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Creating a socket for the client

  • Makale

After initialization, a SOCKET object must be instantiated for use by the client.

To create a socket

  1. Declare an addrinfo object that contains a sockaddr structure and initialize these values. For this application, the Internet address family is unspecified so that either an IPv6 or IPv4 address can be returned. The application requests the socket type to be a stream socket for the TCP protocol.

    struct addrinfo *result = NULL,
                *ptr = NULL,
                hints;

ZeroMemory( &hints, sizeof(hints) );
hints.ai_family   = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;

  2. Call the getaddrinfo function requesting the IP address for the server name passed on the command line. The TCP port on the server that the client will connect to is defined by DEFAULT_PORT as 27015 in this sample. The getaddrinfo function returns its value as an integer that is checked for errors.

    #define DEFAULT_PORT "27015"

// Resolve the server address and port
iResult = getaddrinfo(argv[1], DEFAULT_PORT, &hints, &result);
if (iResult != 0) {
    printf("getaddrinfo failed: %d\n", iResult);
    WSACleanup();
    return 1;
}

  3. Create a SOCKET object called ConnectSocket.

    SOCKET ConnectSocket = INVALID_SOCKET;

  4. Call the socket function and return its value to the ConnectSocket variable. For this application, use the first IP address returned by the call to getaddrinfo that matched the address family, socket type, and protocol specified in the hints parameter. In this example, a TCP stream socket was specified with a socket type of SOCK_STREAM and a protocol of IPPROTO_TCP. The address family was left unspecified (AF_UNSPEC), so the returned IP address could be either an IPv6 or IPv4 address for the server.

    If the client application wants to connect using only IPv6 or IPv4, then the address family needs to be set to AF_INET6 for IPv6 or AF_INET for IPv4 in the hints parameter.

    // Attempt to connect to the first address returned by
// the call to getaddrinfo
ptr=result;

// Create a SOCKET for connecting to server
ConnectSocket = socket(ptr->ai_family, ptr->ai_socktype, 
    ptr->ai_protocol);

  5. Check for errors to ensure that the socket is a valid socket.

    if (ConnectSocket == INVALID_SOCKET) {
    printf("Error at socket(): %ld\n", WSAGetLastError());
    freeaddrinfo(result);
    WSACleanup();
    return 1;
}

The parameters passed to the socket function can be changed for different implementations.

Error detection is a key part of successful networking code. If the socket call fails, it returns INVALID_SOCKET. The if statement in the previous code is used to catch any errors that may have occurred while creating the socket. WSAGetLastError returns an error number associated with the last error that occurred.

Note

More extensive error checking may be necessary depending on the application.

For example, setting hints.ai_family to AF_UNSPEC can cause the connect call to fail. If that happens, then use a specific IPv4 (AF_INET) or IPv6 (AF_INET6) value instead.

WSACleanup is used to terminate the use of the WS2_32 DLL.

Next Step: Connecting to a Socket

Getting Started With Winsock

Initializing Winsock

Winsock Client Application