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From: "Alejandro R. Mosteo" <alejandro@mosteo.com>
Newsgroups: comp.lang.ada
Subject: Re: Interfaces.C.Strings chars_ptr memory management strategy
Date: Wed, 30 May 2018 15:10:23 +0200
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Date: 2018-05-30T15:10:23+02:00
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On 26/05/2018 00:22, NiGHTS wrote:
> I am creating a binding to a C library that requires me to repeat the same function but with a string parameter that changes on each call. I don't want to have to keep creating and destroying string memory for all of these function calls. I would like to create the memory for the string once, allocate enough space so it doesn't need to grow, and reuse that memory for the function call every time I need to pass a new string to it.

I'm currently using this:

https://github.com/mosteo/cstrings

which is not what you want since it allocates on every instance (although 
on the stack). Still, it might give you some ideas.

I'm still unsure if that's 100% guaranteed to be safe; for the experts 
out here, the question is, in a call to a C function like this:

Call_To_C_Function
    (Function_That_Returns_A_Limited_Tagged_Type (...)
       .Subprogram_That_Returns_A_C_Pointer_To_Data_In_The_Tagged_Type);

Is the in-place built limited tagged type guaranteed to live during the 
call to the C function? (In other words, is the pointer safe (as long as 
the C side does not make a copy, of course)?

My suspicion is that once the subprogram returns the pointer, the limited 
type can be optimized away before the call to the C side. It's not what 
I'm seeing now, but I don't want to depend on an erroneous assumption.

Alex.

> 
> The trick here is that whatever strategy I use must be compatible with C, so for instance using a storage pool would not be directly compatible with the C binding.
> 
> Here is just a quick and sloppy idea I had on how to tackle my problem.
> 
> str : chars_ptr := New_String ("                                     ");
> ...
> Update (Item => str, Offset => 0, Str => "Some Param");
> ...
> Update (Item => str, Offset => 0, Str => "Some Other Param");
> ...
> Free (str);
> 
> I find the first line quite ugly. I'm sure there is an easier way to create a large empty string but I can't seem to come up with an elegant way to do it.
> 
> As far as the Update commands, will it act like strcpy() in C? If so I'd guess that this is an efficient technique.
> 
> Thanks for your help!
>