[fpc-pascal] A warning when Blockwriting/reading dynamic array pointers

Jonas Maebe jonas.maebe at elis.ugent.be
Fri Apr 1 18:11:37 CEST 2011


On 01 Apr 2011, at 18:04, J├╝rgen Hestermann wrote:

> 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: http://www.freepascal.org/docs-html/ref/refsu15.html#x39-450003.3.1

"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.

> 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
> 
> fillchar(DynArray,sizeof(DynArray),#0);
> 
> 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.


Jonas


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