Interfacing Generics with C Pointers

[Brian Palmer <bpalmer@elmco.com>]

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