tcp连接的select

tcp的socket不同于udp

tcp：建立socket的fd；bind；listen；accept；send or recv。另一端connect（客户端）

udp：                        ；bind；                        ；send or recv。另一端直接recv or send

select步骤：

1、fd_set(1024位的bitmap，包括清空【FD_ZERO】，把我的TCPsocket句柄放入这个bitmap【FD_SET】这是一个置位操作故不可重复利用，需清空某个位置后在使用【FD_CLR】会把bitmap相对应的socket置为0，最重要的轮询检查这个fd_set里哪些位置被监听【FD_ISSET】)

2、调用select（maxsock+1，fd_set，写，异常，timeout）；//参数一是最大句柄数+1也就是是bitmap的长度，第二个参数标识读文件描述符，这个select只处理读句柄

3、轮询check每一个fd在fd_set中的状态，1表示监听，0表示不监听，对于监听的端口使用socket的recv（）函数接收数据来读，相反，如果是写句柄则调用send（）函数发送数据去写如另一个进程。

4、每次轮询完已有的fd_set，检查是否有新的fd连接进来【FD_ISSET】如果有则bitmap的大小 maxsock++

缺点：

bitmap只有1024位--------->>>>>>poll采用链表把所有的结构体连接起来，对pollfd。revents置位

fd_set不可重用，每次都要创建一个新的---------->>>>>poll

用户态和内核态的切换要开销--------->>>>>epoll不需要切换

再次遍历bitmap的时间复杂度O（n）--------->>>>>epoll改进，epoll_wait()每次把监听端口的epfd排序，放在前面并返回个数，每次遍历返回个数就可以获取全部监听socket

代码：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#define MYPORT 1234    // the port users will be connecting to
#define BACKLOG 5     // how many pending connections queue will hold
#define BUF_SIZE 1024
int fd_A[BACKLOG];    // accepted connection fd
int conn_amount;      // current connection amount
void showclient()
{
    int i;
    printf("client amount: %d/n", conn_amount);
    for (i = 0; i < BACKLOG; i++) {
        printf("[%d]:%d  ", i, fd_A[i]);
    }
    printf("/n/n");
}
int main(void)
{
    int sock_fd, new_fd;             // listen on sock_fd, new connection on new_fd
    struct sockaddr_in server_addr;  // server address information
    struct sockaddr_in client_addr;  // connector's address information
    socklen_t sin_size;
    int yes = 1;
    char buf[BUF_SIZE];
    int ret;
    int i;
    if ((sock_fd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
        perror("socket");
        exit(1);
    }
    if (setsockopt(sock_fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1) {
        perror("setsockopt");
        exit(1);
    }
    server_addr.sin_family = AF_INET;         // host byte order
    server_addr.sin_port = htons(MYPORT);     // short, network byte order
    server_addr.sin_addr.s_addr = INADDR_ANY; // automatically fill with my IP
    memset(server_addr.sin_zero, '/0', sizeof(server_addr.sin_zero));
    if (bind(sock_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) == -1) {
        perror("bind");
        exit(1);
    }
    if (listen(sock_fd, BACKLOG) == -1) {
        perror("listen");
        exit(1);
    }
    printf("listen port %d/n", MYPORT);
    fd_set fdsr;
    int maxsock;
    struct timeval tv;
    conn_amount = 0;
    sin_size = sizeof(client_addr);
    maxsock = sock_fd;
    while (1)
    {
        // initialize file descriptor set
        FD_ZERO(&fdsr);
        FD_SET(sock_fd, &fdsr);  // add fd
        // timeout setting
        tv.tv_sec = 30;
        tv.tv_usec = 0;
        // add active connection to fd set
        for (i = 0; i < BACKLOG; i++) {
            if (fd_A[i] != 0) {
                FD_SET(fd_A[i], &fdsr);
            }
        }
        ret = select(maxsock + 1, &fdsr, NULL, NULL, &tv);
        if (ret < 0) {          // error
            perror("select");
            break;
        } else if (ret == 0) {  // time out
            printf("timeout/n");
            continue;
        }
        // check every fd in the set
        for (i = 0; i < conn_amount; i++)
        {
            if (FD_ISSET(fd_A[i], &fdsr)) // check which fd is ready
            {
                ret = recv(fd_A[i], buf, sizeof(buf), 0);
                if (ret <= 0)
                {        // client close
                    printf("ret : %d and client[%d] close/n", ret, i);
                    close(fd_A[i]);
                    FD_CLR(fd_A[i], &fdsr);  // delete fd
                    fd_A[i] = 0;
                    conn_amount--;
                }
                else
                {        // receive data
                    if (ret < BUF_SIZE)
                        memset(&buf[ret], '/0', 1); // add NULL('/0')
                    printf("client[%d] send:%s/n", i, buf);
                }
            }
        }
        // check whether a new connection comes
        if (FD_ISSET(sock_fd, &fdsr))  // accept new connection
        {
            new_fd = accept(sock_fd, (struct sockaddr *)&client_addr, &sin_size);
            if (new_fd <= 0)
            {
                perror("accept");
                continue;
            }
            // add to fd queue
            if (conn_amount < BACKLOG)
            {
                fd_A[conn_amount++] = new_fd;
                printf("new connection client[%d] %s:%d/n", conn_amount,
                        inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
                if (new_fd > maxsock)  // update the maxsock fd for select function
                    maxsock = new_fd;
            }
            else
            {
                printf("max connections arrive, exit/n");
                send(new_fd, "bye", 4, 0);
                close(new_fd);
                break;   
            }
        }
        showclient();
    }
    // close other connections
    for (i = 0; i < BACKLOG; i++)
    {
        if (fd_A[i] != 0)
        {
            close(fd_A[i]);
        }
    }
    exit(0);
}