Delphi dcu to obj

Question

Is there a way to convert a Delphi .dcu file to an .obj file so that it can be linked using a compiler like GCC? I've not used Delphi for a couple of years but would like to use if for a project again if this is possible.

Answer 1

Delphi can output .obj files, but they are in a 32-bit variant of Intel OMF. GCC, on the other hand, works with ELF (Linux, most Unixes), COFF (on Windows) or Mach-O (Mac).

But that alone is not enough. It's hard to write much code without using the runtime library, and the implementation of the runtime library will be dependent on low-level details of the compiler and linker architecture, for things like correct order of initialization.

Moreover, there's more to compatibility than just the object file format; code on Linux, in particular, needs to be position-independent, which means it can't use absolute values to reference global symbols, but rather must index all its global data from a register or relative to the instruction pointer, so that the code can be relocated in memory without rewriting references.

DCU files are a serialization of the Delphi symbol tables and code generated for each proc, and are thus highly dependent on the implementation details of the compiler, which changes from one version to the next.

All this is to say that it's unlikely that you'd be able to get much Delphi (dcc32) code linking into a GNU environment, unless you restricted yourself to the absolute minimum of non-managed data types (no strings, no interfaces) and procedural code (no classes, no initialization section, no data that needs initialization, etc.)

Answer 2

(answer to various FPC remarks, but I need more room)

For a good understanding, you have to know that a delphi .dcu translates to two differernt FPC files, .ppu file with the mentioned symtable stuff, which includes non linkable code like inline functions and generic definitions and a .o which is mingw compatible (COFF) on Windows. Cygwin is mingw compatible too on linking level (but runtime is different and scary). Anyway, mingw32/64 is our reference gcc on Windows.

The PPU has a similar version problem as Delphi's DCU, probably for the same reasons. The ppu format is different nearly every major release. (so 2.0, 2.2, 2.4), and changes typically 2-3 times an year in the trunk

So while FPC on Windows uses own assemblers and linkers, the .o's it generates are still compatible with mingw32 In general FPC's output is very gcc compatible, and it is often possible to link in gcc static libs directly, allowing e.g. mysql and postgres linklibs to be linked into apps with a suitable license. (like e.g. GPL) On 64-bit they should be compatible too, but this is probably less tested than win32.

The textmode IDE even links in the entire GDB debugger in library form. GDB is one of the main reasons for gcc compatibility on Windows.

While Barry's points about the runtime in general hold for FPC too, it might be slightly easier to work around this. It might only require calling certain functions to initialize the FPC rtl from your startup code, and similarly for the finalize. Compile a minimal FPC program with -al and see the resulting assembler (in the .s file, most notably initializeunits and finalizeunits) Moreover the RTL is more flexible and probably more easily cut down to a minimum.

Of course as soon as you also require exceptions to work across gcc<->fpc bounderies you are out of luck. FPC does not use SEH, or any scheme compatible with anything else ATM. (contrary to Delphi, which uses SEH, which at least in theory should give you an advantage there, Barry?) OTOH, gcc might use its own libunwind instead of SEH.

Note that the default calling convention of FPC on x86 is Delphi compatible register, so you might need to insert proper cdecl (which should be gcc compatible) modifiers, or even can set it for entire units at a time using {$calling cdecl}

On *nix this is bog standard (e.g. apache modules), I don't know many people that do this on win32 though.

About compatibility; FPC can compile packages like Indy, Teechart, Zeos, ICS, Synapse, VST and reams more with little or no mods. The dialect levels of released versions are a mix of D7 and up, with the focus on D7. The dialect level is slowly creeping to D2006 level in trunk versions. (with for in, class abstract etc)