[Caml-list] c-interface question...

[Caml-list] c-interface question...

[Michael Wohlwend]

Hi,

to play with the c-interface I tried to change the digest module to allow 
calculating a md5 sum piece by piece (i.e. making the function init, 
updtae and finalize avalable).
one question about the parameters of

external md5_new : unit -> md5_context = "md5_new"

should it be:

CAMLprim value md5_new()
{
CAMLparam0();
...
CAMLreturn(...);
}

or
CAMLprim value md5_new(value unit)
{
CAMLparam1(unit);
...
CAMLreturn(...);
}


as it receives a unit-paramter?
(the same for returning unit: CAMLreturn0 or CAMLreturn(Val_unit) ? )

thanks for answering,
  Michael




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Re: [Caml-list] c-interface question...

[David Baelde]

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 I'm not an expert, but I can quote the tutorial.
 Here is their code for a unit -> window bound function.

value curses_initscr(value unit)
{
  CAMLparam1 (unit);
  CAMLreturn ((value) initscr()); 
}

 That's why I use CAMLprim f (value unit) { CAMLreturn(Val_unit) }

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Re: [Caml-list] c-interface question...

[David Brown]

On Thu, Sep 18, 2003 at 10:31:20AM +0200, Michael Wohlwend wrote:

> external md5_new : unit -> md5_context = "md5_new"
> 
> should it be:
> 
> CAMLparam1(unit);

> as it receives a unit-paramter?

It won't hurt anything to do this, other than performance.  The only
arguments that need to be declared in a CAMLparamx declaration are those
that can be moved or collected by the garbage collector.  Specifically,
integer values do not need to be declared.

There is also no need to CAMLparam an argument that you do not use, even
if it is a heap value.

An added "optimization" is that if an argument is only used before the
first allocation you call, it also doesn't need to be declared.

It helps to think about what is happening here.  Things that allocate
memory may trigger a garbage collection.  If you have arguments or
variables that might be affected by the garbage collector, you need to
tell the gc about them.  That is what CAMLparamx and CAMLlocal do.

A function that only allocates one thing, and doesn't reference its
variables after the allocation doesn't need to use any of these macros.

Dave Brown

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Re: [Caml-list] c-interface question...

[Richard Jones]

On Thu, Sep 18, 2003 at 11:12:15AM -0700, David Brown wrote:
> An added "optimization" is that if an argument is only used before the
> first allocation you call, it also doesn't need to be declared.
> 
> It helps to think about what is happening here.  Things that allocate
> memory may trigger a garbage collection.  If you have arguments or
> variables that might be affected by the garbage collector, you need to
> tell the gc about them.  That is what CAMLparamx and CAMLlocal do.

Interesting ... so the garbage collector can never run asynchronously?
That should simplify my code ...

Rich.

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