[fpc-pascal] Questions regarding arrays

Graeme Geldenhuys graemeg.lists at gmail.com
Thu Sep 23 09:06:53 CEST 2010

Hi Michael,

On 23 September 2010 00:49, Michael Müller wrote:
> So when the array memory is allocated by the calling function and freed
> by another function and you only what to be able to access the elements
> using the array braces you should use the static array approach. But I
> would define the static array type as follows
> TAtomArray = array[0..MaxInt] of TAtom; // or whatever is the allowed maximum index
> to avoid that Pascal will raise a range checking error when you activate range checking.

Ha, I think a light bulb just came on! :-)

So let me see if I got this right. I defined TAtomArray as a static
array with a single element. In static arrays, all elements follow
each other in sequence in memory. I only know the length of the static
array after the C API call, but because I know the memory address of
the first element in the array, I can access the other elements too.
And because I currently have range-checking disabled, Pascal doesn't
complain that I am going over the bounds of my TAtomArray definition.

If I got that right, I should then also know how to pass such an array
to the C API (which was going to be my next question). The easiest
would be to create a static array with a largish size in Pascal (for
DND mime types, 30-50 elements should be plenty), then pass the
address of element 0 to the C API, and another parameter that
specifies the length of the array (only elements with actual data).

Once again, thank you very much for taking the time to explain things
in such detail. I really helps a lot. As for the differences between
static and dynamic arrays, and a string array - i think I'll annotate
my FPC Language Reference docs with what you mentioned, or see if
Michael van Canneyt could add that info into the official docs so
others can benefit too.

Out of interest. Do you know if dynamic array elements are in sequence
(storage area in memory) too - like static arrays? So could I do the
same as above, but instead of using a static array, use a dynamic
array, and pass the address of the first element to the C API?

As for freeing the array that the C API returned to me... I don't do
the freeing, I simply notify the C library when I am done using the
array, and the library does the freeing for me.

> To prove my explanation you can simply print the memory addresses of @xdndtypes and
> @xdndtypes[0] for a static array and a (initialized?) dynamic array and you'll get the
> same addresses for the static array and different addresses for the dynamic.

Indeed, this is what I got. :-)

  - Graeme -

fpGUI - a cross-platform Free Pascal GUI toolkit

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