高性能网络编程 - epoll机制

select系统调用不仅对描述符的数量有限制，而且在高并发的情况下，哪怕只有一个活跃的套接字，也要轮询全部的fd set，而epoll采用回调的事件通知机制，只需要处理活跃的套接字。比如Nginx服务器采用的就是epoll，下面这个程序（当接收到大于10B的数据时）展示了epoll在边沿触发和电平触发的不同表现，在edge-trigger模式下，需要我们的程序一次将这次的事情处理完成（比如把数据全部读取），因为这个事件下次不会加到内核事件注册表中了，相反level-trigger模式下就跟我们的处理程序有关，如果本次没有读取出全部的数据，下次这个事情仍然会被注册的。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/epoll.h>
#include <errno.h>
#include <libgen.h>

#define MAXEVENTS 1024
#define BUF_SIZE 10

//set this descriptor non blocking
static int setnonblocking(int sfd){
    int flags, s;

    flags = fcntl(sfd, F_GETFL, 0);
    if (flags == -1) {
        perror ("fcntl");
        return -1;
    }   

    flags |= O_NONBLOCK;
    s = fcntl(sfd, F_SETFL, flags);
    if (s == -1) {
        perror ("fcntl");
        return -1;
    }   

    return 0;
}

//
static int create_and_bind (char *port){
    struct addrinfo hints;
    struct addrinfo *result, *rp;
    int s, sfd;

    memset (&hints, 0, sizeof (struct addrinfo));
    hints.ai_family = AF_UNSPEC;     //IPv4 IPv6通用
    hints.ai_socktype = SOCK_STREAM; /* We want a TCP socket */
    hints.ai_flags = AI_PASSIVE;     //监听套接字；

    //根据暗示得到所有可用的addrinfo
    s = getaddrinfo (NULL, port, &hints, &result);
    if (s != 0){
        fprintf (stderr, "getaddrinfo: %s\n", gai_strerror (s));
        return -1;
    }
    //use the first addr to create socket
    for (rp = result; rp != NULL; rp = rp->ai_next) {
        sfd = socket (rp->ai_family, rp->ai_socktype, rp->ai_protocol);
        if (sfd == -1)
            continue;

        s = bind (sfd, rp->ai_addr, rp->ai_addrlen);
        if (s == 0) {
            //bind success
            break;
        }

        close (sfd);//if bind failed we close this socket
    }

  if (rp == NULL) {
        fprintf (stderr, "Could not bind\n");
        return -1;
  }

  freeaddrinfo (result);

  return sfd;
}

// add EPOLLIN event of fd to the epollfd
void addfd(int epollfd, int fd, int enable_et){
    struct epoll_event event;
    event.data.fd = fd;
    event.events = EPOLLIN;
    if(enable_et)
        event.events |= EPOLLET;
    epoll_ctl(epollfd, EPOLL_CTL_ADD, fd, &event);
    setnonblocking(fd);
}

//the level-trigger logic
void lt(struct epoll_event *events, int number, int epollfd, int listenfd){
    char buf[BUF_SIZE];
    int i;
    for(i = 0; i < number; i++){
        int sockfd = events[i].data.fd;
        //new client connection
        if(sockfd == listenfd){
            struct sockaddr_in client_addr;
            socklen_t client_addrlen = sizeof(client_addr);
            int connfd = accept(listenfd, (struct sockaddr *)&client_addr, &client_addrlen);
            if(connfd < 0){
                perror("accept failed..");
                exit(-1);
            }
            //register this event
            addfd(epollfd, connfd, 0);
        }else if(events[i].events & EPOLLIN){
            //if there exists data unreaded , this code will invoke
            printf("sockefd %d read event trigger once\n", sockfd);
            int ret = recv(sockfd, buf, BUF_SIZE, 0);
            if(ret <= 0){
                printf("some read error or client closed.\n");
                close(sockfd); // will unregister
                continue;
            }
            printf("get %d bytes from client: %s\n", ret, buf);
        }else{
            // what we are not interested
            printf("something else happened\n");
        }
    }
}

//the edge-trigger logic
void et(struct epoll_event *events, int number, int epollfd, int listenfd){
    char buf[BUF_SIZE];
    int i;
    for(i = 0; i < number; i++){
        int sockfd = events[i].data.fd;
        //new client connection
        if(sockfd == listenfd){
            struct sockaddr_in client_addr;
            socklen_t client_addrlen = sizeof(client_addr);
            int connfd = accept(listenfd, (struct sockaddr *)&client_addr, &client_addrlen);
            if(connfd < 0){
                perror("accept failed..");
                exit(-1);
            }
            //register this event and set edge trigger mode
            addfd(epollfd, connfd, 1);
        }else if(events[i].events & EPOLLIN){
            //in this mode , we should read all the data out once
            printf("sockefd %d read event trigger once\n", sockfd);
            while(1){
                memset(buf, 0, sizeof(buf));
                int ret = recv(sockfd, buf, BUF_SIZE, 0);
                if(ret < 0){
                    //for the nonblocking IO,the following errors indicate the read complete
                    if(errno == EAGAIN || errno == EWOULDBLOCK){
                        printf("read later\n");
                        break;
                    }
                    printf("some other error\n");
                    close(sockfd); // will unregister
                    break;
                }else if(ret == 0){
                    printf("client closed.\n");
                    close(sockfd);
                    break;
                }else{
                    printf("get %d bytes from client: %s\n", ret, buf);
                }
            }
        }else{
            // what we are not interested
            printf("something else happened\n");
        }
    }
}

int main (int argc, char *argv[]){
    int sfd, s;
    int efd;
    struct epoll_event event;
    struct epoll_event events[MAXEVENTS];

    if (argc < 2){
        fprintf (stderr, "Usage: %s [port]\n", basename(argv[0]));
        exit (EXIT_FAILURE);
    }

//  const char *ip = argv[1];
//  int port = atoi(argv[2]);

    sfd = create_and_bind (argv[1]);
    if (sfd == -1)
        abort ();

    //listen for connection coming
    s = listen (sfd, SOMAXCONN);
    if (s == -1) {
        perror ("listen");
        abort ();
    }

    //create a epoll object
    efd = epoll_create1 (0);
    if (efd == -1) {
        perror ("epoll_create");
        abort ();
    }

    //add the listen socket to the event poll
    addfd(efd, sfd, 1);

    while(1){
        int ret = epoll_wait(efd, events, MAXEVENTS, -1);
        if(ret < 0){
            printf("epoll wait failture..\n");
            break;
        }
        //et(events, ret, efd, sfd);
        lt(events, ret, efd, sfd);
    }

    close (sfd);

    return EXIT_SUCCESS;
}

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