Allocating a C string without heap

Allocating a C string without heap

[Victor Porton]

Let we have an Ada String.

I need to pass it converted to a C string into a C library function.

Now I do it with Interfaces.C.Strings.New_String. But it is slow as it uses 
the heap and requires (not to forget incidentally!) further 
Interfaces.C.Strings.Free.

Is there a better way to do this? I mean that we would probably create a 
nul-terminated char_array (not on the heap but on the stack!) and pass the 
pointer to its first element.

This would be both safer an faster.

Can you elaborate on this?

-- 
Victor Porton - http://portonvictor.org

Re: Allocating a C string without heap

[Dmitry A. Kazakov]

On Sat, 26 Jul 2014 19:01:20 +0300, Victor Porton wrote:

> Let we have an Ada String.
> 
> I need to pass it converted to a C string into a C library function.
> 
> Now I do it with Interfaces.C.Strings.New_String. But it is slow as it uses 
> the heap and requires (not to forget incidentally!) further 
> Interfaces.C.Strings.Free.
> 
> Is there a better way to do this? I mean that we would probably create a 
> nul-terminated char_array (not on the heap but on the stack!) and pass the 
> pointer to its first element.

The canonical method is this (Ada 95):

   procedure Foo (Text : String) is
      procedure Internal (Text : char_array);
      pragma Convention (C, Internal, "foo");
   begin
      Internal (To_C (Text));
   end Foo;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: Allocating a C string without heap

[Victor Porton]

Dmitry A. Kazakov wrote:

> On Sat, 26 Jul 2014 19:01:20 +0300, Victor Porton wrote:
> 
>> Let we have an Ada String.
>> 
>> I need to pass it converted to a C string into a C library function.
>> 
>> Now I do it with Interfaces.C.Strings.New_String. But it is slow as it
>> uses the heap and requires (not to forget incidentally!) further
>> Interfaces.C.Strings.Free.
>> 
>> Is there a better way to do this? I mean that we would probably create a
>> nul-terminated char_array (not on the heap but on the stack!) and pass
>> the pointer to its first element.
> 
> The canonical method is this (Ada 95):
> 
>    procedure Foo (Text : String) is
>       procedure Internal (Text : char_array);
>       pragma Convention (C, Internal, "foo");
>    begin
>       Internal (To_C (Text));
>    end Foo;

Why Internal (Text : char_array); not Internal (Text : char_array_access);?

-- 
Victor Porton - http://portonvictor.org

Re: Allocating a C string without heap

[Dmitry A. Kazakov]

On Sat, 26 Jul 2014 19:30:16 +0300, Victor Porton wrote:

> Dmitry A. Kazakov wrote:
> 
>> On Sat, 26 Jul 2014 19:01:20 +0300, Victor Porton wrote:
>> 
>>> Let we have an Ada String.
>>> 
>>> I need to pass it converted to a C string into a C library function.
>>> 
>>> Now I do it with Interfaces.C.Strings.New_String. But it is slow as it
>>> uses the heap and requires (not to forget incidentally!) further
>>> Interfaces.C.Strings.Free.
>>> 
>>> Is there a better way to do this? I mean that we would probably create a
>>> nul-terminated char_array (not on the heap but on the stack!) and pass
>>> the pointer to its first element.
>> 
>> The canonical method is this (Ada 95):
>> 
>>    procedure Foo (Text : String) is
>>       procedure Internal (Text : char_array);
>>       pragma Convention (C, Internal, "foo");
>>    begin
>>       Internal (To_C (Text));
>>    end Foo;
> 
> Why Internal (Text : char_array); not Internal (Text : char_array_access);?

char_array_access is an Ada side type. You should not pass it to C.

If you want to pass an Ada-managed array down to C it is char_array (Ada
knows how to pass arrays and record types). If you want to pass a C-managed
array it is chars_ptr.

There is no safe way to know which side manages the array. C does not make
any distinction because it is always a pointer in C. Each time you should
read the documentation carefully.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de