External function record array type mapping

[henrikt-ma]

The specification is not very clear when it comes to passing record arrays over the external function interface. The way I read https://specification.modelica.org/master/functions.html#arrays-1 is that at least the mapping of the record type itself is described in https://specification.modelica.org/master/functions.html#records. However,

Records are passed by reference (i.e., a pointer to the record is being passed).

leaves some room for interpretation, especially considering that members of record type are not stored as pointers.

So, if struct ext_C_R is the C mapping of some record type, is the external function receiving an array of such records supposed to be called with (a pointer to) an array of ext_C_R* or (a pointer to) an array of ext_C_R?

[henrikt-ma]

Is the unknown amount of padding at the end of a C struct the ultimate reason why the answer must be that an array of ext_C_R* is passed? If so, how does this work with Modelica's design of storing sub-record members by value and not by reference?

[gkurzbach]

This is unclear for me too.

[HansOlsson]

Is the unknown amount of padding at the end of a C struct the ultimate reason why the answer must be that an array of ext_C_R* is passed? If so, how does this work with Modelica's design of storing sub-record members by value and not by reference?

No, it should just be an array of struct. The size of a struct in C includes that potential unknown padding.

[HansOlsson]

Hmm.. I realized that it is a bit confusing as it currently says "Arrays cannot be mapped." (in 12.9.1.3); which assumedly means array members.

To me it would make sense to (not necessarily in this order):

• Clarify that restriction and clarify exactly which type is used in section 12.9.1.2 (as a (const) pointer to an array of structs), Clarify array of records #3715
• Add annotations for declaring records Propose complete handling of records in external functions. #3198 (going from "working in practice on all known systems" to "correct code").
• Allow returning records from C-functions Return record #3155