linux—epoll

一、epoll服务端实现中需要的3个函数：

• epoll_create：创建保存epoll文件描述符的空间。
• epoll_ctl：向空间注册并注销文件描述符。
• epoll_wait：与select函数类似，等待文件描述符发生变化。

二、示例

回声服务端：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/epoll.h>

#define BUF_SIZE 1024
#define EPOLL_SIZE 50
void error_handling(char * messages);

int main(int argc, char *argv[])
{
    if(argc != 2)
    {
        printf("Usage : %s <port>
", argv[0]);
        exit(1);
    }
    int serverSock, clientSock;
    struct sockaddr_in serverAddr, clientAddr;
    socklen_t clientAddrSize;

    char buf[BUF_SIZE];
    int strLen;

    struct epoll_event *ep_events;
    struct epoll_event event;
    int epfd, event_cnt;

    serverSock =socket(PF_INET, SOCK_STREAM, 0);
    if(serverSock == -1)
        error_handling("socket() error");

    memset(&serverAddr, 0, sizeof(serverAddr));
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = htonl(INADDR_ANY);
    serverAddr.sin_port = htons(atoi(argv[1]));

    if(bind(serverSock, (struct sockaddr*) &serverAddr, sizeof(serverAddr)) == -1)
        error_handling("bind() error");
    if(listen(serverSock, 5) == -1)
        error_handling("listen() error");

    epfd = epoll_create(EPOLL_SIZE);
    ep_events = (struct epoll_event*)malloc(sizeof(struct epoll_event) * EPOLL_SIZE);

    event.events = EPOLLIN;
    event.data.fd = serverSock;
    epoll_ctl(epfd, EPOLL_CTL_ADD, serverSock, &event);

    puts("Server start...");
    while(1){
        event_cnt = epoll_wait(epfd, ep_events, EPOLL_SIZE, -1);
        if(event_cnt == -1){
            puts("epoll_wait() error");
            break;
        }
        for(int i = 0; i < event_cnt; i++){
            if(ep_events[i].data.fd == serverSock){
                clientAddrSize = sizeof(clientAddr);
                clientSock = accept(serverSock, (struct sockaddr*)&clientAddr, &clientAddrSize);
                event.events = EPOLLIN;
                event.data.fd = clientSock;
                epoll_ctl(epfd, EPOLL_CTL_ADD, clientSock, &event);
                printf("connected client: %d
", clientSock);
            }
            else{
                strLen = read(ep_events[i].data.fd, buf, BUF_SIZE);
                if(strLen == 0){
                    epoll_ctl(epfd, EPOLL_CTL_DEL, ep_events[i].data.fd, NULL);
                    close(ep_events[i].data.fd);
                    printf("closed client: %d
", ep_events[i].data.fd);
                }
                else{
                    write(ep_events[i].data.fd, buf, strLen);
                    puts("echo");
                }
            }
        }
    }
    close(serverSock);
    puts("Server close...");
    return 0;
}

void error_handling(char * messages)
{
    puts(messages);
    exit(1);
}

三、条件触发和边缘触发

　　条件触发方式中，只要输入缓冲有数据就会一直通知该事件。

　　边缘触发方式中输入缓冲收到数据时仅注册一次该事件。即使输入缓冲中还留有数据，也不会再进行注册。

　　select模型是以条件触发的方式工作的。

　　epoll默认是以条件触发方式工作的。

　　若将文件（套接字）改为非阻塞模式。调用read&write函数时，无论是否存在数据，都会形成非阻塞文件（套接字）。

　　边缘触发方式下，以阻塞方式工作的read&write函数又可能引起服务器端的长事件停顿。因此，边缘触发方式中一定要采用非阻塞read&write函数。

边缘触发的回声服务端：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/epoll.h>
#include <fcntl.h>
#include <errno.h>

#define BUF_SIZE 4
#define EPOLL_SIZE 50
void setnonblockingmode(int fd);
void error_handling(char * messages);

int main(int argc, char *argv[])
{
    if(argc != 2)
    {
        printf("Usage : %s <port>
", argv[0]);
        exit(1);
    }
    int serverSock, clientSock;
    struct sockaddr_in serverAddr, clientAddr;
    socklen_t clientAddrSize;

    char buf[BUF_SIZE];
    int strLen;

    struct epoll_event *ep_events;
    struct epoll_event event;
    int epfd, event_cnt;

    serverSock =socket(PF_INET, SOCK_STREAM, 0);
    if(serverSock == -1)
        error_handling("socket() error");

    memset(&serverAddr, 0, sizeof(serverAddr));
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = htonl(INADDR_ANY);
    serverAddr.sin_port = htons(atoi(argv[1]));

    if(bind(serverSock, (struct sockaddr*) &serverAddr, sizeof(serverAddr)) == -1)
        error_handling("bind() error");
    if(listen(serverSock, 5) == -1)
        error_handling("listen() error");

    epfd = epoll_create(EPOLL_SIZE);
    ep_events = (struct epoll_event*)malloc(sizeof(struct epoll_event) * EPOLL_SIZE);

    setnonblockingmode(serverSock);
    event.events = EPOLLIN;
    event.data.fd = serverSock;
    epoll_ctl(epfd, EPOLL_CTL_ADD, serverSock, &event);

    puts("Server start...");
    while(1){
        event_cnt = epoll_wait(epfd, ep_events, EPOLL_SIZE, -1);
        if(event_cnt == -1){
            puts("epoll_wait() error");
            break;
        }
        puts("return epoll_wait");
        for(int i = 0; i < event_cnt; i++){
            if(ep_events[i].data.fd == serverSock){
                clientAddrSize = sizeof(clientAddr);
                clientSock = accept(serverSock, (struct sockaddr*)&clientAddr, &clientAddrSize);
                setnonblockingmode(clientSock);
                event.events = EPOLLIN|EPOLLET;
                event.data.fd = clientSock;
                epoll_ctl(epfd, EPOLL_CTL_ADD, clientSock, &event);
                printf("connected client: %d
", clientSock);
            }
            else{
                while(1)
                {
                    strLen = read(ep_events[i].data.fd, buf, BUF_SIZE);
                    if(strLen == 0){
                        epoll_ctl(epfd, EPOLL_CTL_DEL, ep_events[i].data.fd, NULL);
                        close(ep_events[i].data.fd);
                        printf("closed client: %d
", ep_events[i].data.fd);
                        break;
                    }
                    else if(strLen < 0){
                        if(errno == EAGAIN)
                            break;
                        else
                            puts("???????");
                    }
                    else{
                        write(ep_events[i].data.fd, buf, strLen);
                    }
                }
            }
        }
    }
    close(serverSock);
    puts("Server close...");
    return 0;
}

void setnonblockingmode(int fd) // 套接字改成非阻塞的
{
    int flag = fcntl(fd, F_GETFL, 0);
    fcntl(fd, F_SETFL, flag|O_NONBLOCK);
}

void error_handling(char * messages)
{
    puts(messages);
    exit(1);
}