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Re: Buffer access with bounds checking...

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"Mark Wills" <markrobertwills@yahoo.co.uk> wrote in message
news:86bccc99-b093-42c4-ae50-39c308b66254@p12g2000vbm.googlegroups.com...
> While writing about memory buffer overruns in a different thread
> earlier today, I was inspired to have a bash at writing some
> code that would allow safe read/write access to memory buffers.
>
> I came up with the code and wonder if it could be simplified or
> improved any.
>
> It's very simple. When a buffer is created, the first four cells
> are reserved for the following:
> * The pfa of the buffer (i'll explain in a minute)
> * The size of the buffer in bytes
> * The lowest legally accessible address
> * The highest legally accessible address
>

The problem here is the same issue that C has with pointers.  It's the same
reason why Java removed pointers for safety.  C has the issue of determining
whether a pointer points to or into a valid C object, or not.

If you have an address in Forth, you don't know if it points into a buffer,
which buffer, or somewhere else, unless you check the ranges of _all_ known
buffers.  So, let's say you've got over 50 buffers and an address.  Does the
address point into a buffer and which one?  You'll need a dictionary of
buffer entries.  You need to loop through all of them for each address to
determine if the address is within the range of one of the buffers.  As you
demonstrated, this only works _if_ you already know which buffer an address
points into.

The other option is to not allow pointers to point into an object.  Then,
the pointer always points to the object's header, or to invalid data.  But,
that complicates accessing data within an object, such as a character from a
string, or an item from a structure.

Of course, if you don't realize it, what you're doing is implementing part
of a type system.  You're storing all the relevant info a compiler would
need to check the type, but doing so just for your buffer.


Rod Pemberton

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Thread

Buffer access with bounds checking... Mark Wills <markrobertwills@yahoo.co.uk> - 2012-08-31 06:51 -0700
  Re: Buffer access with bounds checking... Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-08-31 11:04 -0500
  Re: Buffer access with bounds checking... Alex McDonald <blog@rivadpm.com> - 2012-08-31 09:20 -0700
    Re: Buffer access with bounds checking... Mark Wills <markrobertwills@yahoo.co.uk> - 2012-08-31 12:57 -0700
      Re: Buffer access with bounds checking... "Rod Pemberton" <do_not_have@notemailnot.cmm> - 2012-08-31 18:46 -0400
        Re: Buffer access with bounds checking... Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-09-01 04:05 -0500
          Re: Buffer access with bounds checking... "Rod Pemberton" <do_not_have@notemailnot.cmm> - 2012-09-01 13:45 -0400
            Re: Buffer access with bounds checking... Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-09-02 04:19 -0500
              Re: Buffer access with bounds checking... "Rod Pemberton" <do_not_have@notemailnot.cmm> - 2012-09-02 16:15 -0400
                Re: Buffer access with bounds checking... Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-09-04 12:02 -0500
  Re: Buffer access with bounds checking... Doug Hoffman <glidedog@gmail.com> - 2012-08-31 14:15 -0400
    Re: Buffer access with bounds checking... Mark Wills <markrobertwills@yahoo.co.uk> - 2012-08-31 12:56 -0700
    Re: Buffer access with bounds checking... Paul Rubin <no.email@nospam.invalid> - 2012-08-31 13:32 -0700
      Re: Buffer access with bounds checking... Doug Hoffman <glidedog@gmail.com> - 2012-08-31 17:45 -0400
        Re: Buffer access with bounds checking... Paul Rubin <no.email@nospam.invalid> - 2012-08-31 15:07 -0700
          Re: Buffer access with bounds checking... Mark Wills <forthfreak@gmail.com> - 2012-09-01 00:49 -0700
            Re: Buffer access with bounds checking... Paul Rubin <no.email@nospam.invalid> - 2012-09-01 14:06 -0700
              Re: Buffer access with bounds checking... Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-09-02 04:21 -0500
              Re: Buffer access with bounds checking... anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-09-02 10:27 +0000
          Re: Buffer access with bounds checking... Doug Hoffman <glidedog@gmail.com> - 2012-09-01 06:59 -0400
  Re: Buffer access with bounds checking... humptydumpty <ouatubi@gmail.com> - 2012-08-31 14:21 -0700
  Re: Buffer access with bounds checking... "Rod Pemberton" <do_not_have@notemailnot.cmm> - 2012-08-31 18:48 -0400
  Re: Buffer access with bounds checking... Bernd Paysan <bernd.paysan@gmx.de> - 2012-09-01 01:41 +0200
    Re: Buffer access with bounds checking... Bernd Paysan <bernd.paysan@gmx.de> - 2012-09-01 02:37 +0200
    Re: Buffer access with bounds checking... mhx@iae.nl (Marcel Hendrix) - 2012-09-01 11:11 +0200

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