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[Caml-list] camlidl and pointer to function
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Date: 2001-11-14 (20:25)
From: Dmitry Bely <dbely@m...>
Subject: Re: [Caml-list] camlidl and pointer to function
Dmitry Bely <> writes:

> "Marcin 'Qrczak' Kowalczyk" <> writes:
> > > The problem is that C library is binary-only (Intel image processing
> > > library), and CallBack type is exactly 
> > > 
> > > typedef int (*CallBack)(int);
> > 
> > Converting a function closure to a C function pointer can't be done
> > portably, but it can be done with lots of ugly magic.
> > 
> > Glasgow Haskell does this (by generating a piece of assembler on
> > the heap) and it's convenient to use from the level of Haskell.
> > Such functions need to be explicitly freed of course.
> > 
> > GNU C does this for local functions, but only "downwards". If the
> > function pointer doesn't need to live longer than the function which
> > installs the callback, the GNU C extension can be used. It generates
> > the piece of assembler on the stack. To use it - just define a function
> > inside a function and take its address.
> Hmm, this trick assumes that the stack is executable (IMHO, the serious
> security hole). Don't know about Linux/x86 page/segment attributes policy,
> but I am almost sure that Windows NT/2000 x86 (which is my platform) will
> not allow to execute the code in data/stack pages.

Sorry, in fact i386 and above does not seems to have page-level execute
permissions, so this trick will work on all i386 flat model-based OSes
(I've tried it under Win2000). Now I would like to see this implemented in
camlidl :-)

Here is the way I am going to use right now:

typedef [abstract] void* SomeType;

// typedef RES (CALLBACK*)(PARAM);
typedef [abstract] void* CALLBACK; 
typedef int RES;
typedef int PARM;

void convert( SomeType* in, SomeType* out, CALLBACK cb)
  {                                                  \n\
    value v = *(value*)cb;                           \n\
    RES _cb(PARM p)                                  \n\
    {                                                \n\
      value _v_p;                                    \n\
      RES _res;                                      \n\
      value _vres;                                   \n\
      _v_p = camlidl_c2ml_cback_PARM(&p,_ctx);       \n\
      _vres = callback(v,p);                         \n\
      camlidl_ml2c_cback_RES(_vres,&_res,_ctx);      \n\
      return _res;                                   \n\
    }                                                \n\
    convert(in,out,_cb);                             \n\
  }                                                  \n\

Unfortinately, I cannot define such macro for camlidl, as it does not allow
C-style string concatenation inside quote statement...

> > It would be nice if OCaml provided this functionality because not
> > all C libraries provide the extra argument for simulating closures.
> > Unfortunately it can't be implemented nicely.
> > 
> > I've once seen a C library which tries to provide it for several
> > platforms but I forgot its name.
> It would be nice if you recall it :-)

Hope to hear from you soon,

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