#include <stdio.h>
#include <string.h>
#include <netinet/in.h>
 
#define MAXLINE 80
#define SERV_PORT 8000
 
int main(void)
{
    struct sockaddr_in servaddr, cliaddr;
    socklen_t cliaddr_len;
    int listenfd, connfd;
    char buf[MAXLINE];
    char str[INET_ADDRSTRLEN];
    int i, n;
 
    listenfd = socket(AF_INET, SOCK_STREAM, 0);
 
        int opt = 1;
        setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
 
    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(SERV_PORT);
    
    bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr));
 
    listen(listenfd, 20);
 
    printf("Accepting connections ...\n");
    while (1) {
        cliaddr_len = sizeof(cliaddr);
        connfd = accept(listenfd, 
                (struct sockaddr *)&cliaddr, &cliaddr_len);
        //while (1) 
                {
            n = read(connfd, buf, MAXLINE);
            if (n == 0) {
                printf("the other side has been closed.\n");
                break;
            }
            printf("received from %s at PORT %d\n",
                   inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)),
                   ntohs(cliaddr.sin_port));
    
            for (i = 0; i < n; i++)
                buf[i] = toupper(buf[i]);
            write(connfd, buf, n);
        }
        //這里故意不關(guān)閉socket,或者是在close之前加上一個sleep都可以
        //sleep(5);
        //close(connfd);
    }
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
 
#define MAXLINE 80
#define SERV_PORT 8000
 
int main(int argc, char *argv[])
{
    struct sockaddr_in servaddr;
    char buf[MAXLINE];
    int sockfd, n;
    char *str;
    
    if (argc != 2) {
        fputs("usage: ./client message\n", stderr);
        exit(1);
    }
    str = argv[1];
    
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
 
    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    inet_pton(AF_INET, "127.0.0.1", &servaddr.sin_addr);
    servaddr.sin_port = htons(SERV_PORT);
    
    connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr));
 
    write(sockfd, str, strlen(str));
 
    n = read(sockfd, buf, MAXLINE);
    printf("Response from server:\n");
    write(STDOUT_FILENO, buf, n);
    write(STDOUT_FILENO, "\n", 1);
 
    close(sockfd);
    return 0;
}

debian-wangyao:~$ ./client a
Response from server:
A
debian-wangyao:~$ ./client b
Response from server:
B
debian-wangyao:~$ ./client c
Response from server:
C
debian-wangyao:~$ netstat -antp | grep CLOSE_WAIT
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
tcp        1      0 127.0.0.1:8000          127.0.0.1:58309         CLOSE_WAIT  6979/server     
tcp        1      0 127.0.0.1:8000          127.0.0.1:58308         CLOSE_WAIT  6979/server     
tcp        1      0 127.0.0.1:8000          127.0.0.1:58307         CLOSE_WAIT  6979/server  

int KeepAliveProbes=1;
int KeepAliveIntvl=2;
int KeepAliveTime=120;
setsockopt(s, IPPROTO_TCP, TCP_KEEPCNT, (void *)&KeepAliveProbes, sizeof(KeepAliveProbes));
setsockopt(s, IPPROTO_TCP, TCP_KEEPIDLE, (void *)&KeepAliveTime, sizeof(KeepAliveTime));
setsockopt(s, IPPROTO_TCP, TCP_KEEPINTVL, (void *)&KeepAliveIntvl, sizeof(KeepAliveIntvl));