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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Interfacing to C
Date: Thu, 31 Dec 2009 18:18:34 -0500
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Date: 2009-12-31T18:18:34-05:00
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Keith Thompson <kst-u@mib.org> writes:

> "Hibou57 (Yannick Duch�ne)" <yannick_duchene@yahoo.fr> writes:
>> On 24 d�c, 10:46, "RasikaSriniva...@gmail.com"
>> <rasikasriniva...@gmail.com> wrote:
>>> I am building an interface to a C library. This library uses structs
>>> like the following:
>>>
>>> typedef struct {
>>> � � x : int ;
>>> � � y : int } �MyStruct ....
>
> (The correct syntax is:
>     typedef struct {
>         int x;
>         int y;
>     } MyStruct;
> )

Right.

>>> While passing variables of this type to functions, the convention in C
>>> is to pass by value (best of my knowledge).
>
> Yes.  In fact, C doesn't have pass by reference as a language feature;
> *all* function arguments are passed by value.
>
> For structs, it's very common in C to do the equivalent of
> pass-by-reference, by explicitly passing the address of the struct
> object.  This is partly because very old versions of C didn't
> support struct parameters.

Which version of C (when?) introduced (by-copy) struct params?

> I find it a bit odd that Ada imposes a pass-by-pointer convention for
> C structs.

It was a mistake.  Partly my fault.  I was thinking that it's common in
C to pass struct params by explicitly passing a pointer-to-struct, so
Ada should mimic that.  Bad idea.  By the time we realized the mistake,
it was too late to fix (compatibility!), so we invented C_Pass_By_Copy
as a workaround.

> (No, C arrays aren't passed by reference.  C array expressions, in
> most contexts, are implicitly converted to pointers; the resulting
> pointer can be passed by value.)

Yeah.  In other words, C arrays aren't passed, period.
That's another bad idea, and it's not my fault.  ;-)

- Bob