采用进程实现文件拷贝

程序要求：
通过fork创建进程，子进程实现文件前半部分的拷贝，父进程实现文件后半部分的拷贝。

程序如下：

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>


int child_copy(int fd_dest,int fd_src,int len)
{
    char buf[100];
    int n = 0,count = 0;

    memset(buf,0,sizeof(buf));

    lseek(fd_src,0,SEEK_SET);
    lseek(fd_dest,0,SEEK_SET);

    while((n = read(fd_src,buf,sizeof(buf))) > 0){
        write(fd_dest,buf,n);
        count += n;

        if(count > len / 2)
            break;
    }

    return 0;
}

int father_copy(int fd_dest,int fd_src)
{
    char buf[100];
    int n = 0,count = 0;

    memset(buf,0,sizeof(buf));

    while((n = read(fd_src,buf,sizeof(buf))) > 0){
        write(fd_dest,buf,n);
    }

    return 0;

}

int main(int argc,char *argv[])
{
    int fd_src,fd_dest;
    pid_t pid;
    int len;

    if(argc < 3){
        fprintf(stderr,"usage:%s srcfile destfile \n",argv[0]);
        exit(-1);
    }

    if((fd_src = open(argv[1],O_RDONLY)) < 0){
        perror("fail to open");
        exit(-1);
    }

    if((fd_dest = open(argv[2],O_WRONLY | O_CREAT | O_TRUNC,0666)) < 0)
    {
        perror("fail to open");
        exit(-1);
    }

    len = lseek(fd_src,0,SEEK_END);

    if(ftruncate(fd_dest,len) < 0){
        perror("fail to change size");
        exit(-1);
    }

    if((pid = fork()) < 0){
        perror("fail to open");
        exit(-1);
    }else if(pid == 0){
        child_copy(fd_dest,fd_src,len);
        close(fd_dest);
        close(fd_src);
        exit(0);
    }else{
        close(fd_dest);
        close(fd_src);

        if((fd_src = open(argv[1],O_RDONLY)) < 0){
            perror("fail to open");
            exit(-1);
        }

        printf("fd_src : %d\n",fd_src);

        if((fd_dest = open(argv[2],O_WRONLY)) < 0)
        {
            perror("fail to open");
            exit(-1);
        }

        lseek(fd_src,len / 2,SEEK_SET);
        lseek(fd_dest,len / 2,SEEK_SET);

        father_copy(fd_dest,fd_src);

        close(fd_dest);
        close(fd_src);

        exit(0);
    }

    return 0;
}