#include <unistd.h> 
#include <stdlib.h> 

int main(int argc, char **argv)
{
    int pid;    

    pid = fork();
    if (pid > 0)
    {
        printf("부모 프로세스\n");
        pause();
    }
    else if (pid == 0)
    {
        printf("자식 프로세스\n");
        pause();
    }
    else if (pid == -1)
    {
        perror("fork error : ");
        exit(0);
    }
}

[yundream@localhost test]# ./fork 
부모 프로세스
자식 프로세스

[yundream@localhost /root]# ps -eo "%U %p %P %c"  | grep fork
yundream      1581  1483 fork
yundream      1582  1581 fork

#include <sys/types.h> 
#include <sys/stat.h> 
#include <sys/socket.h> 
#include <signal.h> 
#include <unistd.h> 
#include <netinet/in.h> 
#include <arpa/inet.h> 
#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 

int main(int argc, char **argv)
{
    int server_sockfd, client_sockfd;
    int state, client_len;
    int pid;

    FILE *fp;
    struct sockaddr_in clientaddr, serveraddr;

    char buf[255];
    char line[255];

    if (argc != 2)
    {
        printf("Usage : ./zipcode [port]\n");
        printf("예    : ./zipcode 4444\n");
        exit(0);  
    } 

    memset(line, '\0', 255);
    state = 0;

    // 주소 파일을 읽어들인다.  
    client_len = sizeof(clientaddr);
    if((fp = fopen("zipcode.txt", "r")) == NULL)
    {  
        perror("file open error : ");
        exit(0); 
    } 

    // internet 스트림 소켓을 만들도록 한다. 
    if ((server_sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        perror("socket error : ");
        exit(0);
    } 
    bzero(&serveraddr, sizeof(serveraddr));
    serveraddr.sin_family = AF_INET;
    serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
    serveraddr.sin_port = htons(atoi(argv[1]));

    state = bind(server_sockfd , (struct sockaddr *)&serveraddr,
            sizeof(serveraddr));
    if (state == -1)
    {
        perror("bind error : ");
        exit(0);
    }
    
    state = listen(server_sockfd, 5);
    if (state == -1)
    {
        perror("listen error : ");
        exit(0);
    }
    
    signal(SIGCHLD, SIG_IGN);
    while(1)
    {
        client_sockfd = accept(server_sockfd, (struct sockaddr *)&clientaddr,
                               &client_len);
        pid = fork();
        if (pid == 0) 
        {
            if (client_sockfd == -1)
               {
                perror("Accept error : ");
                exit(0);
            }
            while(1)
            {
                memset(buf, '\0', 255);
                if (read(client_sockfd, buf, 255) <= 0)
                {
                    close(client_sockfd);
                    exit(0);
                }

                if (strncmp(buf, "quit",4) == 0)
                {
                    write(client_sockfd, "bye bye", 8);
                    close(client_sockfd);
                    exit(0);
                    break;
                }

                while(fgets(line,255,fp) != NULL)
                {
                    if (strstr(line, buf) != NULL)
                    {
                        write(client_sockfd, line, 255);
                    }
                    memset(line, '\0', 255);
                }
                write(client_sockfd, "end", 255);
                printf("send end\n");
                rewind(fp);
            }
        }
        if (pid == -1)
        {
            perror("fork error : ");
        }
    }
    close(client_sockfd);
}