Re: Passing parameters of type 'ARRAY[0..MAXINDEX] OF ElementType' to C

[Rudolf Schubert]

Hi Benjamin,

yes, you're right, it is probably best to use open array parameters to
avoid problems here. It was just that I had used 'none open' array parameters
(with GPM) and did notice that GM2 'disappointed my expectations';-)

BTW, are you sure the hidden parameter is passed BEFORE the open
array parameter? At least with GPM and GM2 I use several C functions
where the (hidden) high value is passed AFTER the open array parameter.
So my C function would look like this:

void p ( char str[], unsigned argc );


BR

Rudolf

--
Rudolf Schubert                 \
Kirchstr. 18a                    \  mailto:rudolf@muc.de
82054 Sauerlach                   > http://www.dose.muc.de
Deutschland                      /   (alpine archkiste)
Tel. 08104/908311               /


On Mon, 29 Jan 2024, Benjamin Kowarsch wrote:

> Gaius will probably answer this with the specific parameter passing and foreign 
> function
> interfacing conventions used by GM2, but I would like to recommend a different 
> approach which is to use open
> array parameters.
>
> The actual number of array components of an open array parameter is passed as a 
> hidden parameter before the
> open array parameter.
> PROCEDURE P ( str : ARRAY OF CHAR );
>
> is actually
>
> PROCEDURE P ( count : CARDINAL; str : ARRAY OF CHAR );
>
> and in C
>
> void p ( unsigned argc, char str[] );
>
> This convention is pretty universal for most if not all Modula-2 compilers and 
> thus its use is portable.
>
> When using specific array types, varying parameter passing conventions will 
> apply depending on the compiler
> and even use cases, for example, a compiler or its optimising back-end may 
> decide to pass fixed arrays
> exceeding a certain size by reference even if they aren't VAR parameters. The 
> code will then become more
> difficult to maintain and non-portable.
>
> regards
> benjamin
>
> On Sun, 28 Jan 2024 at 22:03, Rudolf Schubert <rudolf@muc.de> wrote:
>                High Gaius,
>
>                I'm facing a problem when trying to pass a parameter of typ 
> String256
>                to C, which I've defined like this:
>
>                CONST
>                  len256=                       256;
>                TYPE
>                  String256=                    ARRAY[0..len256-1] OF CHAR;
>
>                In the appended example I've set up three DEFINITION MODULEs
>                (unix1.def, unix2.def, unix3.def) each with a corresponding
>                C file (unix1.c, unix2.c, unix3.c). There is only one simple
>                function in these C files which should print out the length
>                of the passed string and the string proper.
>
>                unix1.def does NOT have 'FOR "C"' whereas unix2.def and unix3.def
>                do have a 'FOR "C"' in the MODULE definition. In unix1.def and 
> unix2.def
>                the PROCEDURE ParamTest has got (my_str: String256) but in 
> unix3.def
>                I'm using 'varargs' (...).
>
>                I'm seeing that only unix3 does give the expected results. unix1 
> and
>                unix2 do either crash the programm or return wrong results.
>
>                I do observe similar behaviour not only for
>
>                type 'ARRAY[0..MAXINDEX] OF CHAR'
>
>                but for instance also for
>
>                type 'ARRAY[0..MAXINDEX] OF CARDINAL'
>
>                Now my question is: how would I pass parameters of this type to
>                C functions? I would suspect that this should not only work for
>                'varargs' but also in the other cases? When using open ARRAYs in
>                the PROCEDURE things are also working fine. It's just when using
>                'non open' ARRAYs that the problem occurs.
>
>       BR
>
>       Rudolf
>
>       --
>       Rudolf Schubert                 \
>       Kirchstr. 18a                    \  mailto:rudolf@muc.de
>       82054 Sauerlach                   > http://www.dose.muc.de
>       Deutschland                      /   (alpine archkiste)
>       Tel. 08104/908311               /