浅谈（一）： WinSocket select（）用法

 

在进行TCP编程时，通常要用到select来管理socket，比如，设置recv接收数据超时时间，对方是否已经非正常关闭连接等。

 

select  函数原型：

 

 

int select (
  int nfds,                                          //用来与Berkely socket兼容， 通常写0
  fd_set* readfds,                            //检查可读性的socket集合
  fd_set* writefds,                           //检查可写性的socket集合
  fd_set* exceptfds, //检查异常的socket集合
  const struct timeval* timeout      // 设置超时时间  

);

返回值：

    readfds, writefds, exceptfds中被检查复合条件的socket总数。

注意：

   由于select每次返回时，readfds, writefds, exceptfds都会被改变，所以，下次调用select前，要重新初始化fds。

对于三种检查各自如下：

readfds:

> 若当前socket处在listen状态，则有个连接到来，调用accept将成功建立连接

>若连接已经建立，则意味着有新数据到来，此时可以调用recv接收数据

>对方连接终止，调用recv，根据返回值判断是否是正常终止： 0 对方正常终止， ErrorCode 对方重置连接或强制断开连接

writefds:

>当前socket调用了connect， 则成功连接

>当前socket调用了send，则能成功发送出去

exceptfds:

>当前socket调用connect，则连接失败

>OOB 数据可读（暂时没研究，OOB是什么个东东）

以下附上简单的测试代码：

server端：

#include "SocketInit.h"
#include "iostream"
using namespace std;

CInitSock sockInit;

int main()
{
	SOCKET s = SocketFactory::getTCPSocket();
	
	hostent *thisHost = gethostbyname("");
	char *localIP = inet_ntoa (*(struct in_addr *)*thisHost->h_addr_list);
	const int localIPInt = inet_addr(localIP);
	const unsigned short linkPort = ntohs(4001);//本地监听

	sockaddr_in servAddr;
	servAddr.sin_family = AF_INET;
	servAddr.sin_port = linkPort;
	servAddr.sin_addr.S_un.S_addr = localIPInt;

	::bind(s, (sockaddr *)&servAddr, sizeof(servAddr));
	::listen(s, 1);

	sockaddr_in clientAddr;
	int lenC = sizeof(clientAddr);

	cout<<"TCP server: "<<endl;
	SOCKET clientSocket = ::accept(s, (sockaddr *)&clientAddr, &lenC);
	cout<<"Recv a connection..."<<endl;

	timeval waitTime;
	waitTime.tv_sec=2;
	waitTime.tv_usec=0;
	while(true)
	{
		fd_set fdSocket;
		FD_ZERO(&fdSocket);
		FD_SET(clientSocket, &fdSocket);

		int nRet = ::select(0, &fdSocket, 0, 0, &waitTime);
		if(nRet<0)
		{
			cout<<"Faild Select()"<<endl;
			exit(-1);
		}
		else if(nRet ==0)
		{
			cout<<"time out()"<<endl;
			continue;
		}
		else 
		{
			if(FD_ISSET(clientSocket, &fdSocket))
			{
				char rBuff[100];
				int nRec=::recv(clientSocket, rBuff, 100, 0);
				cout<<"Recv from client data: "<<nRec<<endl;
				if(nRec < 0)
				{
					cout<<"Not normal close: "<<::GetLastError()<<endl;
					break;
				}
				else if(nRec == 0)
				{
					cout<<"Normal close"<<endl;
					break;
				}
				else
				{
					rBuff[nRec]=0;
					cout<<"Data : "<<rBuff<<endl;
				}

			}
		}
	}

	::closesocket(clientSocket);
	::closesocket(s);

	return 0;
} 

client端：

#include "SocketInit.h"
#include "iostream"
using namespace std;

CInitSock sockInit;
int main()
{
	
	SOCKET s = SocketFactory::getTCPSocket();

	hostent *thisHost = gethostbyname("");
	char *localIP = inet_ntoa (*(struct in_addr *)*thisHost->h_addr_list);
	const int localIPInt = inet_addr(localIP);
	const unsigned short linkPort = ntohs(4001);//本地监听

	sockaddr_in servAddr;
	servAddr.sin_family = AF_INET;
	servAddr.sin_port = linkPort;
	servAddr.sin_addr.S_un.S_addr = localIPInt;

	cout<<"Client : "<<endl;
	if(::connect(s, (sockaddr *)&servAddr, sizeof(servAddr)) == -1)
	{
		printf("Faild connect(): %d/n", ::GetLastError());
		return 0;
	}

	char sendStr[100];
	while(true)
	{
		cout<<"Input send data('q' to normal quit): ";
		cin.getline(sendStr, 100);
		if(sendStr[0]=='q') break;
			::send(s, sendStr, strlen(sendStr), 0);
	}

	::closesocket(s);

	return 0;
}