External, C, AsmName, ...

Sat, 7 Sep 1996 15:00:07 +0200 (MET DST)

Hello folks,

  (* This mail is somehow long.  In short: I want to take
  the directives `Extern', `C', and `AsmName' *out* of GPC
  and extend the syntax of `External' instead.  Okay? :*)

as you know (?;-), we have the directives `External', `Extern'
(same as `External'), `C', and `AsmName' in GPC for specifying 
external procedures.  This is especially useful when using 
libraries written in C, for example:

    Procedure Foo; External;  (* yields "Foo" *)

    Procedure ioCtl ( FilDes, Cmd: Integer; ... ); C;  (* yields "ioctl" *)

    Procedure XOpenDisplay ( Name: __CString__ ); 
    AsmName 'XOpenDisplay';

This turned out not to be enough; we need the same for variables.
Actually we already have

    Var
      Foo: __external__ Integer;   (* yields "foo", not "Foo"! *)

but this does not cover cases where the external variable needs a
name with a capital letter.

As an intermediate solution, I implemented

    Var
      Foo: __asmname__ 'foO' Integer;

(patched source is available on demand), but I think it is not the best 
way to implement these things.

Now I am looking out for reasonable ideas.

Does somebody know what the ISO/ANSI Standards say about this?

My own thoughts follow.

I would suggest to define as few directives as possible and to
drop everything but `External' but extend its syntax to meet our
requirements.  A first approach could be

    Procedure foo; external;  (* yields "Foo" *)

    Var
      x: Integer; external;  (* yields "X" *)

and

    Procedure foo; external 'foO';

    Var
      x: Integer; external 'my_x';

but it would crash with Borland Pascal's use of `External' where
the latter declaration would mean that Procedure foo is contained
in a shared library named `foO'.

Okay, so let's stay for the moment at Borland's syntax.  In the
same context they also define

    Procedure foo; external 'MySharedLib' index 42;

and

    Procedure foo; external 'MySharedLib' name 'foO';

where `foO' would be the external name as defined in the shared
library.

My current idea now would be to define

    Procedure foo; external;  (* yields "Foo" *)

    Var
      x: Integer; external;  (* yields "X" *)

plus

    Procedure foo; external name 'foO';

    Var
      x: Integer; external name 'my_x';

to replace "AsmName 'foO'".  Later we can extend this to allow a
name of a shared library between `external' and `name'.

Once doing this, it could also be nice to make directives out of
the other C-style storage qualifiers, too.  I mean I'd like to
replace

    Var
      x: __static__ Integer;        by       x: Integer; static;
      y: __volatile__ Integer;      by       y: Integer; volatile;

etc..  My plan is to do it in a way that things like

    Var
      static: Integer;

would still work.

I think it would *not* be a good idea to make

    Procedure foO; external;

yield "foO", because (i) it would be difficult to implement and 
(ii) Pascal is a case-insensitive language, so everything outside
string constants *must not* depend on casing at all.

Please tell me your opinion about this.

Yours,

    Peter

      e-mail:  peter.gerwinski@uni-essen.de
home address:  D\"usseldorfer Str. 35, 45145 Essen, Germany
         WWW:  http://agnes.dida.physik.uni-essen.de/~peter

Peter Gerwinski (peter@agnes.dida.physik.uni-essen.de)