Re: GNAT, OS/2, Libraries

[tojst1+@pitt.edu (Tore Joergensen)]

Robert Morphis (morphis@fnalv1.fnal.gov) wrote:
: sigh, I ask a question and then get more confused.  I can guess what a
: binding is but a "thin binding"?  (why do I feel like I am way over my
: head? :)

An example:
   -- This function draws a section of the segment chain.

   function GpiDrawFrom(
      a_hps         : HPS;      -- Presentation-space handle
      lFirstSegment : LONG;     -- First segment to be drawn; it must be
                                -- greater than 0
      lLastSegment  : LONG)     -- Last segment to be drawn; it must be
                                -- greater than 0
   return
      BOOL;                     -- Success indicator (rc)
   pragma Import(
      Convention => C,
      Entity     => GpiDrawFrom,
      Link_Name  => "_GpiDrawFrom");

This was a thin binding. It gives minimum overhead (I'm not sure if it will
generate more code than the same call from a C-program or not), and doesn't
add any protection. A thick binding would probably change it into a 
procedure, and raise an exception if the C-call return 0 (false). It would 
use an Ada-type that protected against values of 0 or lower for the
parameters, or maybe change the parameters completely if that was more
appropriate. One call in a thick binding may result in several calls to
the thin binding. A thick binding may use a controlled type for something,
and use the thin binding's create and delete calls as Initialize and
Finalize. A thick binding involves code, while a thin binding just give you
a name and parameter-list so you can call it from within an Ada-program.
To put it simple, a thin binding gives you Ada functions and procedures that
feels like C-functions (if it is a C-library). A thick binding will give you
Ada look and feel :-).
-- 
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| Tore B. Joergensen      | e-mail : tore@lis.pitt.edu                |
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| Pgh, PA 15232, USA      | Norwegian MSIS-student at Univ. of Pgh.   |
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