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From: "RasikaSrinivasan@gmail.com" <rasikasrinivasan@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Interfacing to C
Date: Thu, 24 Dec 2009 06:26:34 -0800 (PST)
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Date: 2009-12-24T06:26:34-08:00
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On Dec 24, 7:34 am, Niklas Holsti <niklas.hol...@tidorum.invalid>
wrote:
> 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 ....
>
> > While passing variables of this type to functions, the convention in C
> > is to pass by value (best of my knowledge).
>
> Correct, as I understand the C parameter-passing rules.
>
> > however when I import a function like:
>
> > type MyStruct is record ..... end record ;
> > pragma Convention(C,MyStruct);
>
> > procedure process(var : MyStruct);
> > pragma Import(C,process);
>
> > It appears that Ada passes the var by reference.
>
> Yes, by default Ada considers records/struct under convention C as
> pass-by-reference (LRM B.3 (69/2)). C programmers often choose to pass
> structs by reference (pointer to struct) and the Ada Convention (C)
> pragma has been defined accordingly. Or so I guess.
>
> > So the question is - how c(an I force Ada (gnat) to pass a record by
> > value?
>
> Use Pragma Convention (C_Pass_By_Copy, MyStruct). See LRM B.3 (60.14/2)
> and LRM B.3 (68.1/2).
>
> --
> Niklas Holsti
> Tidorum Ltd
> niklas holsti tidorum fi
>        .      @       .

Vadim and Niklas

Thank you so much, srini