I think the sizeof call is irrelevant here (which I believe is what
Jolyon is also saying).
The error occurs on this line:
LogData.LogArgs := CreateConstArray(Args);
LogArgs is a dynamic array as is the CreateConstArray result. Standard
Delphi logic is that the memory for the old dynamic array (ie LogArgs)
will be dereferenced and then freed if no longer used. If LogArgs
points to random memory because it was never initialised then
obviously the dereferencing and subsequent freeing are both extremely
likely to blow up.
The compiler usually works pretty hard to make sure dynamic array
variables are initialised to nil but the GetMem call was bypassing that.
Cheers,
David.
*From:*[email protected]
[mailto:[email protected]] *On Behalf Of *Jolyon Smith
*Sent:* Tuesday, 30 August 2011 12:04 p.m.
*To:* [email protected]; NZ Borland Developers Group - Delphi List
*Subject:* Re: [DUG] [computing] Sizeof record gives error
No, it is not. The compiler is perfectly correct.
A "dynamic array" is a reference type. It is a POINTER to a structure
that describes a dynamically sized and reference counted array, just
as a "String" is a pointer to a structure that describes (in even more
detail) a dynamically sized, reference counted String. In fact, an
"array of Char", plus twiddly bits. :)
sizeof(TObject) = 4
sizeof(Pointer) = 4
sizeof(String) = 4
sizeof(dynamic array) = 4
Is correct and true because all four types are _pointers_ and (on
Win32) a pointer is 32-bits == 4 bytes.
The size of/amount of memory that each variable of those types points
_to_ is different, and includes not only the "user data" (at the
positive offsets from the address that the pointer references) but
also RTL information at *negative* offsets. But "sizeof" is not
concerned with (is in fact entirely ignorant of) this data, only the
size of the type used to hold the *reference* to that data.
type
tfoo = record
bar: array of integer;
end;
var
f: TFoo; // << has a single field holding a *reference* to an
empty array of integer;
begin
// sizeof(f) == 4 at this point - a pointer
SetLength(f.bar, 20); // makes "bar" big enough for 20 integers - 80
bytes
// sizeof(f) is STILL *4* at this point because "bar" is still just
a pointer.
// Furthermore, sizeof() is evaluated when you *compiled* the code
so it cannot know
// how much memory bar points to even if it wanted to.
end;
It is the *initialization* of the memory *referenced* by your dynamic
array variable that is crucial to the correct functioning of the
"compiler magic" that supports such types. If you run this code in
the debugger and place a breakpoint on the "SetLength", when the code
stops on that breakpoint open the CPU view and look back and you will
see that the compiler has injected a call to "InitializeRecord"... if
you look in System.pas you will see that _InitializeRecord in turn
calls _InitializeArray.
It is this sequence of "Initialize" calls that is missing when you
simply allocate a chunk of memory and then set a typed pointer to
point at it without first doing all the housekeeping necessary to make
that chunk of memory *behave* in the way that your typed pointer is
going to expect it to.
By all means do that simple memory allocation, but you are then
responsible for doing the housekeeping... calling "Initialize".
Or use "New" and let the RTL take care of any initialisation that may
be required.
On 30 August 2011 09:10, Rohit Gupta <[email protected]
<mailto:[email protected]>> wrote:
Joylon,
Good point. But I dont think it applies here. Sizeof returns an
incorrect result... Its a compiler flaw.
Rohit
On 29/08/2011 8:10 p.m., Jolyon Smith wrote:
Having just looked up the references for New/GetMem it may not
actually be a question of amount of memory allocated after all, but
rather one of correct initialization of the allocated memory.
GetMem() does not initialize the allocated memory, New() does.
This is explained in the documentation for the "Initialize()"
procedure (it might have been helpful if the documentation for
GetMem() had mentioned this - or the lack there-of - too - LOL)
So you might find that GetMem() would have worked had you also called
Initialise() on the pointer afterward (and Finalize() before calling
FreeMem()). But whatever the underlying reasons it is obviously
simply much easier and safer (less to remember to have to do, less to
get wrong in doing it :)) to use New/Dispose when working with
allocations of variables/structured(typed) memory and use
GetMem/FreeMem when it is strictly speaking *just* unstructured memory
you need (e.g. i/o buffers etc), rather than space for variables.
Glad it helped with your problem in any event. :)
On 29 August 2011 17:40, David Moorhouse <[email protected]
<mailto:[email protected]>> wrote:
I'm aware of the compiler layout behind the scene - and the fact that
regardless of the length of the dynamic array, my call to GetMem (or
New) does NOT have to allocate memory for the dynamic array's
contents, just it's overhead :)
However, the compiler gets the size right using New rather than GetMem
:) So thanks for the tip.
Cheers
D
On 29/08/11 16:23, Jolyon Smith wrote:
Is it a compiler *error* or just a compiler _behaviour_ ?
I haven't looked into it in detail, but dynamic arrays are notoriously
slippery when you are working with them at a low level and alarm bells
started ringing as soon as I saw they were involved.
In particular, a dynamic array is a reference type, like a string. So
whilst their may be additional RTTI at a negative offset from the base
address of the array, the "array" itself may well be a pointer, hence
"sizeof()" will return 4 - the size of a pointer - no matter how many
items may be in the array (as opposed to Length(), obviously).
NOTE: sizeof(String) also yields "4" even though we all know that a
String variable requires many more bytes than that.
As far as this particular example goes, do you get any better results
using the typed New() function rather than GetMem() which knows
nothing about the "type" of memory required by the pointer you are
initialising and just blithely allocates the specified number of bytes...:
Instead of >> LogData := GetMem( ... );
Use >> New( LogData );
And see if you get better results. :)
(Also, don't forget to use "Dispose()" to deallocate the memory
obtained with "New()", rather than FreeMem())
On 28 August 2011 21:33, David Moorhouse (DUG)
<[email protected] <mailto:[email protected]>> wrote:
I believe it is a compiler error and will raise a QA ticket
Thanks for your help
D
On 26/08/11 08:17, Peter Ingham wrote:
> Try filling LogData with binary zeros after the Getmem& before the
assign.
>
> FillChar (LogData^, sizeof(TLogData), 0);
>
> I believe the uninitialized memory is messing up the compiler magic
> associated with the dynamic array.
>
>
> Any reason the local Tlogdata record is referenced via a pointer?
>
> I suspect the following will also work:
> procedure TUserClass.Log(const LogType: TLogType; const Args: array of
> const );
> var
> LogData: TLogData;
> begin
> LogData.LogType := LogType;
> LogData.LogArgs := CreateConstArray(Args);
> // ... do some other stuff with the LogData item finally calling
> end;
>
> Cheers
>
> On 26/08/2011 1:49 a.m., David Moorhouse wrote:
>> Hi Peter
>>
>> Been there done that :)
>>
>> The function call is fine. It is the assignment that causes the AV -
>> because the "bucket" is too small.
>> Assigning it with 16 bytes fixes the problem, regardless of how many
>> items the array holds.
>>
>> I smell compiler magic in the background.
>>
>> Cheers
>
>> D
>>
>> On 25/08/11 17:29, Peter Ingham wrote:
>>> Another attempt to reply...
>>>
>>> First thing to do is determine if the crash occurs in the
procedure call,
>>> on the subsequent assign, or in between.
>>>
>>> Give this a try:
>>> procedure TUserClass.Log(const LogType: TLogType; const Args:
array of
>>> const );
>>> var
>>> LogData: PLogData;
>>> TempArgs : TConstArray;
>>> begin
>>> // size of record TLogData does not work
>>> GetMem(LogData, sizeof(TLogData));
>>> LogData.LogType := LogType;
>>> // blows up on one of these lines
>>> TempArgs := CreateConstArray(Args);
>>> LogData.LogArgs := TempArgs;
>>> // ... do some other stuff with the LogData item finally calling
>>> FreeMem
>>> end;
>>>
>>>
>>> Regarding the size of a dynamic array, like a string variable, the
>>> variable (LogArgs in this case) is the size of a pointer (i.e. 4 bytes
>>> for Win32). If the pointer is non-zero, it points to a structure
which
>>> includes the adjacent array elements preceded by a length.
>>>
>>> One thing to watch out for is that Getmem does not clear the allocated
>>> memory, so LogData after the Getmem call will contain any old rubbish.
>>> The reference to LogData.LogArgs in the assignment may be
>>> dereferencing a non-zero pointer& attempting to use whatever it
>>> contains.
>>>
>>> Cheers
>>>
>>>
>>> On 25/08/2011 11:40 a.m., David Moorhouse (DUG) wrote:
>>>> I have the following code snippet
>>>>
>>>> <code>
>>>> type
>>>> PConstArray = ^TConstArray;
>>>> TConstArray = array of TVarRec;
>>>>
>>>> function CreateConstArray(const Elements: array of const):
TConstArray;
>>>>
>>>> type
>>>> TLogType = (ltError, ltWarn, ltInfo);
>>>> PLogData = ^TLogData;
>>>> TLogData = record
>>>> LogType: TLogType;
>>>> LogArgs: TConstArray;
>>>> end;
>>>>
>>>> ....
>>>>
>>>> procedure TUserClass.Log(const LogType: TLogType; const Args:
array of
>>>> const );
>>>> var
>>>> LogData: PLogData;
>>>> begin
>>>> // size of record TLogData does not work
>>>> GetMem(LogData, sizeof(TLogData));
>>>> LogData.LogType := LogType;
>>>> // blows up on next line
>>>> LogData.LogArgs := CreateConstArray(Args);
>>>> // ... do some other stuff with the LogData item finally calling
>>>> FreeMem
>>>> end;
>>>>
>>>> function CreateConstArray(const Elements: array of const):
TConstArray;
>>>> var
>>>> I: Integer;
>>>> begin
>>>> SetLength(Result, Length(Elements));
>>>> for I := Low(Elements) to High(Elements) do
>>>> Result[I] := // assign a TVarRec here
>>>> end;
>>>> </code>
>>>>
>>>> The code that assigns the memory only assigns 8 bytes - and an access
>>>> violation ensues. If I replace the call to "sizeof" with the
number 16,
>>>> the code works fine.
>>>>
>>>> My understanding of dynamic arrays was that the compiler created a 4
>>>> byte
>>>> field before the first element that contained the length of the
array.
>>>>
>>>> So why does the sizeof function not reflect this ? And why do I
>>>> need 16
>>>> bytes not 12 (4 for LogType + 4 for length of array + 4 for array
>>>> pointer)?
>>>> Also regardless of the number of items in the open array
parameter, 16
>>>> bytes works, so it does not relate the length of the TConstArray.
>>>>
>>>> Your thoughts ?
>>>>
>>>> David
>>>>
>>>>
_______________________________________________
NZ Borland Developers Group - Delphi mailing list
Post: [email protected] <mailto:[email protected]>
Admin: http://delphi.org.nz/mailman/listinfo/delphi
Unsubscribe: send an email to [email protected]
<mailto:[email protected]> with Subject: unsubscribe
_______________________________________________
NZ Borland Developers Group - Delphi mailing list
Post: [email protected]
Admin: http://delphi.org.nz/mailman/listinfo/delphi
Unsubscribe: send an email to [email protected] with
Subject: unsubscribe
