使用Select编写多客户连接的网络服务器

select系统条用允许程同时在多个底层文件描述符上等待输入的到达（或输出的完成）。

一般在linux系统上，标准输入的文件描述符是0；标准输出的文件描述符是1；

标准错误的文件描述符是2；

在Posix定义分别定义为：STDIN_FILENO,STDOUT_FILENO,STDERR_FILENO

他们都包含在unistd.h头文件中

以下代码对于并发连接并不能够同时处理，可以在数据处理部分起多线程处理

服务器端代码如下（客户端代码不做修改）：

#include <sys/types.h>
#include <sys/socket.h>
#include <iostream>
#include <netinet/in.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <string.h>

using namespace std;

const string SERVER_IP = "192.168.1.105";
const short SERVER_PORT = 9527;
const int MAX_CONN_NUM = 5;
const int MAX_DATA_LENGTH = 255;


int main(int argc, char** argv)
{
    int server_sockfd, client_sockfd;
    int server_len, client_len;
    struct timeval timeout;
    struct sockaddr_in server_address;
    struct sockaddr_in client_address;
    int result;
    fd_set readfds, testfds;
    char ch[MAX_DATA_LENGTH] = {0};

    timeout.tv_sec = 2;
    timeout.tv_usec = 500000;

    server_sockfd = socket(AF_INET, SOCK_STREAM, 0);

    server_address.sin_family = AF_INET;
    server_address.sin_addr.s_addr = inet_addr(SERVER_IP.c_str());
    server_address.sin_port = htons(SERVER_PORT);
    server_len = sizeof(server_address);

    bind(server_sockfd, (struct sockaddr*)&server_address, server_len);

    listen(server_sockfd, MAX_CONN_NUM);

    FD_ZERO(&readfds);
    FD_SET(server_sockfd, &readfds);

    while(1)
    {
        int fd;
        int nread;

        testfds = readfds;

        cout << "server waiting..." << std::endl;
        result = select(FD_SETSIZE, &testfds, (fd_set*)0, (fd_set*)0, &timeout); // 此处可以设超时时间为(struct timeval*)0,一般设置为0无限期阻塞
 
        if (result < 1)
        {
            cerr << "select failed!" << endl;
            exit(EXIT_FAILURE);
        }

        for (fd = 0; fd < FD_SETSIZE; fd++)
        {
            if (FD_ISSET(fd, &testfds))
            {
                if (fd == server_sockfd)
                {
                    client_len = sizeof(client_address);
                    client_sockfd = accept(server_sockfd, (struct sockaddr*)&client_address, (socklen_t*)&client_len);
                    FD_SET(client_sockfd, &readfds);
                    cout << "adding client on fd " << client_sockfd << endl;
                } 
                else
                {
                    ioctl(fd, FIONREAD, &nread);

                    if (nread == 0)
                    {
                        close(fd);
                        FD_CLR(fd, &readfds);
                        cout << "removing client on fd " << fd << endl; 
                    }
                    else
                    {
                        read(fd, ch , MAX_DATA_LENGTH);
                        cout << "serving client on fd " << fd << endl;
                        cout << "receive from client is " << ch << std::endl;
                        strncpy(ch, "Hello Client", MAX_DATA_LENGTH);
                        write(fd, ch, MAX_DATA_LENGTH);
                        sleep(5);
                    }
                }
            }
        }
    }
    close(server_sockfd);
}