Question

I am creating a winsock UDP program. code i am using is shown below.

I am always getting port assignment error.

I am not able to understand why port always allocated is zero. If some 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 doesn’t bind to the listening port until you either issue a sendto() or a bind() on the socket. The latter lets you select the port that you want to listen on. Sendto(), on the other hand, will pick an ephemeral port for you. I would expect that the port will remain zero until you do one of these two things.

Clarification

I looked into this a little more after some of the comment. According to the Single UNIX Specification the result of calling socket() is an unbound socket. A socket is bound explicitly by calling bind() or implicitly sendto().

Think of a socket’s name as a tuple containing its (Address Family, Protocol, local IP Address, and local Port Number). The first two are specified in the socket() call and the last two by calling bind(). In the case of connectionless protocols, a call to sendto() on a disconnected socket will result in an implicit bind to an OS chosen port number.

The most surprising thing is that the only reference that I can find to this behavior is in the remarks section of the Microsoft documentation for sendto().

If the socket is unbound, unique values are assigned to the local association by the system and the socket is then marked as bound. An application can use getsockname (Windows Sockets) to determine the local socket name in this case.

The Single UNIX Specification for getsockname() states:

If the socket has not been bound to a local name, the value stored in the object pointed to by address is unspecified.

It seems that a successful return with an unspecified result is the “standard” behavior… hmmm… The implementations that I have tried all return successfully with a socket address of 0.0.0.0:0 which corresponds to INADDR_ANY with an unspecified port. After calling either bind() or sendto(), getsockname() returns a populated socket address though the address portion might still be INADDR_ANY.