Re: Interfacing to C library...

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Eric G. Miller" <egm2@jps-nospam.net> writes:

> adaic has the new one, adahome has the old one...

Thank you.  I printed out the new version of the AARM (on a double-sided
laser printer!) for my own use a while back, but I had forgotten where I
got it.

> I'm not sure I understand your statement about using access parameters
> versus named access types (I'm still pretty green w.r.t. to Ada)....

Probably because I don't really understand what you're trying to do.

Anyway, I'll try to explain:

    type R is record....
    type A is access all R;

    procedure P(X: A; Y: access R);

A is a "named access type".
X is a "parameter of a named access type".
Y is an "access parameter", which is the same thing as
"a parameter of an anonymous access type".

There are various subtle differences between named and anonymous access
types, which I won't explain in detail.  One important difference is
that access parameters allow dispatching (assume R is tagged),
whereas parameter of a named access type do not.

The accessibility rules are different, which is what allows you to say
'Access in some cases with access parameters, where otherwise
'Unchecked_Access would be required.

> Aha! I missed that you couldn't mix "in/out" w/ "access" in a subprogram
> parameter list declaration/definition. Okay, definitely nicer than defining
> 40 access to record types...

You *can* mix 'in out' with 'access':

    procedure P(X: in out R; Y: access R);

is legal.  Perhaps what you're thinking of is the fact that functions do
not allow 'in out' parameters, so sometimes people use access parameters
instead, when they want a function to have side effects on its
parameters.  C functions do that all the time -- e.g., you pass a
"char *", and the function writes upon the thing pointed-to, and *also*
returns some sort of status code.

It seems to me that on the C side, in your Get/Set example, you have
struct parameters passed by copy, and pointer-to-struct parameters.  On
the Ada side, the former can correspond to an 'in' mode parameter of a
record type, and the latter can correspond to an access parameter,
or to a parameter of a named access type, or to a parameter of type
System.Address.

I really think the best advice in writing bindings is to look at some
examples.  Somebody suggested CLAW, which is a good example.  The people
who wrote it have much more experience in this area than I do.  If you
don't understand some detail, ask about it here.

- Bob