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From: "Jeffrey R. Carter" <jrcarter@acm.org>
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Subject: Re: Interfacing with C - pointer problem
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Date: 2007-01-31T23:53:27+00:00
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Adam Beneschan wrote:
>>    type LPSTR is access Interfaces.C.Char;
>>    procedure C_Func (x : LPSTR);  -- expects an address (C-pointer, 32-bit)
>> and then
>>    data : Interfaces.C.char_array := Interfaces.C.To_C("ABC");
>> and
>>    C_Func (data(data'first)'unchecked_access);
> 
> Or perhaps better:
> 
>    procedure C_Func (x : Interfaces.C.Strings.chars_ptr);
> 
>    data : aliased Interfaces.C.char_array := Interfaces.C.To_C
> ("ABC");
> 
>    C_Func (Interfaces.C.Strings.To_Chars_Ptr (data'unchecked_access));
> 
> May as well use the stuff Ada provides for this exact purpose.

Right. Therefore, even better:

procedure C_Func (X : in Interfaces.C.Char_Array);
pragma Import (C, C_Func, ...);

Data : Interfaces.C.Char_Array := Interfaces.C.To_C ("ABC);

C_Func (X => Data);

The language passes this a a pointer to Data (Data'First). See ARM B.3 
(70): "An Ada parameter of an array type with component type T, of any 
mode, is passed as a t* argument to a C function, where t is the C type 
corresponding to the Ada type T."

-- 
Jeff Carter
"Alms for an ex-leper!"
Monty Python's Life of Brian
75