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

[Benjamin Kowarsch]

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

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