unix下利用creat函数与write函数 创建复制文件

文件描述符（file descriptor）：1 标准输出；2 标准错误；0 标准输入

fd = file descriptor

exit（0） = 正常退出

argc代表命令行输入参数的个数
argv存储了所有的命令行参数

#include<stdio.h>#include<fcntl.h>#include<string.h>#include<unistd.h>#include<dirent.h>#include<stdlib.h>#include<sys/types.h>#include<sys/stat.h>#define BUFFER_SIZE 1024int main(int argc,char **argv){    int from_fd,to_fd;    long file_len = 0;    int ret = 1;    char buffer[BUFFER_SIZE];    char *ptr;    if(argc!= 3)//文件描述符 0+1+2     {        printf("usage:%s fromfile tofile !\n",argv[0]);        exit(1);    }    if((from_fd = open(argv[1],O_RDONLY|O_CREAT))==-1)   //测试文件偏移量，因为文件偏移量可能为负值 所以用-1 来判断测试    {        printf("open %s error!\n",argv[1]);        exit(1);    }    if((to_fd = open(argv[2],O_WRONLY|O_CREAT))==-1)    {        printf("open %s error! \n",argv[2]);        exit(1);    }    file_len = lseek(from_fd,0,SEEK_END);      lseek(from_fd,0,SEEK_SET);//off_t lseek(int fd ,off_t offset,int whence);    printf("from file size is %d!\n",file_len);    while(ret)        {        ret = read(from_fd,buffer,BUFFER_SIZE);//read(int fd, void *buf ,size_t nbytes);        if(ret == -1)        {            printf("read error!\n");            exit(1);        }            write(to_fd,buffer,ret);        file_len =ret;        bzero(buffer ,BUFFER_SIZE);    }    printf("there are %d buyes data left without copy!\n",file_len);    close(from_fd);    close(to_fd);    exit(0);}