O_ACCMODE<0003>：读写文件操作时，用于取出flag的低2位

O_RDONLY<00>：只读打开

O_WRONLY<01>：只写打开

O_RDWR<02>：读写打开

可以很好的理解下面的程序：

1#include <sys/types.h>

2 #include <fcntl.h>

3 #include "ourhdr.h"

4

5 int

6 main(int argc, char *argv[])

7 {

8 int accmode, val;

9 if(argc !=2)

10 err_quit("usage: a.out<descriptor#>");

11

12 if((val=fcntl(atoi(argv[1]),F_GETFL, 0))<0)

13 err_sys("fcntl error for fd %d", atoi(argv[1]));

14

15 accmode=val & O_ACCMODE;

16 if (accmode==O_RDONLY) printf("read only");

17 else if (accmode==O_WRONLY) printf("write only");

18 else if (accmode==O_RDWR) printf("read write");

19 else err_dump("unknown access mode");

20

21 if(val & O_APPEND) printf(", append");

22 if(val & O_NONBLOCK) printf(", nonblocking");

23 #if !defined(_POSIX_SOURCE) && defined(O_SYNC)

if(val & O_SYNC) printf(", synchronous writes");

25 #endif

26 putchar('\');

27 exit(0);

28 }