Re: Bug in select_tut

Re: Bug in select_tut

[Michael Kerrisk]

Hi Sebastian,

On Sun, Jan 18, 2009 at 3:02 AM, Sebastian Kienzl <seb-O0gdCS+Gcp8@public.gmane.org> wrote:
> Hello!
>
> There's a bug in the select_tut man-page in the chapter "Combining Signal
> and Data Events", which should illustrate pselect()-usage.
>
> See the attachments for detailed information, proof-code and a proposed fix.
>
> Regards,
> Seb.
>
> /*
> This is a demonstration of how the code provided
> in the Linux man-page select_tut(2) / "Combining Signal and Data Events"
> for counting SIGCHLD-events is not correct.
>
> The problem is that the example-code assumes that
> normal signals get queued somehow, which is not the case; quote from
> signal(7):
>
>  "By contrast, if multiple instances of a standard signal are delivered
>  while that signal is currently blocked, then only one instance is queued."
>
> Bottom line:
>
> Setting a global flag when a signal has occured will do the job
> and that's what pselect() is intended for -- but counting the occurences
> of a standard signal as shown in the example DOES NOT WORK.

Thanks for your very thorough report.  I agree that the example in its
current form seems to be confused.  (And indeed, there are many things
taht I do not really like about this page, and I plan to clean some of
them up.)

> Compile and run this file to see how the example code will result in zombies
> if multiple SIGCHLDs are delivered at the same time;
> compile with -DCORRECT to get the correct solution for this case.
>
> See select_tut-badcount-fix.patch for a proposal to fix the documentation,.

However, what I am inclined to do is fix the program, rather than the
text.  I will send you a revised version in a moment; please take a
look at it.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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Re: Bug in select_tut

[Michael Kerrisk]

Hello Sebastian,

Sebastian Kienzl wrote:
> Hello!
> 
> There's a bug in the select_tut man-page in the chapter "Combining
> Signal and Data Events", which should illustrate pselect()-usage.
> 
> See the attachments for detailed information, proof-code and a proposed
> fix.
> 
> Regards,
> Seb.

What do you think of the revised (but still incomplete)
program below?

Cheers,

Michael

       static volatile sig_atomic_t got_SIGCHILD = 0;

       void
       child_sig_handler(int sig)
       {
           got_SIGCHILD = 1;
       }

       int
       main(int argc, char *argv[])
       {
           sigset_t sigmask, empty_mask;
           struct sigaction sa;
           fd_set readfs, writefds, exceptfds;
           int r;

           sigemptyset(&sigmask);
           sigaddset(&sigmask, SIGCHLD);
           if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == -1) {
               perror("sigprocmask");
               exit(EXIT_FAILURE);
           }

           sa.sa_flags = 0;
           sa.sa_handler = child_sig_handler;
           sigemptyset(&sa.sa_mask);
           if (sigaction(SIGCHILD, &sa, NULL) == -1) {
               perror("sigaction");
               exit(EXIT_FAILURE);
           }

           sigemptyset(empty_mask);

           for (;;) {          /* main loop */
               /* Initialize readfds, writefds, and exceptfds
                  before the pselect() call. (Code omitted.) */

               r = pselect(nfds, &readfs, &writefds, &exceptfd,
                           0, &empty_mask);
               if (r == -1 && errno != EINTR) {
                   /* Handle error */
               }

               if (got_SIGCHILD) {
                   got_SIGCHILD = 0;

                   /* Handle signalled event here; e.g., wait() for all
                      terminated children. (Code omitted.) */
               }

               /* main body of program */
           }
       }
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Re: Bug in select_tut

[Michael Kerrisk]

On Mon, Jan 26, 2009 at 12:02 AM, Michael Kerrisk
<mtk.manpages-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org> wrote:
> Hello Sebastian,
>
> Sebastian Kienzl wrote:
>> Hello!
>>
>> There's a bug in the select_tut man-page in the chapter "Combining
>> Signal and Data Events", which should illustrate pselect()-usage.
>>
>> See the attachments for detailed information, proof-code and a proposed
>> fix.
>>
>> Regards,
>> Seb.
>
> What do you think of the revised (but still incomplete)
> program below?

Hmmm -- obviously, s/SIGCHILD/SIGCHLD/ throughout the example code I
sent earlier.

Cheers,

Michael

> Michael
>
>       static volatile sig_atomic_t got_SIGCHILD = 0;
>
>       void
>       child_sig_handler(int sig)
>       {
>           got_SIGCHILD = 1;
>       }
>
>       int
>       main(int argc, char *argv[])
>       {
>           sigset_t sigmask, empty_mask;
>           struct sigaction sa;
>           fd_set readfs, writefds, exceptfds;
>           int r;
>
>           sigemptyset(&sigmask);
>           sigaddset(&sigmask, SIGCHLD);
>           if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == -1) {
>               perror("sigprocmask");
>               exit(EXIT_FAILURE);
>           }
>
>           sa.sa_flags = 0;
>           sa.sa_handler = child_sig_handler;
>           sigemptyset(&sa.sa_mask);
>           if (sigaction(SIGCHILD, &sa, NULL) == -1) {
>               perror("sigaction");
>               exit(EXIT_FAILURE);
>           }
>
>           sigemptyset(empty_mask);
>
>           for (;;) {          /* main loop */
>               /* Initialize readfds, writefds, and exceptfds
>                  before the pselect() call. (Code omitted.) */
>
>               r = pselect(nfds, &readfs, &writefds, &exceptfd,
>                           0, &empty_mask);
>               if (r == -1 && errno != EINTR) {
>                   /* Handle error */
>               }
>
>               if (got_SIGCHILD) {
>                   got_SIGCHILD = 0;
>
>                   /* Handle signalled event here; e.g., wait() for all
>                      terminated children. (Code omitted.) */
>               }
>
>               /* main body of program */
>           }
>       }
>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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