mycp.c

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#define BUFSIZE 1024
/*
   ./mycp  /etc/passwd   /home/passwd
   diff /etc/passwd  /home/passwd
*/
void copyfile(const char *s,const char *d)
{
    int fd1,fd2,rnum,wnum;
    char buf[BUFSIZE];
    fd1 = open(s,O_RDONLY);
    //if(fd1 == -1).........
    fd2 = open(d,O_WRONLY|O_CREAT|O_TRUNC,0644);
    //if(fd2 == -1)........
    while((rnum = read(fd1,buf,BUFSIZE))>0)
    {
        wnum = write(fd2,buf,rnum);
        if(wnum!=rnum)
            printf("write num less than rnum\n");
    }
    close(fd1);
    close(fd2);
}
int main(int argc,char**argv)
{
    if(argc != 3)
    {
        printf("arg num error\n");
        exit(1);
    }
    copyfile(argv[1],argv[2]);
    exit(0);
}