Purpose of pthread_cleanup_push_defer_np()

[Sebastian Huber <sebastian dot huber at embedded-brains dot de>]

Hello,

what is the purpose of pthread_cleanup_push_defer_np()?  From the man page I have:

http://man7.org/linux/man-pages/man3/pthread_cleanup_pop_restore_np.3.html

       This sequence of calls:

           pthread_cleanup_push_defer_np(routine, arg);
           pthread_cleanup_pop_restore_np(execute);

       is equivalent to (but shorter and more efficient than):

           int oldtype;

           pthread_cleanup_push(routine, arg);
           pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
           ...
           pthread_setcanceltype(oldtype, NULL);
           pthread_cleanup_pop(execute);

Why is the cancel type set to PTHREAD_CANCEL_DEFERRED after the 
pthread_cleanup_push()?  In case the thread gets canceled asynchronously 
between the pthread_cleanup_push() and pthread_setcanceltype() the routine is 
invoked.  This is problematic in case the routine requires some conditions set 
by the ... body.  Here is an example:

http://www.sourceware.org/pthreads-win32/manual/pthread_cleanup_push.html

I thought that the pthread_cleanup_push_defer_np() is a short cut to deal with 
this.

In "nptl/sysdeps/pthread/pthread.h" we have:

/* Remove a cleanup handler as pthread_cleanup_pop does, but also
   restores the cancellation type that was in effect when the matching
   pthread_cleanup_push_defer was called.  */
#  define pthread_cleanup_pop_restore_np(execute) \
      do { } while (0);/* Empty to allow label before pthread_cleanup_pop.  */\
    } while (0);							      \
    __pthread_unregister_cancel_restore (&__cancel_buf);		      \
    if (execute)							      \
      __cancel_routine (__cancel_arg);					      \
  } while (0)

The __cancel_routine is invoked after the restore sequence, thus in case the 
thread is asynchronously canceled after the 
__pthread_unregister_cancel_restore() call, the __cancel_routine is not 
invoked.  Is this really intentional?

--
Sebastian Huber, embedded brains GmbH

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