WSAOVERLAPPED 구조체

typedef struct _WSAOVERLAPPED {
  ULONG_PTR Internal;
  ULONG_PTR InternalHigh;
  union {
    struct {
      DWORD Offset;
      DWORD OffsetHigh;
    } ;
    PVOID  Pointer;
  } ;
  HANDLE    hEvent;
} WSAOVERLAPPED, *LPWSAOVERLAPPED;

#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdio.h>
#include <windows.h>
#pragma comment(lib, "wininet.lib")

int __cdecl main()
{

    //---------------------------------------------
    // Declare and initialize variables
    WSADATA wsaData;
    WSABUF DataBuf;

    WSAOVERLAPPED Overlapped = { 0 };
    SOCKET SendToSocket = INVALID_SOCKET;

    struct sockaddr_in RecvAddr;
    struct sockaddr_in LocalAddr;
    int RecvAddrSize = sizeof (RecvAddr);
    int LocalAddrSize = sizeof (LocalAddr);

    int Port = 27015;
    struct hostent *localHost;
    char *ip;

    char SendBuf[1024] = "Data buffer to send";
    int BufLen = 1024;
    DWORD BytesSent = 0;
    DWORD Flags = 0;

    int rc, err;
    int retval = 0;

    //---------------------------------------------
    // Initialize Winsock
    // Load Winsock
    rc = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (rc != 0) {
        fprintf(stderr, "Unable to load Winsock: %d\n", rc);
        return 1;
    }
    // Create an event handle and setup an overlapped structure.
    Overlapped.hEvent = WSACreateEvent();
    if (Overlapped.hEvent == NULL) {
        fprintf(stderr, "WSACreateEvent failed: %d\n", WSAGetLastError());
        WSACleanup();
        return 1;
    }
    //---------------------------------------------
    // Create a socket for sending data
    SendToSocket =
        WSASocket(AF_INET, SOCK_DGRAM, IPPROTO_UDP, NULL, 0,
                  WSA_FLAG_OVERLAPPED);
    if (SendToSocket == INVALID_SOCKET) {
        fprintf(stderr, "socket failed: %d\n", WSAGetLastError());
        WSACloseEvent(Overlapped.hEvent);
        WSACleanup();
        return 1;
    }
    //---------------------------------------------
    // Set up the RecvAddr structure with the IP address of
    // the receiver (in this example case "123.123.123.1")
    // and the specified port number.
    RecvAddr.sin_family = AF_INET;
    RecvAddr.sin_port = htons(Port);
    RecvAddr.sin_addr.s_addr = inet_addr("123.123.123.1");

    //---------------------------------------------
    // Set up the LocalAddr structure with the local IP address
    // and the specified port number.
    localHost = gethostbyname("");
    ip = inet_ntoa(*(struct in_addr *) *localHost->h_addr_list);

    LocalAddr.sin_family = AF_INET;
    LocalAddr.sin_addr.s_addr = inet_addr(ip);
    LocalAddr.sin_port = htons(Port);

    //---------------------------------------------
    // Bind the sending socket to the LocalAddr structure
    // that has the internet address family, local IP address
    // and specified port number.  
    rc = bind(SendToSocket, (struct sockaddr *) &LocalAddr, LocalAddrSize);
    if (rc == SOCKET_ERROR) {
        fprintf(stderr, "bind failed: %d\n", WSAGetLastError());
        WSACloseEvent(Overlapped.hEvent);
        closesocket(SendToSocket);
        WSACleanup();
        return 1;
    }
    //---------------------------------------------
    // Send a datagram to the receiver
    printf("Sending a datagram...\n");
    DataBuf.len = BufLen;
    DataBuf.buf = SendBuf;
    rc = WSASendTo(SendToSocket, &DataBuf, 1,
                   &BytesSent, Flags, (SOCKADDR *) & RecvAddr,
                   RecvAddrSize, &Overlapped, NULL);

    if ((rc == SOCKET_ERROR) && (WSA_IO_PENDING != (err = WSAGetLastError()))) {
        fprintf(stderr, "WSASendTo failed: %d\n", err);
        WSACloseEvent(Overlapped.hEvent);
        closesocket(SendToSocket);
        WSACleanup();
        return 1;
    }

    rc = WSAWaitForMultipleEvents(1, &Overlapped.hEvent, TRUE, INFINITE, TRUE);
    if (rc == WSA_WAIT_FAILED) {
        fprintf(stderr, "WSAWaitForMultipleEvents failed: %d\n",
                WSAGetLastError());
        retval = 1;
    }

    rc = WSAGetOverlappedResult(SendToSocket, &Overlapped, &BytesSent,
                                FALSE, &Flags);
    if (rc == FALSE) {
        printf("WSASendTo operation failed: %d\n", WSAGetLastError());
        retval = 1;
    }

    //---------------------------------------------
    // When the application is finished sending, close the socket.
    printf("Finished sending. Closing socket.\n");
    WSACloseEvent(Overlapped.hEvent);
    closesocket(SendToSocket);
    printf("Exiting.\n");

    //---------------------------------------------
    // Clean up and quit.
    WSACleanup();
    return (retval);
}