[fpc-pascal] ReAllocMem problem (?)

Marcel Martin mm10 at ellipsa.net
Wed Jan 19 00:57:44 CET 2005

Marcel Martin a écrit :
> Hello,
> The following function SysReAllocMem comes from the file
> /rtl/inc/heap.inc (FPC 1.9.7 / Win32)
> function SysReAllocMem(var p: pointer; size: ptrint):pointer;
> [...]
>    { Resize block }
>    if not SysTryResizeMem(p,size) then
>    begin
>      minsize := MemoryManager.MemSize(p);
>      if size < minsize then
>        minsize := size;
>      p2 := MemoryManager.AllocMem(size);
>      if p2<>nil then
>        Move(p^,p2^,minsize);
>      MemoryManager.FreeMem(p);
>      p := p2;
>    end;
> [...]
> Maybe you have good reasons to write it like you did but is
> the instruction "MemoryManager.FreeMem(p);" intentionally
> always executed?
> I thought that, in case it cannot increase the size of the
> memory allocated to p^, the memory manager would set p to nil
> but not that it would free it.
> Suppose OneClass has a field FP which points to an array of
> pointers. In case of problem when attempting to increase the
> size of FP^, FP is freed and OneClass can no more destroy the
> pointed objects it created.
> Up to now, I believed the code was something like
>   if p2<>nil then
>   begin
>     Move(p^,p2^,minsize);
>     MemoryManager.FreeMem(p);
>   end;
>   p := p2; <- p might be set to nil but it is not freed
> and, in OneClass, one could do
>   Q := FP;
>   ReAllocMem(FP, NewSize (greater than current size));
>   if FP = nil then
>   begin
>     FP := Q;
>     Signal the 'Out of Memory' problem
>   end;
> Was your programming intentional? And if so, why?
> Thanks.

Considering the numerous answers I received, I am going to ask the
questions otherwise: Is it a bug or not? Should I wait for a fixed
version or should I manage to use it as it is because it will stay
as it is?  :-)

There is an other thing I didn't notice yesterday when I sent my 
post. SysReAllocMem makes use of MemoryManager.AllocMem instead of 
MemoryManager.GetMem, i.e., SysReAllocMem fills up with 0s the 
new allocated memory (whereas the ReAllocMem doc says "only the 
used memory is initialized, extra memory will not be zeroed out").


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