup();

    exit(1);
}

Answer 1

UDP does not bind to a listening port until you select sendto()or bind()in a socket. The latter allows you to select the port you want to listen to. sendto(), on the other hand, will choose the ephemeral port for you. I would expect the port to remain null until you did one of these two things.

Explanation

. Single UNIX Specification socket() . , bind() sendto().

, ( , , IP- ). socket() , bind(). sendto() OS-.

, , , Microsoft sendto().

, , . getsockname (Windows Sockets) .

UNIX getsockname() :

, , , , .

, - "" ... hmmm... , , 0.0.0.0:0, INADDR_ANY . bind(), sendto(), getsockname() , INADDR_ANY.