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One more overlooked area where the thread group ID has to be used.
Compile and run the following code.  The program should always exit with
value zero, but currently it doesn't if a parameter is passed (i.e., if
the semaphore is modified in a thread other than the main one).  If
somebody wants to add the code to a kernel test suite, feel free.

The attached patch is against the current 2.5 tree.

#define _GNU_SOURCE
#include <errno.h>
#include <error.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/sem.h>
#include <sys/wait.h>


union semun
{
  int val;
};

static int sem;

static void
eh (void)
{
  semctl (sem, 0, IPC_RMID);
}

static void
second_process (void)
{
  int val = semctl (sem, 0, GETPID);
  printf ("parent PID: %ld\nsem PID: %d\n", (long int) getppid (), val);
  exit (getppid () != val);
}

static void
setval (void)
{
  union semun su = { .val = 1 };
  if (semctl (sem, 0, SETVAL, su) == -1)
    error (EXIT_FAILURE, errno, "semctl failed");
}

static void *
tf (void *arg)
{
  setval ();
  return NULL;
}

int
main (int argc, char *argv[])
{
  sem = semget (IPC_PRIVATE, 1, IPC_CREAT | IPC_EXCL | 0666);
  if (sem == -1)
    error (EXIT_FAILURE, errno, "semget failed");

  atexit (eh);

  if (argc == 1)
    setval ();
  else
    {
      pthread_t th;
      int r = pthread_create (&th, NULL, tf, NULL);
      if (r != 0)
        error (EXIT_FAILURE, r, "pthread_create failed");
      r = pthread_join (th, NULL);
      if (r != 0)
        error (EXIT_FAILURE, r, "pthread_join failed");
    }

  pid_t pid = fork ();
  if (pid == -1)
    error (EXIT_FAILURE, errno, "fork failed");
  if (pid == 0)
    second_process ();

  int status;
  if (TEMP_FAILURE_RETRY (waitpid (pid, &status, 0)) != pid)
    error (EXIT_FAILURE, errno, "waitpid failed");

  if (!WIFEXITED (status))
    error (EXIT_FAILURE, 0, "child process didn't exit normally");

  return WEXITSTATUS (status);
}


- --
- --------------.                        ,-.            444 Castro Street
Ulrich Drepper \    ,-----------------'   \ Mountain View, CA 94041 USA
Red Hat         `--' drepper at redhat.com `---------------------------
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