以poll为例实现一个简单的聊天室程序

该聊天室程序能让所有用户同时在线群聊，它分为客户端和服务器端两个部分。
客户端程序有两个功能：一是从标准输入终端读入用户数据，并将数据发送至服务器；二是往标准输出终端打印服务器发送给它的数据。
服务器的功能是接受客户数据，并把客户数据发送给每一个登录到该服务器的客户端（数据发送者除外）

客户端

客户端程序使用poll同时监听用户输入和网络连接，并利用splice函数将用户输入内容直接定向到网络连接上发送，从而实现数据零拷贝，提高了程序执行效率。

#define _GNU_SOURCE 1
#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 <string.h>
#include <stdlib.h>
#include <poll.h>
#include <fcntl.h>

#define BUFFER_SIZE 64

int main(int argc, char* argv[])
{
    if(argc <= 2)
    {
        printf("usage: %s ip_address port_number\n", basename(argv[0]));
    }
    const char* ip = argv[1];
    int port = atoi(argv[2]);

    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);
    if(connect(sockfd, (struct sockaddr*)&server_address, sizeof(server_address)) < 0)
    {
        printf("connection filed\n");
        close(sockfd);
        return 1;
    }
    /* 注册文件描述符0（标准输入） 和文件描述符sockfd上的可读事件 */
    struct pollfd fds[2];
    fds[0].fd = 0;
    fds[0].events = POLLIN;
    fds[0].revents = 0;
    fds[1].fd = 0;
    fds[1].events = POLLIN | POLLRDHUP;
    fds[1].revents = 0;

    char read_buf[BUFFER_SIZE];
    int pipefd[2];
    int ret = pipe(pipefd);
    assert(ret != -1);
    while(1)
    {
        ret = poll(fds , 2, -1);
        if(ret < 0)
        {
            printf("poll failure\n");
            break;
        }
        if(fds[1].revents & POLLRDHUP)
        {
            printf("server close the connection\n");
            break;
        }
        else if(fds[1].revents & POLLIN)
        {
            memset(read_buf, '\0', BUFFER_SIZE);
            recv(fds[1].fd, read_buf, BUFFER_SIZE-1, 0);
            printf("%s\n", read_buf);
        }

        if(fds[0].revents & POLLIN)
        {
            /* 使用splice将用户输入的数据直接写到sockfd上（零拷贝） */
            ret = splice(0, NULL, pipefd[1], NULL, 32768, SPLICE_F_MORE | SPLICE_F_MOVE);
            ret = splice(pipefd[0], NULL, sockfd, NULL, 32768, SPLICE_F_MORE | SPLICE_F_MOVE);
        }
    }
    close(sockfd);
    return 0;
}

服务器

服务器程序使用poll同时管理监听socket和连接socket，并且使用牺牲空间换取时间的策略来提高服务器性能

#define _GNU_SOURCE 1
#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 <stdlib.h>
#include <poll.h>
#include <fcntl.h>

#define USER_LIMIT 5    //最大用户数量
#define BUFF_SIZE 64    //读缓冲区的大小
#define FD_LIMIT 65535  //文件描述符数量限制

struct client_data
{
    struct sockaddr_in address;
    char* write_buf;
    char buf[BUFF_SIZE];
};

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;
}

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(listen >= 0);

    ret = bind(listenfd, (struct sockaddr*)&address, sizeof(address));
    assert(listenfd != -1);

    ret = listen(listenfd, 5);
    assert(ret != -1);

    /* 创建users数组，分配FD_LIMIT个client_data对象。可以预期：每个可能的socket连接
    都可以获得一个这样的对象，并且socket的值可以直接用来索引（作为数组的下标）socket连接
    对应的client_data对象，这是将socket和客户数据关联的简单而高效的方式 */
    struct client_data* users = (struct client_data*)malloc(sizeof(struct client_data) * FD_LIMIT);
    /* 尽管我们分配了足够多的client_data对象，但为了提高poll的性能，仍然有必要限制用户数量 */
    struct pollfd fds[USER_LIMIT + 1];
    int user_counter = 0;
    for(int i =1; i <= USER_LIMIT; ++i)
    {
        fds[i].fd = -1;
        fds[i].events = 0;
    }
    fds[0].fd = listenfd;
    fds[0].events = POLLIN | POLLERR;
    fds[0].revents = 0;

    while(1)
    {
        ret = poll(fds, user_counter + 1, -1);
        if(ret < 0)
        {
            printf("poll failure\n");
            break;
        }

        for(int i = 0; i < user_counter + 1; ++i)
        {
            if((fds[i].fd == listenfd) && (fds[i].revents & POLLIN))
            {
                struct sockaddr_in client_address;
                socklen_t client_addrlength = sizeof(client_address);
                int connfd = accept(listenfd, (struct sockaddr*)&client_address, &client_addrlength);
                if(connfd < 0)
                {
                    printf("errno is: %d\n",errno);
                    continue;
                }
                //如果请求过多，则关闭新到的连接
                if(user_counter >= USER_LIMIT)
                {
                    const char* info = "too many users\n";
                    printf("%s",info);
                    send(connfd, info, strlen(info), 0);
                    close(connfd);
                    continue;
                }
                /* 对于新的连接，同时修改fds和users数组。前文提到，users[connfd]对应于
                新连接文件描述符connfd的客户数据 */
                ++user_counter;
                users[connfd].address = client_address;
                setnonblocking(connfd);
                fds[user_counter].fd = connfd;
                fds[user_counter].events = POLLIN | POLLRDHUP | POLLERR;
                fds[user_counter].revents = 0;
                printf("comes a new user, now have %d users\n", user_counter);
            }
            else if(fds[i].revents & POLLERR)
            {
                printf("get an error from %d\n", fds[i].fd);
                char errors[100];
                memset(errors, '\0', 100);
                socklen_t length = sizeof(errors);
                if(getsockopt(fds[i].fd, SOL_SOCKET, SO_ERROR, &errors, &length) < 0)
                {
                    printf("get socket option failed\n");
                }
                continue;
            }
            else if(fds[i].revents & POLLRDHUP)
            {
                /* 如果客户端关闭连接，则服务器端也关闭对应的连接，并将用户总数减1 */
                users[fds[i].fd] = users[fds[user_counter].fd];
                close(fds[i].fd);
                fds[i] = fds[user_counter];
                --i;
                --user_counter;
                printf("a client left\n");
            }
            else if(fds[i].revents & POLLIN)
            {
                int connfd = fds[i].fd;
                memset(users[connfd].buf, '\0', BUFF_SIZE);
                ret = recv(connfd , users[connfd].buf, BUFF_SIZE-1, 0);
                printf("get %d bytes of client data %s from %d\n",ret, users[connfd].buf, connfd);
                if(ret < 0)
                {
                    //如果读操作出错，则关闭连接
                    if(errno != EAGAIN)
                    {
                        close(connfd);
                        users[fds[i].fd] = users[fds[user_counter].fd];
                        fds[i] = fds[user_counter];
                        --i;
                        --user_counter;
                    }
                }
                else if(ret == 0)
                {

                }
                else
                {
                    //如果接收到客户数据，则通知其他socket连接准备写数据
                    for(int j = 1; j <= user_counter; ++j)
                    {
                        if(fds[j].fd == connfd)
                        {
                            continue;
                        }
                        fds[j].events |= ~POLLIN;
                        fds[j].events |= POLLOUT;
                        users[fds[j].fd].write_buf = users[connfd].buf;
                    }
                }
            }
            else if(fds[i].revents & POLLOUT)
            {
                int connfd = fds[i].fd;
                if(!users[connfd].write_buf)
                {
                    continue;
                }
                ret = send(connfd, users[connfd].write_buf, strlen(users[connfd].write_buf),0);
                users[connfd].write_buf = NULL;
                fds[i].events |= ~POLLOUT;
                fds[i].events |= POLLIN;
            }
        }
    }
    free(users);
    close(listenfd);
    return 0;
}