select io 多路复用

最新推荐文章于 2025-04-22 15:41:37 发布

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本文介绍了一种基于select机制的简单服务器模型，用于处理多个客户端连接请求，并通过实例展示了如何利用select进行文件描述符（fd）的检测，以及如何实现带超时的读取操作。

select主要用于检测多个fd状态，可检测fd大小收内核编译宏__FD_SETSIZE限制，默认为1024。

当打开的fd较多时，select的效率会降低。可修改下面简单服务器模型为仅接受连接，当fd数量超过20000时，select返回明显变慢。

1.简单单线程同时处理多连接的服务器模型

void testSelectSocket()
{
	int nListenFd; 
	int nLen; 
	struct sockaddr_in ServerAddress,ClientAddress; 
	int result; 
	fd_set readfds, testfds; 
	nListenFd = socket(AF_INET, SOCK_STREAM, 0);
	ServerAddress.sin_family = AF_INET; 
	ServerAddress.sin_addr.s_addr = htonl(INADDR_ANY); 
	ServerAddress.sin_port = htons(1234); 
	nLen = sizeof(ServerAddress); 
	bind(nListenFd, (struct sockaddr *)&ServerAddress, nLen); 
	listen(nListenFd, 5);

	printf("listen %d\r\n",nListenFd);	

	fd_set readFds;
	std::set<int> ClientFds;
	while(1)
	{
		FD_ZERO(&readFds);
		FD_SET(nListenFd,&readFds);

		std::set<int>::const_iterator it(ClientFds.begin()),itEnd(ClientFds.end());
		for(;it != itEnd;++it)
			FD_SET(*it,&readFds);

		int nRet = select(FD_SETSIZE,&readFds,NULL,NULL,NULL);
		if(nRet == -1)
		{
			printf("select error.");
			return;
		}
		else if(nRet > 0)
		{
			for(int fd=0;fd != FD_SETSIZE;++fd)
			{
				if(FD_ISSET(fd,&readFds))
				{
					if(fd == nListenFd)
					{
						socklen_t nLen = sizeof(ClientAddress); 
						int client_sockfd = accept(nListenFd, (struct sockaddr *)&ClientAddress, &nLen);
						printf("accept %d,\r\n",client_sockfd); 
						ClientFds.insert(client_sockfd);
					}
					else
					{
						printf("%d has thing.\r\n",fd);
						int nRead;

						//test if there some data to read;
						ioctl(fd, FIONREAD, &nRead);
						char ch; 
						if(nRead != 0)
							nRead = read(fd, &ch, 1); 
						if(nRead == 0) 
						{ 
							close(fd); 
							//FD_CLR(fd, &readFds); 
							ClientFds.erase(fd);
							printf("removing client on fd %d\r\n", fd); 
						} 
						else
						{
							if(ch != '\r' && ch != '\n')
								printf("serving client on fd %d %c %d \r\n", fd,ch,ch); 
						} 
	
					}

				}
			}
		}
		else
		{
			printf("select time out");
			return;
		}	
	}
}

2.实现对fd的time read

int Read(int fd, char * pBuffer, unsigned int nBufferLend, unsigned int nMillisecondTimeOut)
{
	struct timeval TimeOut;
	TimeOut.tv_sec  = nMillisecondTimeOut / 1000;
	TimeOut.tv_usec = (nMillisecondTimeOut%1000)*1000;
	int nReadLen = 0;
	char* ptr = pBuffer;
	fd_set readFds;
	FD_ZERO(&readFds);
	FD_SET(fd,&readFds);

	while(nReadLen < nBufferLend)
	{
		int nRet = select(fd+1,&readFds,NULL,NULL,&TimeOut);
		if(nRet == -1)										
		{
			nReadLen = -1;
			perror("select error");
			break;
		}
		else if(nRet > 0)
		{
			nRet = read(fd,ptr,1);
			printf("remain time,sec: %d,usec: %d\n",TimeOut.tv_sec,TimeOut.tv_usec);
			if(nRet < 0)
			{
				perror("read err:");
				break;
			}
			else if(nRet == 0)
				break;
			nReadLen += nRet;
			ptr += nRet;
		}
		else													
		{
			nReadLen = -1;
			printf("timeout!\n");
			break;
		}
	}

	return nReadLen;
}