Howto create C-Header for Delphi/Free Pascal/Lazarus DLL - data types

Question

For my application I need to create a DLL from Delphi (to be more precise Delphi compatible code written within Lazarus IDE compiled by free pascal under linux) using stdcall.
When using that DLL (for example in Matlab or so) one needs of course the meta information for passing the arguments - often realised with a header file. I'm searching for a tool do to that running on the delphi source code. Something like h2pas-reverse.
My research yielded no results. As I think, there is no such tool, I'd like to find a table or other information, how the Delphi/Pascal data types are mapped to C types and how to work with records.

Answer 1

I have used the below construct to generate header files compatible with the C-mode compiler of Visual C++ 6 from Delphi 5 code when Delphi had the -JPH switch (see notes below).

Note that I have not used this since Delphi 5, but the switch has since then been expanded:

Somewhere along the line, the JPHNE switch has been added to the dcc32 command-line compiler:

  -JPHNE = Generate C++ .obj file, .hpp file, in namespace, export all

Rudy Velthuis has a nice article using the JPHNE switch.

It certainly does not handle all types, and you will need quite a bit of HPPEMIT and EXTERNALSYM directives.

I uploaded my Delphi 5 to Visual C++ 6 HPP conversion from back then to BitBucket.

It generates the .hpp files to import the DLL that was written in Delphi.

Notes from the Delphi 5 era:

{ Visual C++ 6 does not like nested structs/unions the way that BC++ can handle them.
  Visual C++ 6 requires the "typedef" to be present AND the typename AFTER the struct definition.
  You will see this when defining TConversationId, TReturnKey and other types that depend on nested structs/unions

  The trick is to perform these steps each and every time this unit changes:
    - Generate this unit once with all the EXTERNALSYM disabled.
      - Then the DCC32 -JPH will generate the basic structs for them.
    - Copy all the nested struct and union definitions to this file
    - Embed the definitions with (*$HPPEMIT '' *)
    - Append the typename at the end of the struct definition
    - Enable all the EXTERNALSYM again
    - Regenerate the HPP files by using DCC32 -JPH
  To make this process easier, we have introduced two new conditional defines:
    - BCB - disable EXTERNALSYM, disable HPPEMIT
    - VC6 - enable EXTERNALSYM, enable HPPEMIT

  A similar thing is with these constructions that VC6 does not like those either:
    - short strings (BCB defines them as "SmallString<##>" which VC6 does not like).
    - short integers (BCB defines them as "Shortint" so we have included an HPPEMIT for that)
}

{$ifdef win32}
  { important! Makes sure that the all enumerated types fit in exactly one byte each! }
  {$Z1}

  { force the C/C++ HPP header files to have the C/C++ compiler pack structure elements on byte boundaries }
  {$ifdef BCB}
    {$HPPEMIT '#pragma option push -a1' }
  {$endif BCB}
{$endif win32}