Interfacing Generics with C Pointers

Interfacing Generics with C Pointers

[Brian Palmer]

I've got a problem where I get in a pointer to a block of memory that was 
allocated in C++.  In addition to the pointer, I get the number of bytes 
that were allocated.  The Ada type that the memory comes in as is a pointer 
to an unconstrained byte array.  Whenever I try to dereference the pointer, 
I get a CONSTRAINT ERROR.  I think the problem is due to the compiler 
trying to access a storage location within the type that tells how many 
bytes are there.  I feel like I could constrain the base type and get 
around the problem, however, this mechanism is used for several different 
size blocks of memory, several hundred times per second, and I don't want 
to have to allocate more memory than necessary.  Any ideas how to access 
unconstrained memory without the compiler looking for that built in 
element?

Any help would be appreciated.

-Brian Palmer

Re: Interfacing Generics with C Pointers

[Robert A Duff]

Brian Palmer <bpalmer@elmco.com> writes:

> I've got a problem where I get in a pointer to a block of memory that was 
> allocated in C++.  In addition to the pointer, I get the number of bytes 
> that were allocated.  The Ada type that the memory comes in as is a pointer 
> to an unconstrained byte array.  Whenever I try to dereference the pointer, 
> I get a CONSTRAINT ERROR.  I think the problem is due to the compiler 
> trying to access a storage location within the type that tells how many 
> bytes are there.

Probably true.  In general, unconstrained arrays don't work well when
interfacing to C, because C doesn't know about the dope.

>...  I feel like I could constrain the base type and get 
> around the problem, however, this mechanism is used for several different 
> size blocks of memory, several hundred times per second, and I don't want 
> to have to allocate more memory than necessary.  Any ideas how to access 
> unconstrained memory without the compiler looking for that built in 
> element?

One way is to lie to the Ada compiler about the length of the array.
E.g., "type T is array(Natural) of ...;" then you get back a pointer to
T, and you can refer to X.all(I).  Just make sure I doesn't go out of
the "real" ounds.  Presumably the real bounds are available via some
other means.

As soon as possible, start dealing with the data in a safer way -- e.g.,
pass the appropriate slice of X.all off the to rest of the code.

- Bob