EPOLLONESHOT事件

epoll模式中事件可能被触发多次，比如socket接收到数据交给一个线程处理数据，在数据没有处理完之前又有新数据达到触发了事件，另一个线程被激活获得该socket，从而产生多个线程操作同一socket，即使在ET模式下也有可能出现这种情况。采用EPOLLONETSHOT事件的文件描述符上的注册事件只触发一次，要想重新注册事件则需要调用epoll_ctl重置文件描述符上的事件，这样前面的socket就不会出现竞态。

一个采用EPOLLONETSHOT的例子：

epoll_oneshot._server.cpp服务端程序

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#include<sys/types.h>  
#include<sys/socket.h>  
#include<netinet/in.h>  
#include<arpa/inet.h>  
#include<assert.h>  
#include<stdio.h>  
#include<unistd.h>  
#include<errno.h>  
#include<string.h>  
#include<fcntl.h>  
#include<stdlib.h>  
#include<sys/epoll.h>  
#include<pthread.h>  
#include<iostream>  
#define MAX_EVENT_NUMBER 1024//最大事件连接数  
#define BUFFER_SIZE 1024//接收缓冲区大小  
using namespace std;  
struct fds{//文件描述符结构体，用作传递给子线程的参数  
    int epollfd;  
    int sockfd;  
};  
int setnonblocking(int fd){//设置文件描述符为非阻塞  
    int old_option=fcntl(fd,F_GETFL);  
    int new_option=old_option|O_NONBLOCK;  
    fcntl(fd,F_SETFL,new_option);  
    return old_option;  
}  
void addfd(int epollfd,int fd,bool oneshot){//为文件描述符添加事件  
    epoll_event event;  
    event.data.fd=fd;  
    event.events=EPOLLIN|EPOLLET;  
    if(oneshot){//采用EPOLLONETSHOT事件  
        event.events|=EPOLLONESHOT;  
    }  
    epoll_ctl(epollfd,EPOLL_CTL_ADD,fd,&event);  
    setnonblocking(fd);  
}  
void reset_oneshot(int epollfd,int fd){//重置事件  
    epoll_event event;  
    event.data.fd=fd;  
    event.events=EPOLLIN|EPOLLET|EPOLLONESHOT;  
    epoll_ctl(epollfd,EPOLL_CTL_ADD,fd,&event);  
}  
void* worker(void* arg){//工作者线程(子线程)接收socket上的数据并重置事件  
    int sockfd=((fds*)arg)->sockfd;  
    int epollfd=((fds*)arg)->epollfd;//事件表描述符从arg参数(结构体fds)得来  
    cout<<"start new thread to receive data on fd:"<<sockfd<<endl;  
    char buf[BUFFER_SIZE];  
    memset(buf,'\0',BUFFER_SIZE);//缓冲区置空  
    while(1){  
        int ret=recv(sockfd,buf,BUFFER_SIZE-1,0);//接收数据  
        if(ret==0){//关闭连接  
            close(sockfd);  
            cout<<"close "<<sockfd<<endl;  
            break;  
        }  
        else if(ret<0){  
            if(errno==EAGAIN){//并非网络出错，而是可以再次注册事件  
                reset_oneshot(epollfd,sockfd);  
                cout<<"reset epollfd"<<endl;  
                break;  
            }  
        }  
        else{  
            cout<<buf;  
            sleep(5);//采用睡眠是为了在5s内若有新数据到来则该线程继续处理，否则线程退出  
        }  
    }  
    cout<<"thread exit on fd:"<<sockfd;  
    //_exit(0);//这个会终止整个进程！！  
    return NULL;  
}  
int main(int argc,char* argv[]){  
    if(argc<=2){  
        cout<<"argc<=2"<<endl;  
        return 1;  
    }  
    const char* ip=argv[1];  
    int port=atoi(argv[2]);  
    int ret=0;  
    struct sockaddr_in address;  
    bzero(&address,sizeof(address));  
    address.sin_family=AF_INET;  
    inet_pton(AF_INET,ip,&address.sin_addr);  
    address.sin_port=htons(port);  
    int listenfd=socket(PF_INET,SOCK_STREAM,0);  
    assert(listenfd>=0);  
    ret=bind(listenfd,(struct sockaddr*)&address,sizeof(address));  
    assert(ret!=-1);  
    ret=listen(listenfd,5);  
    assert(ret!=-1);  
    epoll_event events[MAX_EVENT_NUMBER];  
    int epollfd=epoll_create(5);  
    assert(epollfd!=-1);  
    addfd(epollfd,listenfd,false);//不能将监听端口listenfd设置为EPOLLONESHOT否则会丢失客户连接  
    while(1){  
        int ret=epoll_wait(epollfd,events,MAX_EVENT_NUMBER,-1);//等待事件发生  
        if(ret<0){  
            cout<<"epoll error"<<endl;  
            break;  
        }  
        for(int i=0;i<ret;i++){  
            int sockfd=events[i].data.fd;  
            if(sockfd==listenfd){//监听端口  
                struct sockaddr_in client_address;  
                socklen_t client_addrlength=sizeof(client_address);  
                int connfd=accept(listenfd,(struct sockaddr*)&client_address,&client_addrlength);  
                addfd(epollfd,connfd,true);//新的客户连接置为EPOLLONESHOT事件  
            }  
            else if(events[i].events&EPOLLIN){//客户端有数据发送的事件发生  
                pthread_t thread;  
                fds fds_for_new_worker;  
                fds_for_new_worker.epollfd=epollfd;  
                fds_for_new_worker.sockfd=sockfd;  
                pthread_create(&thread,NULL,worker,(void*)&fds_for_new_worker);//调用工作者线程处理数据  
            }  
            else{  
                cout<<"something wrong"<<endl;  
            }  
        }  
    }  
    close(listenfd);  
    return 0;  
}

epoll_oneshot_client.cpp客户端程序

#include<iostream>  
#include<assert.h>  
#include<sys/socket.h>  
#include<netinet/in.h>  
#include<arpa/inet.h>  
#include<stdio.h>  
#include<unistd.h>  
#include<string.h>  
#include<stdlib.h>  
#include<sys/types.h>  
#include<sys/stat.h>  
#include<errno.h>  
#include<fcntl.h>  
#define BUFFER_SIZE 512  
using namespace std;  
int main(int argc,char* argv[]){  
    if(argc<=2){  
        cout<<"argc<=2"<<endl;  
        return 1;  
    }  
    const char* ip=argv[1];  
    int port=atoi(argv[2]);  
    char buf[BUFFER_SIZE];  
    struct sockaddr_in server_address;  
    bzero(&server_address,sizeof(server_address));  
    server_address.sin_family=AF_INET;  
    inet_pton(AF_INET,ip,&server_address.sin_addr);  
    server_address.sin_port=htons(port);  
    int sockfd=socket(PF_INET,SOCK_STREAM,0);  
    assert(sockfd>=0);  
    int fd=open("epoll_oneshot_server.cpp",O_RDONLY);//打开一个文件并向服务端不断发送数据  
    if(connect(sockfd,(struct sockaddr*)&server_address,sizeof(server_address))<0){  
        cout<<"connect error "<<strerror(errno)<<endl;  
    }  
    else{  
        while(read(fd,buf,BUFFER_SIZE-1)>0){  
            send(sockfd,buf,strlen(buf),0);  
        }  
    }  
    close(sockfd);  
    return 0;  
}

补充：软件开发 , C++ ,