关于getsockname函数的使用

getsockname可以获得一个与socket相关的地址。
服务器端可以通过它得到相关客户端地址。
而客户端也可以得到当前已连接成功的socket的ip和端口。

第二种情况在客户端不进行bind而直接连接服务器时，而且客户端需要知道当前使用哪个ip进行通信时比较有用（如多网卡的情况）。

笔者分别通过TCP和UDP协议进行测试，测试结果表明：
对于TCP连接的情况，如果不进行bind指定IP和端口，那么调用connect连接成功后，使用getsockname可以正确获得当前正在通信的socket的IP和端口地址。
而对于UDP的情况，无论是在调用sendto之后还是收到服务器返回的信息之后调用，都无法得到正确的ip地址：使用getsockname得到ip为0，端口正确。

测试用例如下：
1. TCP
//client
int _tmain(int argc, _TCHAR* argv[])
{
WSADATA wsadata;
int ret = WSAStartup ( 0x0202 , & wsadata );
if ( ret != 0 || 0x0202 != wsadata.wVersion )
{
  cout<<"WSAStartup Error!";
  return 0;
}
SOCKET mySock = INVALID_SOCKET;
mySock = socket(AF_INET,SOCK_STREAM,0);
if (mySock == INVALID_SOCKET)
{
  cout<<"Create Socket Error!";
  return 0;
}

sockaddr_in addrDest;
memset(&addrDest,0,sizeof(addrDest));
addrDest.sin_family = AF_INET;
addrDest.sin_addr.s_addr = inet_addr("10.10.44.76");
addrDest.sin_port = htons(9000);

ret = connect(mySock,(sockaddr*)&addrDest,sizeof(addrDest));
if (ret == -1)
{
cout<<GetLastError()<<endl;
return 0;
}

cout<<"Connect to Server Success!"<<endl;

sockaddr_in addrMy;
memset(&addrMy,0,sizeof(addrMy));
int len = sizeof(addrMy);

ret = getsockname(mySock,(sockaddr*)&addrMy,&len);
if (ret != 0)
{
cout<<"Getsockname Error!"<<endl;
return 0;
}

cout<<"Current Socket IP:"<<inet_ntoa(addrMy.sin_addr)<<":"<<ntohs(addrMy.sin_port)<<endl;

getchar();

closesocket(mySock);
WSACleanup();

return 0;
}
//server
int _tmain(int argc, _TCHAR* argv[])
{
WSADATA wsadata;
int ret = WSAStartup ( 0x0202 , & wsadata );
if ( ret != 0 || 0x0202 != wsadata.wVersion )
{
  cout<<"WSAStartup Error!";
  return 0;
}
SOCKET listenSock = INVALID_SOCKET;
listenSock = socket(AF_INET,SOCK_STREAM,0);
if (listenSock == INVALID_SOCKET)
{
  cout<<"Create Socket Error!";
  return 0;
}

sockaddr_in addrBind;
memset(&addrBind,0,sizeof(addrBind));
addrBind.sin_family = AF_INET;
addrBind.sin_addr.s_addr = INADDR_ANY;
addrBind.sin_port = htons(9000);

ret = bind(listenSock,(sockaddr*)&addrBind,sizeof(addrBind));
if (ret == SOCKET_ERROR)
{
  cout<<"Bind Error"<<endl;
  return 0;
}

ret = listen(listenSock,5);
if (ret != 0)
{
  cout<<"Listen Error"<<endl;
  return 0;
}

SOCKET conSock;
sockaddr_in addrCon;
int len = sizeof(addrCon);
while (true)
{
  conSock = accept(listenSock,(sockaddr*)&addrCon,&len);
  if (conSock == INVALID_SOCKET)
  {
   cout<<"Accept Error"<<endl;
   return 0;
  }
  cout<<inet_ntoa(addrCon.sin_addr)<<" Connect to Server!"<<endl;

}

closesocket(conSock);
closesocket(listenSock);

WSACleanup();

return 0;
}
2. UDP
//client
int _tmain(int argc, _TCHAR* argv[])
{
WSADATA wsadata;
int ret = WSAStartup ( 0x0202 , & wsadata );
if ( ret != 0 || 0x0202 != wsadata.wVersion )
{
  cout<<"WSAStartup Error!";
  return 0;
}
SOCKET mySock = INVALID_SOCKET;
mySock = socket(AF_INET,SOCK_DGRAM,0);
if (mySock == INVALID_SOCKET)
{
  cout<<"Create Socket Error!";
  return 0;
}

sockaddr_in addrDest;
memset(&addrDest,0,sizeof(addrDest));
addrDest.sin_family = AF_INET;
addrDest.sin_addr.s_addr = inet_addr("10.10.44.76");
addrDest.sin_port = htons(9000);

//set non-blocking
int nMode = 1;
ioctlsocket(mySock,FIONBIO,(u_long FAR*) &nMode);

char sendchar[20];
strcpy_s(sendchar,"login");

sendto(mySock,sendchar,strlen(sendchar)+1,0,(sockaddr*)&addrDest,sizeof(sockaddr));

//get send socket ip
sockaddr_in addrMy;
memset(&addrMy,0,sizeof(addrMy));
int leng = sizeof(addrMy);

ret = getsockname(mySock,(sockaddr*)&addrMy,&leng);
if (ret != 0)
{
cout<<"Getsockname Error!"<<endl;
return 0;
}

cout<<"Current Socket IP:"<<inet_ntoa(addrMy.sin_addr)<<":"<<ntohs(addrMy.sin_port)<<endl;

char recvchar[20];
sockaddr_in addrRev;
memset(&addrRev,0,sizeof(addrRev));
memset(recvchar,0,20);
int len = sizeof(sockaddr);
while (true)
{
  if (recvfrom(mySock,recvchar,20,0,(sockaddr*)&addrRev,&len) == SOCKET_ERROR)
  {
   if (GetLastError() == WSAEWOULDBLOCK || GetLastError() == 10054)
   {
    continue;
   }
   else
   {
    cout<<"Receive Error!"<<GetLastError()<<endl;
    return 0;
   }
  }

cout<<"login success!"<<endl;

  //get send socket ip
  sockaddr_in addrMy;
  memset(&addrMy,0,sizeof(addrMy));
  int leng = sizeof(addrMy);

  ret = getsockname(mySock,(sockaddr*)&addrMy,&leng);
  if (ret != 0)
  {
   cout<<"Getsockname Error!"<<endl;
   return 0;
  }

cout<<"Current Socket IP:"<<inet_ntoa(addrMy.sin_addr)<<":"<<ntohs(addrMy.sin_port)<<endl;
}

return 0;
}
//server
int _tmain(int argc, _TCHAR* argv[])
{
WSADATA wsadata;
int ret = WSAStartup ( 0x0202 , & wsadata );
if ( ret != 0 || 0x0202 != wsadata.wVersion )
{
  cout<<"WSAStartup Error!";
  return 0;
}
SOCKET mySock = INVALID_SOCKET;
mySock = socket(AF_INET,SOCK_DGRAM,0);
if (mySock == INVALID_SOCKET)
{
  cout<<"Create Socket Error!";
  return 0;
}

sockaddr_in addrBind;
memset(&addrBind,0,sizeof(addrBind));
addrBind.sin_family = AF_INET;
addrBind.sin_addr.s_addr = INADDR_ANY;
addrBind.sin_port = htons(9000);

ret = bind(mySock,(sockaddr*)&addrBind,sizeof(addrBind));
if (ret == SOCKET_ERROR)
{
cout<<"Bind Error"<<endl;
return 0;
}

//set non-blocking
int nMode = 1;
ioctlsocket(mySock,FIONBIO,(u_long FAR*) &nMode);

sendto(mySock,recvchar,strlen(recvchar)+1,0,(sockaddr*)&addrRev,sizeof(sockaddr));
}

return 0;
}

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