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"    
4.   
5. int main(int argc, char* argv[])  
6. {  
7.         if( argc != 2 )  
8.         {  
9.                 err_quit("usage: a .out <pathname>");  
10.         }  
11.   
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.         }  
20.   
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.         }  
29.   
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。