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Date: Mon, 13 Sep 2021 17:48:15 -0700 (PDT)
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Subject: Re: Custom Storage Pool questions
From: Jere <jhb.chat@gmail.com>
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On Monday, September 13, 2021 at 7:12:43 AM UTC-4, J-P. Rosen wrote:
> Le 13/09/2021 =C3=A0 02:53, Jere a =C3=A9crit :=20
> > I was learning about making user defined storage pools when=20
> > I came across an article that made me pause and wonder how=20
> > portable storage pools actually can be. In particular, I assumed=20
> > that the Size_In_Storage_Elements parameter in the Allocate=20
> > operation actually indicated the total number of storage elements=20
> > needed.=20
> >=20
> > procedure Allocate(=20
> > Pool : in out Root_Storage_Pool;=20
> > Storage_Address : out Address;=20
> > Size_In_Storage_Elements : in Storage_Elements.Storage_Count;=20
> > Alignment : in Storage_Elements.Storage_Count) is abstract;=20
> >=20
> > But after reading the following AdaCore article, my assumption is now=
=20
> > called into question:=20
> > https://blog.adacore.com/header-storage-pools=20
> >=20
> > In particular, the blog there advocates for separately counting for=20
> > things like unconstrained array First/Last indices or the Prev/Next=20
> > pointers used for Controlled objects. Normally I would have assumed=20
> > that the Size_In_Storage_Elements parameter in Allocate would account=
=20
> > for that, but the blog clearly shows that it doesn't
> [...]=20
>=20
> That blog shows a special use for Storage_Pools, where you allocate=20
> /user/ data on top of the requested memory. When called by the compiler,=
=20
> it is up to the compiler to compute how much memory is needed, and your=
=20
> duty is to just allocate that.=20
>=20
Yes, but if you look at that blog, they are allocating space for the /user/=
 data=20
and for the Next/Prev for controlled types and First/Last for unconstrained
arrays in addition to the size specified by allocate. =20

I agree I feel it is up to the compiler to provide the correct size to Allo=
cate,=20
but the blog would indicate that GNAT does not (or did not..old blog..so=20
who knows?).  Does the RM require that an implementation pass the full
amount of memory needed to Allocate when new is called? =20