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[Caml-list] CamlIDL documentation and COM issues
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Date: 2002-06-04 (17:25)
From: Dmitry Bely <dbely@m...>
Subject: Re: [Caml-list] CamlIDL documentation and COM issues
Florian Hars <florian@hars.de> writes:

> I am trying to write an interface to an existing C library and I
> intend to use CamlIDL for this task, but I am missing some more
> HOWTO-like documentation.
> The library uses a pattern that should be common enough to supported
> in CamlIDL (and explained in the hints on writing IDL files): there is
> a struct that is only of internal relavance, and there is an open and
> a close function that returns or frees a pointer to such a struct, and
> several functions that operate on such a handle:
> typedef struct { /* things */ } Foo;
> typedef Foo * FooHandle;
> FooHandle FooOpen(args...);
> int FooOperate(FooHandle fhdl, args...);
> void FooClose(FooHandle fhdl);
> Now all I need is a way to put this handle into a Caml variable (and,
> optionally, to call FooClose automatically whenever the handle runs
> out of scope). It looks like the [abstract] and [ptr] attributes,
> either alone or in combination, might be relevant for this task. But
> [ptr] requires the Com module, which I do not want to use. (OK, it is
> tiny under unix, but still...

It's easy:

quote(c,"#include <foo-native-definition.h>");
typedef [abstract,finalize(FooClose)] void* FooHandle;
FooHandle FooOpen(args...);
int FooOperate(FooHandle fhdl, args...);
void FooClose(FooHandle fhdl);

$camlidl -no-include interface.idl

FooClose() will be called automatically when Foo instance is

> what happens if I ever recompile the
> program under windoze? Shouldn't the generally useful features for
> interfacing to C/C++ and the COM-specific stuff be somehow separated,
> like in a module Idl and another module Com? This would certainly help
> to reduce the prevailing confusion over the possible uses of
> CamlIDL. Or is it irrelevant for my problem?)

Com module works under unix and windows (under Unix some COM stuff is
emulated), so there is no such problem.

> An example for the use of errorcheck(fn) and a remark on the type of
> fn in errorcheck might be helpful, too.

If you write IDL definition

typedef [errorcheck(checkFoo)] something Foo;

then checkFoo() should have C prototype

void check(Foo);

> Oh, and shouldn't the example on page 24 of the Manual (1.0.4) read
>   double time() quote(call, "_res=....; ");
>                       ^^^^^^
> Or is "call" implicit if no ident is given in a quote in a function
> definition?

Yes, "call" can be ommited:

        { ("call", $4) :: $1 }
        { ($4, $6) :: $1 }
  | /* empty */
        { [] }

But you are right, this doesn't seem to be documented in the manual. Ask
Xavier Leroy for the correction.

Hope to hear from you soon,

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