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Re: free a pointer and have it point to NULL

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Robert Wessel <robertwessel2@yahoo.com> wrote:
> On Tue, 29 Apr 2014 12:41:07 -0400, James Kuyper
> <jameskuyper@verizon.net> wrote:

(snip)
>>I left out a key step: on such systems it is often the case that loading
>>an invalid into an address register is sufficient to cause your program
>>abort. Dereferencing the pointer isn't necessary. This is a safety
>>measure: the designers felt that any program written badly enough to
>>treat an invalid address as if were a valid one, is sufficiently
>>dangerous that it's safest to have a policy of stopping such programs as
>>soon as possible.

More specifically, the way x86 processors starting with the 80286
work, is that when you load a segment selector into a segment
register, the processor also loads a segment descriptor from the
appropriate entry in a descriptor table. If the descriptor is 
invalid, an interrupt occurs.

> While not specifically a C example, we supported the Realia Cobol
> compiler in the 16-bit era, and on protected mode systems, you'd have
> exactly that problem.

C compilers I know tend to avoid loading segment registers until
they are pretty much ready to use them. For example, pointer
assignment and pointer comparison are not done using a segment
register. (I don't believe that there are compare instructions
for segment registers. Assignment could be done through load
and store, though.)
 
> Cobol is mostly pass-by-reference (in those days it was all
> pass-by-ref, there is some pass-by-value in the modern standards), and
> the Realia calling convention put the (pointers to the) first two
> parameters in DS:SI and ES:DI.  Merely calling a subroutine with a
> pointer to storage that had been returned to the OS would cause an
> abend when the caller tried to load the appropriate segment register.

There have been many systems where you could load from any memory
address, such as real mode MS-DOS. Some programmers for those
systems tend to ignore fetch from bad addresses, such as past the
end of arrays. (In most cases, just after the end of an array is
still part of your address space.) 

Much code for languages that don't require short-circuit IF
evaluation works because the out of bounds fetch doesn't cause
any problems.

And, by the way, all the segment selector logic is still in
current, and likely future, x86 processors.  It isn't some strange
system that died out years ago, now only found in a museum.
(But there are some of those, too.)

-- glen

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free a pointer and have it point to NULL G G <gdotone@gmail.com> - 2014-04-22 14:22 -0700
  Re: free a pointer and have it point to NULL James Kuyper <jameskuyper@verizon.net> - 2014-04-22 17:32 -0400
  Re: free a pointer and have it point to NULL G G <gdotone@gmail.com> - 2014-04-22 14:36 -0700
  Re: free a pointer and have it point to NULL Keith Thompson <kst-u@mib.org> - 2014-04-22 14:37 -0700
    Re: free a pointer and have it point to NULL Johannes Bauer <dfnsonfsduifb@gmx.de> - 2014-04-29 13:10 +0200
      Re: free a pointer and have it point to NULL Richard <rgrdev_@gmail.com> - 2014-04-29 13:14 +0200
        Re: free a pointer and have it point to NULL Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-04-29 13:19 +0100
          Re: free a pointer and have it point to NULL Kaz Kylheku <kaz@kylheku.com> - 2014-04-29 13:57 +0000
      Re: free a pointer and have it point to NULL Kaz Kylheku <kaz@kylheku.com> - 2014-04-29 13:43 +0000
      Re: free a pointer and have it point to NULL James Kuyper <jameskuyper@verizon.net> - 2014-04-29 09:59 -0400
        Re: free a pointer and have it point to NULL Keith Thompson <kst-u@mib.org> - 2014-04-29 07:58 -0700
          Re: free a pointer and have it point to NULL Keith Thompson <kst-u@mib.org> - 2014-04-29 11:50 -0700
            Re: free a pointer and have it point to NULL James Kuyper <jameskuyper@verizon.net> - 2014-04-29 15:47 -0400
        Re: free a pointer and have it point to NULL James Kuyper <jameskuyper@verizon.net> - 2014-04-29 12:41 -0400
          Re: free a pointer and have it point to NULL Robert Wessel <robertwessel2@yahoo.com> - 2014-04-29 13:15 -0500
            Re: free a pointer and have it point to NULL glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-29 21:29 +0000
      Re: free a pointer and have it point to NULL glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-29 15:26 +0000
  Re: free a pointer and have it point to NULL jt@toerring.de (Jens Thoms Toerring) - 2014-04-22 21:43 +0000
    Re: free a pointer and have it point to NULL Keith Thompson <kst-u@mib.org> - 2014-04-22 14:53 -0700
      Re: free a pointer and have it point to NULL Öö Tiib <ootiib@hot.ee> - 2014-04-24 00:38 -0700
        Re: free a pointer and have it point to NULL James Kuyper <jameskuyper@verizon.net> - 2014-04-24 07:52 -0400
          Re: free a pointer and have it point to NULL <william@wilbur.25thandClement.com> - 2014-04-24 16:13 -0700
            Re: free a pointer and have it point to NULL Ian Collins <ian-news@hotmail.com> - 2014-04-25 12:02 +1200
              Re: free a pointer and have it point to NULL <william@wilbur.25thandClement.com> - 2014-04-24 22:03 -0700
                Re: free a pointer and have it point to NULL Ian Collins <ian-news@hotmail.com> - 2014-04-25 17:22 +1200
                Re: free a pointer and have it point to NULL <william@wilbur.25thandClement.com> - 2014-04-24 23:00 -0700
          Re: free a pointer and have it point to NULL Öö Tiib <ootiib@hot.ee> - 2014-04-27 08:32 -0700
    Re: free a pointer and have it point to NULL Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-04-22 16:19 -0600
      Re: free a pointer and have it point to NULL Kaz Kylheku <kaz@kylheku.com> - 2014-04-22 22:41 +0000
        Re: free a pointer and have it point to NULL Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-04-22 18:17 -0600
          Re: free a pointer and have it point to NULL Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-04-24 08:48 -0600
    non-NULL pointers which have the value 0 (Was: Re: free a pointer and have it point to NULL) Andrew Cooper <root@127.0.0.1> - 2014-04-23 03:28 +0100
      Re: non-NULL pointers which have the value 0 (Was: Re: free a pointer and have it point to NULL) Kaz Kylheku <kaz@kylheku.com> - 2014-04-23 04:37 +0000
      Re: non-NULL pointers which have the value 0 glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-23 06:14 +0000
  Re: free a pointer and have it point to NULL Kaz Kylheku <kaz@kylheku.com> - 2014-04-22 22:18 +0000
  Re: free a pointer and have it point to NULL Michael Angelo Ravera <maravera@prodigy.net> - 2014-05-03 01:00 -0700

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