- Hardware and DLL interfacing in BlackBox

From: [at]} <Wojtek>
Date: Mon, 21 Feb 2005 16:08:26 -0500 (EST)

----boundary-LibPST-iamunique-903506249_-_-
Content-type: text/plain

On Mon, 21 Feb 2005, CFB Software wrote:

> An excellent presentation!

Thank you, I appreciate. I will keep adding to it ;-)

> I don't believe that this (i.e. ".NET is beta") was true even back in
> February 2004. It certainly is not true now.

I stand corrected. I will change the slide accordingly.

> 2. Slide 44 of 90:
>
> I believe the advice to use the statement
>
> arr := NIL;
>
> is misguided and the results are potentially dangerous.

Not sure why misguided? I do not see how it is dangerous.
Please tell me me why it is dangerous?

> The whole point
> of garbage collection is to let the system, rather than the programmer,
> decide when it is safe to reclaim memory.

I definitely agree....

> Assigning NIL to pointers is
> essentially the same as "de-allocating by hand" and as unsafe as using
> Dispose (or its equivalent) always was.

I agree on one and disagree on the other account.

 (1) Assigning a NIL to a pointer is indeed "essentially the same as
de-allocation by hand", in the sense that it make the variable
inaccessible and thus disappear from the sight (if that was the last
reference to that variable). This will eventually trigger garbage
collection, though exact point in time when the GC is called is
not exactly specified.

 (2) Assigning a NIL to a pointer is "not as unsafe as using Dispose".
Au contraire, it is safe.

The language report says in appendix D "Memory cannot be deallocated
manually", but it does not say that one cannot or should not assign
NIL to a pointer.

The run-time intergrity rests on the following invariant: either the
variable "arr" is pointing to a variable of a proper type, in which case
it can be dererenced and used, or it points to NIL, in which case it
also can be dereferenced, what however will trigger a TRAP. We need
to agree that a TRAP is permissible, though not pleasant. The run-tme
integrity is not compromised by TRAPs. In other words, TRAPs are safe
for the run-time system, though not necessarily safe for your accounts
receivable. (BlackBox only guarantees the first kind of safety.)

The variable "arr" can thus transition between two states, both of
which are safe and legal: state #1 in which it points to NIL,
and a state #2 in which it points to a variable of a proper type,
operations on which are programmatically safe.

The integrity of the run-time is guaranteed by the semantics of the
assignment operator ":=" or a NEW operator (NEW is a special case
of ":=") that both perform transitions from one safe state
to another safe state of the variable.

The run-time inegrity is potentially compromised by SYSTEM-level
assignments to "arr". One can do such unsafe assignments with SYSTEM.VAL
and SYSTEM.ADR. After using the SYSTEM the state of "adr" may no longer be
legal, and the run-time may crash. The SYSTEM-level programming is as
unsafe as C/C++, what is clearly stated in the documentation.

However, as long as we we do not use SYSTEM, the CPascal syntax and
compiler guarantee that we only transition between legal and safe states.
The assignment "arr := NIL" is therefore safe and cannot lead to system
instability down the road, though it may lead to TRAPs.

W.

--- BlackBox
--- send subject HELP or UNSUBSCRIBE to blackbox{([at]})nowhere.xy



----boundary-LibPST-iamunique-903506249_-_-
Content-type: application/rtf
Content-transfer-encoding: base64
Content-Disposition: attachment; filename="rtf-body.rtf"
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----boundary-LibPST-iamunique-903506249_-_---
Received on Mon Feb 21 2005 - 22:08:26 UTC

This archive was generated by hypermail 2.3.0 : Thu Sep 26 2013 - 06:28:23 UTC