Unix下文件的訪問權限

access Function:

When we open a file, the kernel performs its access tests based on the effective user and group IDs. There are times when a process wants to test accessibility based on the real user and group IDs. This is useful when a process is running as someone else, using either the set-user-ID or the set-group-ID feature. Even though a process might beset-user-ID to root, it could still want to verify that the read user can access a given file. The access function bases its tests on the real user and group IDs.

1. #include <fcntl.h>
2. #include "apue.h"
3. #include "my_err.h"
5. int main(int argc, char* argv[])
6. {
7. if( argc != 2 )
8. {
9. err_quit("usage: a .out <pathname>");
10. }
12. if( access( argv[1], R_OK) < 0 )
13. {
14. err_ret("access error for %s", argv[1]);
15. }
16. else
17. {
18. printf("read access ok\n");
19. }
21. if( open( argv[1], O_RDONLY) < 0 )
22. {
23. err_ret("open error for %s", argv[1]);
24. }
25. else
26. {
27. printf("open for reading OK\n");
28. }
30. exit(0);
31. }

說明： a.out本來沒有權限訪問/etc/shadow文件，該文件只有root用戶才有權限，但是我們把a.out改成root的文件，再增加S屬性，雖然用的是普通的用戶去執行root用戶的文件，由於a.out 文件有S屬性，所以，它的有效用戶id還是root的id，也只有這樣，a.out 才能訪問/etc/shadow文件。