Description

   This routine pushes the specified routine onto the calling
   thread's cleanup handler stack. The cleanup handler routine is
   popped from the stack and called with the arg argument when any
   of the following actions occur:

   o  The thread calls pthread_cleanup_pop() and specifies a nonzero
      value for the execute argument.

   o  The thread calls pthread_exit().

   o  The thread is canceled.

   o  An exception is raised and is caught when the Threads Library
      unwinds the calling thread's stack to the lexical scope of the
      pthread_cleanup_push() and pthread_cleanup_pop()  pair.

   This routine and pthread_cleanup_pop() are implemented as macros
   and must appear as statements and in pairs within the same
   lexical scope. You can think of the pthread_cleanup_push() macro
   as expanding to a string whose first character is a left brace
   ({) and pthread_cleanup_pop() as expanding to a string containing
   the corresponding right brace (}). This routine and pthread_
   cleanup_pop() are implemented as exceptions, and may not work
   in a C++ environment. (See <REFERENCE>(exceptions_chap) for more
   information.)