C++网络编程（3）

**重点内容**EPOLLONESHOT模式

即使我们使用ET模式，一个socket上的某个事件还是可能被触发多次。这在并发程序中就会引起一个问题。比如一个线程（或进程，下同）在读取完某个socket上的数据后开始处理这些数据，而在数据的处理过程中该socket上又有新数据可读（EPOLLIN再次被触发），此时另外一个线程被唤醒来读取这些新的数据。于是就出现了两个线程同时操作一个socket的局面。这当然不是我们期望的。我们期望的是一个socket连接在任一时刻都只被一个线程处理。这一点可以使用epoll的EPOLLONESHOT事件实现。

对于注册EPOLLONESHOT事件的文件描述符，操作系统最多触发其上注册的一个可读、可写或者异常事件。这样，当一个线程在处理某个socket时，其他线程是不可能有机会操作该socket的。但反过来思考，注册了EPOLLONESHOT事件的socket一旦被某个线程处理完毕，该线程就应该立即重置这个socket上的EPOLLONESHOT事件，以确保这个socket下次一次可读时，其EPOLLIN事件能被触发，进而让其他工作线程有机会继续处理这个socket。

#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>#define MAX_EVENT_NUMBER 1024#define BUFFER_SIZE 1024struct fds{   int epollfd;   int sockfd;};int setnonblocking( int fd ){    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 ){    epoll_event event;    event.data.fd = fd;    event.events = EPOLLIN | EPOLLET;    if( oneshot ) //只监听一次事件，当监听完这次事件之后，如果还需要继续监听这个socket的话，需要再次把这个socket加入到EPOLL队列里    {        event.events |= EPOLLONESHOT;    }    epoll_ctl( epollfd, EPOLL_CTL_ADD, fd, &event );    setnonblocking( fd );}void reset_oneshot( int epollfd, int fd ){    epoll_event event;    event.data.fd = fd;    event.events = EPOLLIN | EPOLLET | EPOLLONESHOT;    epoll_ctl( epollfd, EPOLL_CTL_MOD, fd, &event ); //将fd重新添加到epoll队列中}void* worker( void* arg ){    int sockfd = ( (fds*)arg )->sockfd;    int epollfd = ( (fds*)arg )->epollfd;    printf( "start new thread to receive data on fd: %d\n", sockfd );    char buf[ BUFFER_SIZE ];    memset( buf, '\0', BUFFER_SIZE );    while( 1 )    {        int ret = recv( sockfd, buf, BUFFER_SIZE-1, 0 );        if( ret == 0 )        {            close( sockfd );            printf( "foreiner closed the connection\n" );            break;        }        else if( ret < 0 )        {            if( errno == EAGAIN )            {                reset_oneshot( epollfd, sockfd ); //将事件重新添加到epoll监听列表中                printf( "read later\n" );                break;            }        }        else        {            printf( "get content: %s\n", buf ); //添加线程执行计划            sleep( 5 );        }    }    printf( "end thread receiving data on fd: %d\n", sockfd );}int main( int argc, char* argv[] ){    if( argc <= 2 )    {        printf( "usage: %s ip_address port_number\n", basename( argv[0] ) );        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 );    while( 1 )    {        int ret = epoll_wait( epollfd, events, MAX_EVENT_NUMBER, -1 );        if ( ret < 0 )        {            printf( "epoll failure\n" );            break;        }        for ( int i = 0; i < ret; i++ )        {            int sockfd = events[i].data.fd;            if ( sockfd == listenfd )            {                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 );            }            else if ( events[i].events & EPOLLIN )            {                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 );            }            else            {                printf( "something else happened \n" );            }        }    }    close( listenfd );    return 0;}