linux多路IO复用

前言

IO操作在计算机系统中是比较耗时和占用资源的，所以提高IO操作性能显得尤为重要。这里介绍select和poll两种IO复用的方法。

一、select和poll 对比

	select	poll
数据结构	3个数组:读集合、写集合、出错集合	1个数组
维护fd数量	1024	自定义数量
拷贝	数组在用户态和内核态来回拷贝	数组在用户态和内核态来回拷贝
判断读写事件的方式	底层循环判断每个fd是否就绪	底层循环判断每个fd事件
缺点	数量越多，性能却差	数量越多，性能却差

二、两种IO实现

1.select

int main()
{
    int server_fd = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in servaddr;
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(9999);
        
    if(0 > bind(server_fd, (struct sockaddr *)&servaddr, sizeof(struct sockaddr))){
        printf("bind failed:%s\n", strerror(errno));
        return -1;
    }

    listen(server_fd, 128);
    socklen_t len = sizeof(struct sockaddr);
    struct sockaddr_in clientaddr;
    
    fd_set rfds, rset; //用户读集合、传入内核的读集合
    FD_ZERO(&rfds);
    FD_SET(server_fd, &rfds); //服务端fd放进集合
    int max_fd = server_fd;
    int client_fd = -1;

    while(1)
    {
        rset = rfds;
        int nready = select(max_fd + 1, &rset, NULL, NULL, NULL);
        
        //1.先判断服务端fd就绪状态
        if (FD_ISSET(server_fd, &rset)){
            client_fd = accept(server_fd, (struct sockaddr *)&clientaddr, &len);
            printf("accept:%d\n", client_fd);
            FD_SET(client_fd, &rfds);
            if(max_fd < client_fd)
                max_fd = client_fd;
            
            if(--nready==0) continue;  //只有一个服务端fd,终止此次循环
        }
        //2.再循环读取客户fd就绪状态
        for (int i = server_fd + 1; i <= max_fd; i++)
        {
            if(FD_ISSET(i,&rset)){
                char buf[BUF_SIZE]={0};
                int n = read(client_fd, buf, BUF_SIZE);
                if(n <= 0){
                    printf("read failed:%s\n", strerror(errno));
                    close(client_fd);
                    break;
                }
                printf("fd:%d,read buf:%s\n", i,buf);
            }
        }
    }
    return 0;
}

2.poll

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/errno.h>
#include <sys/poll.h>
#include <sys/socket.h>
#include <sys/types.h>

#define BUF_SIZE 1024

int main()
{
    int server_fd = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in servaddr;
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(9999);
        
    if(0 > bind(server_fd, (struct sockaddr *)&servaddr, sizeof(struct sockaddr))){
        printf("bind failed:%s\n", strerror(errno));
        return -1;
    }

    listen(server_fd, 128);
    socklen_t len = sizeof(struct sockaddr);
    struct sockaddr_in clientaddr;
    
    int max_fd = server_fd;
    int client_fd = -1;
    struct pollfd fds[BUF_SIZE];

    fds[server_fd].fd = server_fd;
    fds[server_fd].events = POLLIN;


    while(1)
    {
        int nready = poll(fds, max_fd + 1, -1);
        if(nready<=0) continue;

        if(fds[server_fd].revents & POLLIN)
        {
            client_fd = accept(server_fd, (struct sockaddr*)&clientaddr, &len);
            if(client_fd>0)
            {
                printf("accept:%d\n", client_fd);
                fds[client_fd].fd = client_fd;
                fds[client_fd].events = POLLIN;

                if(max_fd<client_fd)
                    max_fd = client_fd;
            }

            if(--nready==0)
                continue;
        }

        for (int i = server_fd + 1; i <= max_fd; i++){
            if(fds[i].revents & POLLIN){
                char buf[BUF_SIZE] = {0};
                if(0 >= read(client_fd, buf, BUF_SIZE)){
                    printf("%d disconnect\n", client_fd);
                    fds[client_fd].fd = -1;
                    fds[client_fd].events = 0;
                    close(client_fd);
                    continue;
                }
                printf("read buf:%s\n", buf);
            }
        }
    }
    return 0;
}

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总结

这次先总结下select和poll两种方式的区别和相同点，下次说下epoll优点和实现。