How select() works(select()的实现机制)

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How select() works

1. 
   
2. A file descriptor is associated with a file structure.
   
3. In the file structure, there is a set of operations supported by this file type called file_operations.
   
4. In the file_operations structure, there is an entry named poll.
   
5. What the generic select() call does is call this poll() function to get status of a file (or socket or whatever) as the name suggests.
   
6. 
   
7. In general, the select() works like
   
8. 
   
9.     while(1)
   
10. 
    
11.     {
    
12. 
    
13.          for each file descriptor in the set
    
14. 
    
15.          {
    
16. 
    
17.              call file's poll() to get mask.
    
18. 
    
19.              if(mask & can_read or mask & can_write or mask & exception)
    
20. 
    
21.              {
    
22. 
    
23.                  set bit for this fd that this file is readable/writable or there is an
    
24.                  exception.
    
25. 
    
26.                  retval++;
    
27. 
    
28.              }
    
29. 
    
30.          }
    
31. 
    
32.          if(retval != 0)
    
33. 
    
34.              break;
    
35. 
    
36.          schedule_timeout(__timeout);
    
37. 
    
38.     }
    
39. 
    
40. For detailed implementation of select(), please take a look at sys_select() and do_select() in fs/select.c. of standard kernel source code.
    
41. 
    
42. Another thing required to understand is poll_wait(). What it does is put current process into a wait
    
43. queue provided by each kernel facilities such as file or pipe or socket or in our case, message queue.
    
44. 
    
45. Please note that the current process may wait on several wait queues by calling select()
    

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kj501

呵呵，有意思，看来有机会看看linux的kernel源代码还是很有必要的。