Linux网络编程之 Select实现 并发 server

1. 使用 select 实现的 并发 TCP server 端 小程序。
有一点需要注意的是在 调用 select 函数之前 输入的 形参。
比如 第二个形参 readfds，是一个输入输出参数。
输入的是(ˇˍˇ） 想监听的 read 描述符，输出的是实际发生变化的 read 描述符。
所以 第二次调用 select的时候 不能将第一次返回的 实参继续作为第二次的输入，否则会造成其他描述符监控不到。
code 如下：

/*description:
  1. multi connecion tcp server: support multiple connection from different client
  2. send the received data back to client
  3. one client close doesn't have impact on other connection */
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <vector>
using namespace std;

ssize_t readn(int fd, void *buf, size_t count)
{
    int n_left = count;
    int n_read;
    char * readbuf = (char *)buf;
    while (n_left > 0)
    {
        if ((n_read = read(fd, readbuf, n_left)) < 0)
        {
            if (errno == EINTR)
                continue;
            return -1;
        }
        else if (n_read == 0)
        {
            printf ("client has closed\n");
            break;
        }
        printf ("1. receive data is %s, length is %d,lef data:%d\n", readbuf, n_read, n_left - n_read);
        readbuf += n_read;
        n_left -= n_read;
        printf("left data is %d\n", n_left);
    }
    return count - n_left;
}
ssize_t writen(int fd, const void *buf, size_t count)
{
    int n_left = count;
    int n_written;
    char * writebuf = (char *)buf;
    while (n_left > 0)
    {
        if ((n_written = write(fd, writebuf, n_left)) < 0)
        {
            if (errno == EINTR)
                continue;
            return -1;
        }
        else if (n_written == 0)
        {
            continue;
        }
        writebuf += n_written;
        n_left -= n_written;
    }
    return count - n_left;
}
ssize_t readline(int fd, void *buf, size_t count)
{
    int n_left = count;
    int n_read = 0;
    int ret = 0;
    int loop_index;
    char *readbuf = (char *) buf;
    while (1)
    {
        if ((n_read = recv(fd, buf, n_left, MSG_PEEK)) < 0)
        {
            perror("read error\n");
            exit(-1);
        }
        else if (n_read == 0)
        {
            printf("client closed connection\n");
            return 0;
        }
        for (loop_index=0; loop_index < n_read; loop_index++)
        {
            if (readbuf[loop_index] == '\n')
            {
                printf("get \n from buffer\n");
                n_read = readn(fd, buf, loop_index + 1);
                if (n_read != loop_index + 1)
                {
                    perror("read error");
                    exit(-1);
                }
                return n_read;
            }
        }
        if (n_read > n_left)
            exit(EXIT_FAILURE);
        ret = readn(fd, buf, n_read);
        if(ret != n_read)
        {
            perror("read error");
            exit(-1);
        }
        n_left -= n_read;
        buf += n_read;
    }
    return -1;
}
int main()
{
    int listenfd = 0;
    if((listenfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
    {
        perror("socket error\n");
        exit(1);
    }
    printf("creat socket success\n");
    struct sockaddr_in seraddr;
    memset(&seraddr, 0, sizeof(seraddr));
    seraddr.sin_family = AF_INET;
    seraddr.sin_port = htons(51888);
    seraddr.sin_addr.s_addr = inet_addr("127.0.0.1");
    if (bind(listenfd, (struct sockaddr *)&seraddr,sizeof(seraddr)) < 0)
    {
        perror("bind error\n");
        exit(1);
    }
    printf("bind success\n");
    if (listen(listenfd, 50) < 0)    
    {
        perror("listen error\n");
        exit(1);
    }
    printf("listen success\n");
    struct sockaddr_in peer_addr;

    int conid;
   /*int select(int nfds, fd_set *readfds, fd_set *writefds,
                  fd_set *exceptfds, struct timeval *timeout); */ 
    int maxfds = listenfd;
    int numfd = 0;
    vector<int> conid_set;
    fd_set read_set;
    fd_set real_read_set;
    FD_ZERO(&read_set);
    FD_SET(listenfd, &read_set);
    int i;
    while (1)
    {
        real_read_set = read_set;
        printf("start one new select\n");
        numfd = select(maxfds + 1, &real_read_set, NULL, NULL, NULL);
        if (numfd == 0)
        {
            continue;
        }
        else if (numfd == -1)
        {
            perror("select error");
            exit(EXIT_FAILURE);
        }
        if (FD_ISSET(listenfd, &real_read_set))
        {
            socklen_t addr_len = sizeof(peer_addr);
            memset(&peer_addr, 0, sizeof(peer_addr));
            conid = accept(listenfd, (struct sockaddr *)&peer_addr, &addr_len);
            if (conid < 0)
            {
                perror("accept error");
                exit(EXIT_FAILURE);
            }
            printf("accept success\n");
            printf("peer address is %s, port is %d\n", inet_ntoa(peer_addr.sin_addr),ntohs(peer_addr.sin_port));
            conid_set.push_back(conid);
            FD_SET(conid, &read_set);
            if (conid > maxfds)
            {
                maxfds = conid;
            }
            if (--numfd <= 0)
                continue;
        }
        for (i = 0; i < conid_set.size(); i ++)
        {
            if (FD_ISSET(conid_set[i], &real_read_set))
            {
                char recbuf[50];
                memset(recbuf, 0, sizeof(recbuf));
                printf("start one new readline\n");
                int ret = readline(conid_set[i], recbuf, sizeof(recbuf));
                if (ret == 0)
                {
                    printf("client close");
                    close(conid_set[i]);
                    FD_CLR(conid_set[i], &read_set);
                    conid_set.erase(conid_set.begin() + i);
                    continue;
                }
                printf ("receive data is %s, length is %d\n", recbuf, ret);
                writen(conid_set[i], recbuf, ret);
                printf ("send data is %s to client,length is %d\n", recbuf, ret);
                if (--numfd < 0)
                    break;
            }
            
        }
    }
    #if 0
    if ((conid = accept(listenfd, (struct sockaddr *)&peer_addr, &addr_len)) < 0) 
    {
        perror("listen error\n");
        exit(1);
    }
    printf("accept success\n");
    printf("peer address is %s, port is %d\n", inet_ntoa(peer_addr.sin_addr),ntohs(peer_addr.sin_port));
    char recbuf[50];
    while (1)
    {
        memset(recbuf, 0, sizeof(recbuf));
        printf("start one new readline\n");
        int ret = readline(conid, recbuf, sizeof(recbuf));
        if (ret == 0)
        {
            break;
        }
        printf ("receive data is %s, length is %d\n", recbuf, ret);
        writen(conid, recbuf, ret);
        printf ("send data is %s to client,length is %d\n", recbuf, ret);
    }
    close(listenfd);
    close(conid);
    #endif
    return 0;
}