Bug in select_tut

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.

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,.

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.

Hello Sebastian,

Quoted 5 lines · Sebastian Kienzl

Hello!

There's a bug in the select_tut man-page in the chapter "Combining Signal and Data Events", which should illustrate pselect()-usage.

Quoted 5 lines

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?

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 */
          }
      }