Calling D from C - What's the present status?

[Daniel Keep]

hauptmech wrote:
> I'd like to maintain an internal (to my company) code library in D
> but provide customers with a libxxx.a and *.h files so they can use C.
> 
> I'd like to provide access to classes and members. My C library code
> is more or less object oriented (ei pass in a pointer to data as the
> first arg to a "member" function) so customers can deal with that.
> 
> I read the short snippet that said functions attributed as extern(C)
> can be called... and I vaguely rememeber experimenting a year ago and
> being able to call class members with a little pointer abuse...
> 
> Is there a compiler switch that will generate a nice *.h file?!?
> 
> Or perhapse more reasonably; does anyone know of any projects out
> there geared towards providing C bindings for D libraries in an
> automated way?
> 
> hauptmech

AFAIK, there is no automated mechanism for generating .h files -- 
historically, we've been more worried about calling back to C and C++ 
than the other way around :)

Here's what I *do* know on the subject:

"extern(C)" lets you use perfectly normal C calling conventions.  So, 
for example, if you compile this:

extern(C) int add(int a, int b) { return a + b; }

to an object file, and link it into a C program that has the following 
declaration somewhere:

int add(int, int);

Then it should all work as expected.  The trouble comes in because of 
D's additional features such as the garbage collector (used by default 
for all heap allocations), module constructors/destructors, etc.

As for exposing D classes to C, you would have to do what you would 
normally do with C++: build a "flat api" wrapper around the classes for 
C to use.

I'm afraid I can't really offer anything more; if you check out the 
phobos source code (which comes with DMD), it contains the main function 
used by D programs (which in turn calls the main function defined in 
your code).  This includes details on how to do things like start up the 
garbage collector and do initialisation, etc.

Best of luck to you :)

	-- Daniel