udp例子

对于UDP协议~因为不像TCP协议那样事先有connect~所以调用recvfrom前必须先用sendto, WSASendTo, or WSAJoinLeaf.来明确服务器端在哪里~（UDP没有bind 没有connect）MSDN上说：recvfrom的第一个参数（即套接字）必须是绑定的~（Descriptor identifying a bound socket.）（the socket can become bound implicitly to a local address through sendto, WSASendTo, or WSAJoinLeaf.）所以得recvfrom的第一个参数必须是已经调用过sendto的套接字myclient，用没调用过sendto的套接字sockrecv是错的~~~错误10022
//Server

#include <WinSock2.h>
#include <iostream>
#pragma comment(lib,"ws2_32.lib")
#ifdef _DEBUG
#define new DEBUG_NEW
#endif

// The one and only application object

using namespace std;

int main()
{
        WSADATA wsaData;
        int err;
        err = WSAStartup(MAKEWORD(2,2), &wsaData );
        SOCKET Srv=socket(AF_INET,SOCK_DGRAM,0);
        SOCKADDR_IN addSrv;
        addSrv.sin_addr.S_un.S_addr=htonl(INADDR_ANY);
        addSrv.sin_family=AF_INET;
        addSrv.sin_port=htons(4567);

        bind(Srv,(SOCKADDR*)&addSrv,sizeof(SOCKADDR));
        char buf[100];
        char buf2[100]="hello 客服端，我是你的服务器";
        SOCKADDR_IN Client;
        int len=sizeof(SOCKADDR);
        recvfrom(Srv,buf,100,0,(SOCKADDR*)&Client,&len);
        printf("%s\n%s\n",buf,inet_ntoa(Client.sin_addr));
        while(TRUE)
        {
                Sleep(1000);
                if(sendto(Srv,buf2,strlen("hello 客服端，我是你的服务器")+1,0,(SOCKADDR*)&Client,len)==SOCKET_ERROR )//把addSrv改成Client 这样才能发送成功
                {
                        printf("send failed\n");
                }
        }

        closesocket(Srv);
        WSACleanup( );
        return 0;
}

//client

#include <WinSock2.h>
#include <iostream>
#pragma comment(lib,"ws2_32.lib")
#ifdef _DEBUG
#define new DEBUG_NEW
#endif


// The one and only application object

using namespace std;

int main()
{
        WORD wVersionRequested;
        WSADATA wsaData;
        int err;
        wVersionRequested = MAKEWORD( 2, 2 );
        err = WSAStartup(wVersionRequested,&wsaData );
        if ( err != 0 )
        {
                return 0;
        }
        if ( LOBYTE( wsaData.wVersion ) != 2 ||        HIBYTE( wsaData.wVersion ) != 2 )
        {
                        WSACleanup( );
                        return 0;
        }
        SOCKET myclient=socket(AF_INET,SOCK_DGRAM,0);
        SOCKADDR_IN addrcl;
        addrcl.sin_addr.S_un.S_addr=inet_addr("127.0.0.1");
        addrcl.sin_family=AF_INET;
        addrcl.sin_port=htons(4567);
        char buf[100]="hello 服务器，我是你的客服端";
        int len=sizeof(SOCKADDR);
        sendto(myclient,buf,strlen("hello 服务器，我是你的客服端")+1,0,(SOCKADDR*)&addrcl,len);
        //把这句sendto注释掉也会出现跟下面myclient错写成sockrecv一样的错误10022 //说明recvfrom是跟sendto关联??//为什么？？？直接recvfrom为什么不行？？
        memset(&addrcl,0,sizeof(addrcl));
        char buf2[100]=" ";
        while(TRUE)
        {
                if(recvfrom(myclient,buf2,100,0,(SOCKADDR*)&addrcl,&len)==SOCKET_ERROR)
                {
                        printf("recvfrom failed\n");
                        printf("%d\n",WSAGetLastError());
                }//为什么这里没办法接收到服务器端发来的字符串？//懂了sockrecv是错的，得改成 myclient
                //sockrecv与 myclient的唯一差别是sockrecv没有sendto二myclient有 //为什么这样？？
                else
                {
                        printf("%s\n%s\n",buf2,inet_ntoa(addrcl.sin_addr));
                }
        }
        closesocket(myclient);
        WSACleanup( );
        return 1;
}