[fpc-pascal] GetMem question

Daniël Mantione daniel.mantione at freepascal.org
Fri Dec 1 09:07:13 CET 2006

Op Fri, 1 Dec 2006, schreef Graeme Geldenhuys:

> I got this question from a Delphi 2006 user regarding a code change I
> made in the tiOPF project.  tiOPF is compiled with FPC using the
> Delphi compiler mode. FPC 2.0.4 and FPC 2.1.1 is used.
> ---------------------------------------------
> procedure tiGetPropertyNames(AClass : TtiBaseObjectClass;
>     AStringList : TStringList; APropFilter : TTypeKinds = ctkSimple);
> var
> lCount : integer;
> lSize : integer;
> lList : PPropList;
> i : integer;
> lPropFilter : TTypeKinds;
> begin
> lList := nil; // <<== New line, I expect to fix a FPC warning
> ...
> GetMem(lList, lSize);
> But it's created a warning in BDS2006
> Variable assigned to LList never used
> ---------------------------------------------
> Quote from the FPC's GetMem help:
>  Getmem reserves Size bytes memory on the heap, and returns a pointer
> to this memory in p. If no more memory is available, nil is returned.

This is incorrect. This behaviour depends on ReturnNilIfGrowHeapFails. If 
set to true, nil is returned, otherwise, a runtime error is generated.

> Quote from Kylix 3 help:
>  If there isn't enough memory available to allocate the dynamic
> variable, an EOutOfMemory exception is raised.
> Question 1:
>  I set lList := nil; to get rid of a FPC compiler warning saying I am
> using a variable that hasn't been initialized.  Is that warning needed
> in such a case, because GetMem will initialize it for me, correct?
> Ignoring the compiler warning is also a option I guess. :)

Getmem should initialize the variable.

> Question 2:
> Reading the help, I thought there would be no harm setting lList :=
> nil, because if there is no memory available, GetMem would set it to
> nil as well.  But this brings me to the second question.  Under Delphi
> and Kylix, an EOutOfMemory exception is raised, but under FPC the
> pointer is set to nil.  Does FPC also raise an EOutOfMemory exception?
> If it does, the help doesn't mention it.

If sysutils is used, FPC converts runtime errors into exceptions, and 
EOutOfMemory is generated.


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