linux下cp命令的简单实现方式：
#include
#include
#include
#include
#define BUFFERSIZE 4096
#define COPYMODE 0644
int oops(char *, char *);
main(int ac, char *av[])
{
int in_fd, out_fd, n_chars;
char buf[BUFFERSIZE];
/* check args */
if ( ac != 3 ){
fprintf( stderr, "usage: %s source destination\n", *av);
exit(1);
}