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Date: Thu, 24 Dec 2009 14:34:17 +0200
From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Organization: Tidorum Ltd
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Newsgroups: comp.lang.ada
Subject: Re: Interfacing to C
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Date: 2009-12-24T14:34:17+02:00
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RasikaSrinivasan@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
       .      @       .