winsock select I/O模型初探

    windows对socket封装得有点异类，其典型的I/O模型有blocking,select,WSAAsyncSelect,WSAEventSelect,Overlapped,complete port,从本期开始逐步对各种I/O模型进行分析。

    select 模型是继承了berkeley的接口，在微软的MSDN中描述如下：

The select function determines the status of one or more sockets, waiting if necessary, to perform synchronous I/O.

int select(
int nfds,
fd_set* readfds,
fd_set* writefds,
fd_set* exceptfds,
const struct timeval* timeout
);
Parameters

nfds
[in] Ignored. The nfds parameter is included only for compatibility with Berkeley sockets.
readfds
[in, out] Optional pointer to a set of sockets to be checked for readability.
writefds
[in, out] Optional pointer to a set of sockets to be checked for writability.
exceptfds
[in, out] Optional pointer to a set of sockets to be checked for errors.
timeout
[in] Maximum time for select to wait, provided in the form of a TIMEVAL structure. Set the timeout parameter to null for blocking operations.

显然select模型是一种轮询的机制，在处理数据之前需要将感兴趣的socket添加到相应的集合中。它的优点是能够在一个线程中监听多个端口，由于引入了时间的参数，不会导致某个socket在没有事件到来时，长时间处于阻塞状态，但缺点也是明显的：一个是效率上的，轮询需要遍历socket集合，消耗大量的cpu时间，二是并发量上，监听的端口数量有限，winsock2默认是64个套接字，当然可以通过重定义宏FD_SIZE来修改最大监听数目，但是系统的性能会受到影响（还未测试，后续会补上）。综上可以看出select模型在服务器端用途非常有限，可能是当时为了解决阻塞问题临时提供的一套解决方案吧。

    下面提供一套客户端调用select模型的代码：

client端代码：  

#include "stdafx.h"
#include <winsock2.h>
#pragma comment(lib,"WS2_32.lib")

#define DEF_INVALID_SOCKET 0XFFFFFF
#define DEF_MAX_CONNECT 10  
#define FALSE 0
#define TRUE  1

BOOL CreateContext()
{
    WSADATA wsaData;

    if (WSAStartup(MAKEWORD(2,1),&wsaData)) //call Windows Sockets DLL
    {
        printf("Winsock can't initiate,error code: %d !\n",WSAGetLastError ());
        WSACleanup();
        return FALSE;
    }

    return TRUE;
}

int main(int argc, char* argv[])
{
    printf("Hello World!\n");
    SOCKET lszSocket[DEF_MAX_CONNECT] = {0XFFFFFF};
    printf("my client program! \n");

    if(!CreateContext())
    {
        return 0;
    }

    for(int i = 0;i < DEF_MAX_CONNECT;i++)
    {
         lszSocket[i] = socket(AF_INET,SOCK_STREAM,0);
         if( 0 > lszSocket[i])
         {
            printf("create socket error! The %d-th£¬error code: %d \n",i+1,WSAGetLastError ());            
            return 1;
         }
    }
    
    //bind socket
    for( i = 0;i < DEF_MAX_CONNECT;i++)
    {
        sockaddr_in lstLocalAddr;
        lstLocalAddr.sin_family = AF_INET;
        lstLocalAddr.sin_port = htons(0);
        lstLocalAddr.sin_addr.s_addr = htonl(INADDR_ANY);
        if(SOCKET_ERROR == bind(lszSocket[i],(SOCKADDR*)&lstLocalAddr,sizeof(lstLocalAddr)))
        {
             printf("bind socket error! The %d-th£¬error code: %d \n",i+1,WSAGetLastError());

             return 1;
        }
        else
        {
            sockaddr_in lstServerAddr;
            lstServerAddr.sin_family = AF_INET;
            lstServerAddr.sin_port = htons(5700);
            lstServerAddr.sin_addr.s_addr = inet_addr("127.0.0.1");            
            if(0 > connect(lszSocket[i],(sockaddr*)&lstServerAddr,sizeof(lstServerAddr)))
            {
                printf("connect socket error!The %d-th£¬error code: %d  \n",i+1,WSAGetLastError());
            }
        }
    }
   
    int liRet = 0;
    fd_set lstReadFd;
    timeval lstTime;
    lstTime.tv_sec = 0;
    lstTime.tv_usec = 10;
    while(1)
    {
        FD_ZERO(&lstReadFd);
        for(int i = 0;i < DEF_MAX_CONNECT;i++)
        {
            FD_SET(lszSocket[i],&lstReadFd);
        }

        liRet = select(DEF_MAX_CONNECT +1,&lstReadFd,NULL,NULL,&lstTime);
        switch(liRet)
        {
        case -1:
            printf("select socket error! error code: %d \n",WSAGetLastError());
            return 1;
            break;
        case 0:
            break;
        default:
            for( i = 0;i < DEF_MAX_CONNECT;i++)
            {                
                 if(FD_ISSET(lszSocket[i],&lstReadFd))
                 {
                     char lszRecvBuff[512] = {0};
                    int liRcvLen = recv(lszSocket[i],lszRecvBuff,512,0);
                    if(0 > liRcvLen)
                    {
                        printf(" %d socket,receive failed! \n",i+ 1);
                    }
                    printf("%d socket receive : %s \n", i+ 1, lszRecvBuff);
                 }
            }
        }     
    }

    for( i = 0;i < DEF_MAX_CONNECT;i++)
    {
        closesocket(lszSocket[i]);
    }

    //free resource
    WSACleanup();
    return 1;
}

服务端的代码相对简单，粘贴如下：

#include "stdafx.h"
#include <winsock2.h>
#pragma comment(lib,"WS2_32.lib")

int main(int argc, char* argv[])
{

	printf("Hello World!\n");
	WSADATA wsaData;
	if (WSAStartup(MAKEWORD(2,1),&wsaData))
	{
		printf("Winsock can't initiate, error code: %d !\n",WSAGetLastError());
		WSACleanup();
		return 1;
	}
	SOCKET lServerSocket = socket(AF_INET,SOCK_STREAM,0);
	sockaddr_in lstAddr;
	lstAddr.sin_family = AF_INET;
	lstAddr.sin_port = htons(5700);
	lstAddr.sin_addr.s_addr = htonl(INADDR_ANY);
	if(0 > bind(lServerSocket,(sockaddr*) &lstAddr,sizeof(lstAddr)))
	{
        printf("bind socket error ,code : %d \n",WSAGetLastError());
	}

	if( 0 != listen(lServerSocket,1000))
	{
        printf("listen socket error ,code : %d \n",WSAGetLastError());
	}
	while(1)
	{
		sockaddr_in lClientAddr;
		printf("Begin accept! \n");
		char lszSndBuff[1024];
		SOCKET lRecvSocket;
		int liLen = sizeof(lClientAddr);
		if(INVALID_SOCKET  == (lRecvSocket = accept(lServerSocket,(sockaddr*)&lClientAddr,&liLen)))
		{
			printf("receive error : %d \n",WSAGetLastError());
		}
		printf("client addr: %s ,port: %d  \n",inet_ntoa(lClientAddr.sin_addr),ntohs(lClientAddr.sin_port));
		sprintf(lszSndBuff,"hello, %d \n",ntohs(lClientAddr.sin_port));
		int liSndLen = send(lRecvSocket,lszSndBuff,strlen(lszSndBuff),0);
		if(liSndLen <= 0)
		{
			printf("snd %d hello failed ! code: %d \n",ntohs(lClientAddr.sin_port),WSAGetLastError());
		}
		Sleep(300);
	}
	return 0;
}