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digitalmars.D - Calling D functions from C

reply D. Trebbien <D._member pathlink.com> writes:
I know that it is possible to call C functions from D, but is it possible to
call D functions from C? What steps would I need to take to allow for this?
Jun 09 2005
parent reply "Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:
"D. Trebbien" <D._member pathlink.com> wrote in message 
news:d8atov$2v54$1 digitaldaemon.com...

I know that it is possible to call C functions from D, but is it possible 
to
 call D functions from C? What steps would I need to take to allow for 
 this?


Since C doesn't know about D, you can't call D functions from C.  The 
calling conventions are incompatible.

However, you can use the extern(C) function linkage in D to make a function 
C-compatible:

extern(C) export int something()
{
    // blah
    return 10;
}

The problem is that it's not just this simple.  D is garbage collected, 
something that C doesn't understand.  If you create some kind of GC'ed 
object on the heap with 'new' in your D function, and return it without 
storing the pointer to that memory in the D library, it could be garbage 
collected without the C code knowing about it.  You must keep the pointer to 
the memory allocated by the D function in the D library.  The alternative is 
to have the C code that calls the function allocate the memory for the 
function.

Unfortunately, the "Interfacing to C" part of the D website seems to have 
disappeared.  That page had a lot more info on this.
Jun 09 2005
parent reply =?ISO-8859-1?Q?Anders_F_Bj=F6rklund?= <afb algonet.se> writes:
Jarrett Billingsley wrote:


 The problem is that it's not just this simple.  D is garbage collected, 
 something that C doesn't understand.  If you create some kind of GC'ed 
 object on the heap with 'new' in your D function, and return it without 
 storing the pointer to that memory in the D library, it could be garbage 
 collected without the C code knowing about it.  You must keep the pointer to 
 the memory allocated by the D function in the D library.  The alternative is 
 to have the C code that calls the function allocate the memory for the 
 function.


You also need to do some C tricks, if you want to use the
D arrays (with length) and the D objects (implicit pointers)

But that doesn't mean that it isn't doable, or anything:

  C:
====
#include <stdlib.h>

struct Darray
{
   size_t length;
   void   *ptr;
};

extern void hello(struct Darray who);

int main(int argc, char *argv[])
{
   struct Darray str = { 5 , "World" };

   hello(str);

   return EXIT_SUCCESS;
}

  D:
====
import std.stdio;

extern(C) void hello(char[] who)
{
   writefln("Hello, %s!", who);
}

You also need to link with Phobos, just as you would have to
link with "stdc++" if you were using a C++ function from C ?

(recls is one library written in C++, with a C extern API)


Mixing languages like that quickly gets ugly, though...
For instance, unsure what happens if "hello" resizes it

Probably better to use regular lengths/pointers args ?

--anders
Jun 10 2005
parent D. Trebbien <D._member pathlink.com> writes:
What about hiding (compiled) D code in a DLL; maybe with extern(C) around the
functions?

In article <d8bntp$sa9$1 digitaldaemon.com>,
=?ISO-8859-1?Q?Anders_F_Bj=F6rklund?= says...

Jarrett Billingsley wrote:


 The problem is that it's not just this simple.  D is garbage collected, 
 something that C doesn't understand.  If you create some kind of GC'ed 
 object on the heap with 'new' in your D function, and return it without 
 storing the pointer to that memory in the D library, it could be garbage 
 collected without the C code knowing about it.  You must keep the pointer to 
 the memory allocated by the D function in the D library.  The alternative is 
 to have the C code that calls the function allocate the memory for the 
 function.


You also need to do some C tricks, if you want to use the
D arrays (with length) and the D objects (implicit pointers)

But that doesn't mean that it isn't doable, or anything:

  C:
====
#include <stdlib.h>

struct Darray
{
   size_t length;
   void   *ptr;
};

extern void hello(struct Darray who);

int main(int argc, char *argv[])
{
   struct Darray str = { 5 , "World" };

   hello(str);

   return EXIT_SUCCESS;
}

  D:
====
import std.stdio;

extern(C) void hello(char[] who)
{
   writefln("Hello, %s!", who);
}

You also need to link with Phobos, just as you would have to
link with "stdc++" if you were using a C++ function from C ?

(recls is one library written in C++, with a C extern API)


Mixing languages like that quickly gets ugly, though...
For instance, unsure what happens if "hello" resizes it

Probably better to use regular lengths/pointers args ?

--anders
Jun 10 2005