Re: Returning data from Ada to C

[Dave Thompson <david.thompson1@worldnet.att.net>]

On Wed, 04 May 2005 04:49:09 GMT, Jeffrey Carter <spam@spam.com>
wrote:

> igor.kh@gmail.com wrote:
> >   /* BTW, is it OK to use Ada allocated memory after `adafinal()'? */
> 
> No.
> 
> >   sols : C_Double_Array; -- This is probably wrong, but `sols' will be
> >                          -- a C_Double_Array of some size.
> 
> This won't compile. C_Double_Array is an unconstrained array type. 
> Objects must be constrained. You probably need
> 
> Sols : aliased C_Double_Array (<your constraint here);
> 
> > -- Now I want to return `rcnt' and `sols'
> >   rcnt := rc;                -- This is the easy part.
> >   solvec := ???              -- What goes here?
> 
> Solvec := Sols'Unchecked_Access;

Not if Sols is a procedure-local variable, as in the OP's code, and
the (C) caller intends to dereference the resulting pointer, as it
does. For the storage to remain valid for the caller to access, it
needs either to be (in a package) at library level, or dynamically
allocated in Ada by 'new' (and some provision e.g. another callable
Ada routine for deallocating it if necessary) or by hand-built Ada
call to C's malloc in which case the C code can safely free() it.

These considerations are the same if the called code were C instead of
Ada, except that C has file-scope static instead of package, and the
additional very slightly different option of procedure-local static.

- David.Thompson1 at worldnet.att.net