[fpc-pascal] A warning when Blockwriting/reading dynamic array pointers
juergen.hestermann at gmx.de
Fri Apr 1 18:44:55 CEST 2011
Jonas Maebe schrieb:
>> If you Blockwrite a dynamic array pointer to file (i.e. because it
is part of a large record) and read it back later with Blockread then
the pointer value is invalid of course. BUT, if you now try to set it to
nil as in
>> DynArray := nil;
>> then Free Pascal seems to free the memory where the pointer points to!
> That is in fact more or less documented:
> "As remarked earlier, dynamic arrays are reference counted: if in one
of the previous examples A goes out of scope and B does not, then the
array is not yet disposed of: the reference count of A (and B) is
decreased with 1. As soon as the reference count reaches zero the
memory, allocated for the contents of the array, is disposed of."
> Setting a dynamic array pointer to nil also decreases the reference
count, since it removes a reference to the array data.
I would never expect an assignmet to nil changing anything else than
just the value of the pointer. And for (all?) other pointers this
expectation is valid.
>> This creates hard to find access violations later in your program.
This cost me days of headache because never in my life I would have
expected that a simple assignment with nil would do more than just
setting bytes to zero. Instead of the above assignment I had to use
>> This solved all problems.
> It mainly creates memory leaks. If you want to keep an extra
reference to a dynamic array's data, simply use another dynamic array.
No. Be aware that my dyn array pointer is part of a larger record
structure which I blockwrite/-read to and from file. When I read back
the structure the pointer is not valid. It's just the pointer at the
time I wrote the file (could be months ago). Now I have to rebuild the
stucture again. To do this I have to the pointer to nil so that
following calls to SetLength have the correct assumption that no data
has been allocated yet.
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