关于c:Linux网络开发必学教程5基于多路复用的服务端

问题：应用 select() 函数能够扩大服务端性能吗? 如果能够，具体怎么实现？

目前服务端的瓶颈剖析

服务端大多数时候处于期待状态 ，无奈施展主机（设施）的最大性能

while (1) {
    // 阻塞，期待客户端连贯
    client = accept(server, (struct sockaddr*)&caddr, &asize);

    printf("client: %d\n", client);

    do {
        // 阻塞，期待客户端数据
        r = recv(client, buf, sizeof(buf), 0);
        if (r > 0) {printf("Receive: %s\n", buf);

            if (strcmp(buf, "quit") != 0) {len = send(client, buf, r, 0);
            }
            else {break;}
        } 
    } while (r > 0);

    close(client);
}

解决方案：阻塞变轮询

• 通过 select() 函数首先监听服务端 server_fd, 指标事件为“连贯”（读）
• 当事件产生（客户端连贯），则调用 accept() 承受连贯
• 将 client_fd 退出监听范畴，指标事件为“数据接管”（读）
• 循环查看各个被监听的文件描述符是否有事件产生

实现形式

实现逻辑

while (1) {
    rset = reads;

    num = select(max + 1, &rset, 0, 0, &timeout);

    if (num > 0) {
        int i = 0;
        for (i=1; i<=max; ++i) {  // 留神，0 被命令行占用，下标从 1 开始遍历
            if (FD_ISSET(i, &rset)) {if (i == server) {// accept and add client to fd_set} else {// read data from client by i (fd)
                }
            }
        }
    }
}

实现要害

• 动静调整须要监督的文件描述符

  • 当接管到客户端连贯时，将客户端文件描述符退出监听变量 (fd_set) 中
  • 当发现客户端断开时，在监听变量 (fd_set) 剔除客户端文件描述符

// 增加监听
if (client > -1) {FD_SET(client, &reads);
    max = (client > max) ? client : max;
    printf("client: %d\n", client);
}

// 剔除监听
if (r == -1) {FD_CLR(i, &reads);
    close(i);
}

• 动静调整须要监督的文件描述符数量
• 保障每个须要监督的文件描述符可能被轮询
• max = (client > max) ? client : max

编程试验：改良后的服务端

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/select.h>
#include <netinet/in.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>

int server_handler (int server)
{struct sockaddr_in addr = {0};
    socklen_t asize = sizeof(addr);

    return accept(server, (struct sockaddr*)&addr, &asize);
}

int client_handler(int client)
{char buf[32] = {0};
    int ret = read(client, buf, sizeof(buf) - 1);

    if (ret > 0) {buf[ret] = 0;

        printf("Receive: %s\n", buf);

        if (strcmp(buf, "quit") != 0) {ret = write(client, buf, ret);
        } else {return -1;}
    }

    return ret;
}

int main()
{
    int server = 0;
    struct sockaddr_in saddr = {0};
    int max = 0;
    int num = 0;
    fd_set reads = {0};
    fd_set temps = {0};
    struct timeval timeout = {0};

    server = socket(PF_INET, SOCK_STREAM, 0);

    if (server == -1) {printf("server socket error\n");
        return -1;
    }

    saddr.sin_family = AF_INET;
    saddr.sin_addr.s_addr = htonl(INADDR_ANY);
    saddr.sin_port = htons(8888);

    if (bind(server, (struct sockaddr*)&saddr, sizeof(saddr)) == -1) {printf("server bind error\n");
        return -1;
    }

    if (listen(server, 1) == -1) {printf("server listen error\n");
        return -1;
    }

    printf("server start success\n");

    FD_ZERO(&reads);
    FD_SET(server, &reads);

    max = server;

    while (1) {
        temps = reads;

        timeout.tv_sec = 0;
        timeout.tv_usec = 10000;

        num = select(max+1, &temps, 0, 0, &timeout);

        if (num > 0) {
            int i = 0;

            for (i=1; i<=max; ++i) {if (FD_ISSET(i, &temps)) {if (i == server) {int client = server_handler(server);

                        if (client > -1) {FD_SET(client, &reads);

                            max = (client > max) ? client : max;

                            printf("accept client: %d\n", client);
                        }
                    }
                    else {int r = client_handler(i);

                        if (r == -1) {FD_CLR(i, &reads);

                            close(i);
                        }
                    }
                }
            }
        }
    }

    return 0;
}

思考：改良后的服务端是否还有优化的空间？select() 是 Linux() 零碎特有的吗？