RM B.3.1 (1) may have something important to say by
what it does _not_ say, viz. :

The package Interfaces.C.Strings declares types and
subprograms allowing an Ada program to allocate,
reference, update, and free C-style strings. In
particular, the private type chars_ptr corresponds
to a common use of �char *� in C programs, and an
object of this type can be passed to a subprogram to
which pragma Import(C,...) has been applied, and for
which �char *� is the type of the argument of the C
function.

What is does _not_ say is that an object of this
type can be passed to a  subprogram to which pragma
Export(C,...) has been applied, and for which
�char *� is the type of the argument of the C
function.

So... the question is.. if one wishes to have a
procedure with a profile of, say:

procedure My_Proc (Result : out Interfaces.C.Strings.Chars_Ptr);

is it legal to export it like so:

pragma Export (C, My_Proc, "My_Proc");

where the C prototype is void My_Proc (char * result);

The results I'm having are that if the exported procedure
is called from Ada, everything works as expected.

If called from C, a Segmentation fault occurs at the
point where the out mode parameter is assigned its
value in the Ada procedure My_Proc.

What say ye?