echo_server.c

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#include <stdlib.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>

#define MAXLINE 1024
void handle(int conn_fd);

int main(int argc, char **argv)
{
    int listen_fd, conn_fd;
    int server_port = 53101;
    int listen_q = 1024;
    pid_t child_pid;
    socklen_t socklen;

    struct sockaddr_in cli_addr, serv_addr;
    socklen = sizeof(cli_addr);

    bzero(&serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    serv_addr.sin_port = htons(server_port);

    listen_fd = socket(AF_INET, SOCK_STREAM, 0);
    if(0 > listen_fd)
    {
        perror("socket error");
        return listen_fd;
    }

    if(0 > bind(listen_fd, (struct sockaddr *) &serv_addr, socklen))
    {
        perror("bind error");
        return -1;
    }

    if(0 > listen(listen_fd, listen_q))
    {
        perror("listen error");
        return -1;
    }

    printf("server startup, listen on port:%d\n", server_port);

    for( ; ; )
    {
        conn_fd = accept(listen_fd, (struct sockaddr *) &cli_addr, &socklen);
        if(0 > conn_fd)
        {
            perror("accept error");
            continue;
        }

        // sprintf(buf, "accept from %s:%d\n", inet_ntoa(cli_addr.sin_addr), cli_addr.sin_port);
        printf("accept from %s:%d\n", inet_ntoa(cli_addr.sin_addr), ntohs(cli_addr.sin_port));
        
        child_pid = fork();
        if(0 == child_pid)
        {
            // child process
            close(listen_fd);
            handle(conn_fd);
            exit(0);
        }
        else if (0 < child_pid)
        {
            // parent process
            close(conn_fd);
        }
        else
        {
            perror("fork error");
        }
    }

    if (listen_fd)
    {
        close(listen_fd);
    }
    return 0;
}

void handle(int conn_fd)
{
    size_t n;
    char buf[MAXLINE];

    for( ; ; )
    {
        n = read(conn_fd, buf, MAXLINE);
        if(0 > n)
        {
            if(EINTR != errno)
            {
                perror("read error");
                break;
            }
        }
        if(0 == n)
        {
            close(conn_fd);
            printf("client quit\n");
            break;
        }
        if(0 == strncmp("exit", buf, 4))
        {
            close(conn_fd);
            printf("client offline\n");
            break;
        }
        write(conn_fd, buf, n);
    }
}