Re: select() bug

[Paul Marquis <pmarquis@iname.com>]

It shows that even though select() says a file descriptor is not
writable, a write() can still succeed.  This code is not used anywhere
in the real world, or at least my real world :-P.  It just demonstrates
"the bug".

Richard B. Johnson" wrote:
> Not as I see it. This is clearly a code bug. Look:
> iteration 1
> select says pipe is writable
> write succeeded - 1 bytes written
> iteration 2
> select returned 0 -- pipe not writable
> 
>       for (i = 0; i < 4; i++)
>       {
>          printf("iteration %d\n", i + 1);
> 
>          status = 0;
> 
>          /* do select */
>          FD_ZERO(&write_fds);
>          FD_SET(fd[1], &write_fds);
>          tv.tv_sec = 0;
>          tv.tv_usec = 0;
>          switch (select(fd[1] + 1, NULL, &write_fds, NULL, &tv))
>          {
>             case -1:
>                /* should probably check for EINTR and/or EWOULDBLOCK */
>                printf("select error - %s\n", strerror(errno));
>                break;
>             case 0:
>                /* no I/O */
>                puts("select returned 0 -- pipe not writable");
>                break;
> 
>             The BREAK breaks out of the switch-statement ---
>                                                            |
> |-----------------------------------------------------------
> |
> |            default:
> |               /* make sure our fd is writable */
> |               if (FD_ISSET(fd[1], &write_fds))
> |               {
> |                  puts("select says pipe is writable");
> |                  status = 1;
> |               }
> |               else
> |                  puts("select says pipe is not writable");
> |               break;
> |         }
> |
> |         /* do write */
> |------->  n = write(fd[1], buf + (i % 2), 1);
> 
> Then you write it anyway!
> 
> write succeeded - 1 bytes written
> select bug
> 
> Cheers,
> Dick Johnson
> 
> Penguin : Linux version 2.2.17 on an i686 machine (801.18 BogoMips).
> 
> "Memory is like gasoline. You use it up when you are running. Of
> course you get it all back when you reboot..."; Actual explanation
> obtained from the Micro$oft help desk.

--
Paul Marquis
pmarquis@iname.com
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