UDP(面向无连接)的socket编程

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服务器端（接收端）程序：
1、创建套接字（socket）。
2、将套接字绑定到一个本地地址和端口上（bind）。
3、等待接收数据（recvfrom）。
4、关闭套接字。

客户端（发送端）程序：
1、创建套接字（socket）。
2、向服务器发送数据（sendto）。
3、关闭套接字。

1)recvfrom
The recvfrom function receives a datagram and stores the source address.

int recvfrom(SOCKET s,                   char FAR* buf,              int len,                    int flags,                  struct sockaddr FAR *from,  int FAR *fromlen            );

Parameters
s
[in] Descriptor identifying a bound socket.
buf
[out] Buffer for the incoming data.
len
[in] Length of buf.
flags
[in] Indicator specifying the way in which the call is made.
from
[out] Optional pointer to a buffer that will hold the source address upon return.
fromlen
[in, out] Optional pointer to the size of the from buffer.

2)sendto
The sendto function sends data to a specific destination.

int sendto(SOCKET s,                        const char FAR *buf,            int len,                         int flags,                       const struct sockaddr FAR *to,  int tolen                        );

Parameters
s
[in] Descriptor identifying a (possibly connected) socket.
buf
[in] Buffer containing the data to be transmitted.
len
[in] Length of the data in buf.
flags
[in] Indicator specifying the way in which the call is made.
to
[in] Optional pointer to the address of the target socket.
tolen
[in] Size of the address in to.

eg:
头文件：Winsock2.h、stdio.h，连接ws2_32.lib。
服务器端：
void main()
{
WORD wVersionRequested;
WSADATA wsaData;
int err;

wVersionRequested = MAKEWORD( 1, 1 );//ver1.1err = WSAStartup( wVersionRequested, &wsaData );//加载winsocket库if ( err != 0 ) {                               //版本协商    return;}if ( LOBYTE( wsaData.wVersion ) != 1 ||       //low ver    HIBYTE( wsaData.wVersion ) != 1 )         //high ver{           WSACleanup( );//释放winsocket库    return; }SOCKET sockSrv=socket(AF_INET,SOCK_DGRAM,0);//创建数据报socketSOCKADDR_IN addrSrv;//定义地址结构体变量addrSrv.sin_addr.S_un.S_addr=htonl(INADDR_ANY);addrSrv.sin_family=AF_INET;//地址族addrSrv.sin_port=htons(10002);//端口bind(sockSrv,(SOCKADDR*)&addrSrv,sizeof(SOCKADDR));//绑定SOCKADDR_IN addrClient;//定义地址结构变量int len=sizeof(SOCKADDR);char recvBuf[100];recvfrom(sockSrv,recvBuf,100,0,(SOCKADDR*)&addrClient,&len);printf("%s\n",recvBuf);closesocket(sockSrv);//关闭套接字WSACleanup();//释放winsocket库

}

客户端：
void main()
{
WORD wVersionRequested;
WSADATA wsaData;
int err;

wVersionRequested = MAKEWORD( 1, 1 );//ver1.1err = WSAStartup( wVersionRequested, &wsaData );//加载winsocket库if ( err != 0 ) {                               //版本协商    return;}if ( LOBYTE( wsaData.wVersion ) != 1 ||       //low ver    HIBYTE( wsaData.wVersion ) != 1 )         //high ver{           WSACleanup( );//释放winsocket库    return; }SOCKET sockClient=socket(AF_INET,SOCK_DGRAM,0);//创建数据报socketSOCKADDR_IN addrSrv;//定义地址结构体变量addrSrv.sin_addr.S_un.S_addr=inet_addr("127.0.0.1");addrSrv.sin_family=AF_INET;addrSrv.sin_port=htons(10002);sendto(sockClient,"Hello",strlen("Hello")+1,0,(SOCKADDR*)&addrSrv,sizeof(SOCKADDR));closesocket(sockClient);//关闭套接字WSACleanup();//释放winsocket库

}

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