Re: memory leakages with Ada?

[Mark Johnson <mark_h_johnson@raytheon.com>]

"Frank J. Lhota" wrote:
> 
> Yes, it is true that instantiations of Ada.Unchecked_Deallocation are
> required to do nothing if called with a null access value. This is also true
> of the C/C++ equivalents, i.e. the C call "free(NULL)" should also do
> nothing, according to the standard.
> 
> I'm not sure why this is bad.

A little history. I have an old copy of K&R (Chapter 8, p 177) and the
implementation in that book for alloc/free will blow up if you provide a
NULL pointer to free. It is coded something like the following...
  free(ap)
  char *ap;
  {
    register HEADER *p, *q;
    p = (HEADER *)ap-1;
[at this point, p generally has an invalid address if ap is NULL]
There MAY still be some broken implementations using this code sequence
.... :-)

That is not to say that a general method of...
  free(p)
  p = NULL;
(or the Ada equivalent) is one way of making your code far more robust
than keeping the dangling pointers around. I use a method like that
to...
 o allow for a check for NULL pointers to determine if the data is valid
 o get an immediate exception when I forget to check & dereference a
NULL pointer (many machines)
 o it prevents a subsequent free to that pointer - a known bad operation
on several systems
That it has the additional benefit of being a "no operation" to free is
a side effect I would just as soon not exercise.
  --Mark