On 09/28/2014 05:45 PM, Lag Programming wrote:
Hi! I need some help in understanding three things viewed in heap.inc.
...
3) I'm not interested in the importance of the presented function. Why
the "existing" code design is preferred over "alternative" code?
Existing code:
function SysAllocMem(size: ptruint): pointer;
begin
result := MemoryManager.GetMem(size);
if result<>nil then
FillChar(result^,MemoryManager.MemSize(result),0);
end;
Alternative code:
function SysAllocMem(size: ptruint): pointer;
begin
result:=MemoryManager.GetMem(size);
if result<>nil then FillChar(result^,size,0);
end;
The only things I could find to support the "existing" code over
"alternative" code are that(now or in the future):
a) Maybe there is a possibility that the value of variable size gets
changed after "result:=MemoryManager.GetMem(size);"
This cannot happen, because the 'size' parameter to MemoryManager.GetMem
is passed by value, not by reference.
b) Maybe there is a possibility that following
"result:=MemoryManager.GetMem(size);" variable result would point to a
memory block that has allocated a size different than variable
"size"'s content.
This could happen, at least in theory. Memory managers usually align
memory block size up to some given granularity, so they often allocate
more memory than you requested. The question is whether they save and
return the original size in MemSize or the aligned (i.e. bigger) one. If
they store the aligned size, it's a good idea, just in case, to clear
all the memory up to that size.
Nikolay
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