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Date: 4 May 93 19:39:26 GMT
From: cis.ohio-state.edu!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!m
agnus.acs.ohio-state.edu!usenet.ins.cwru.edu!cert.org!news.sei.cmu.edu!ajpo.sei
.cmu.edu!progers@ucbvax.Berkeley.EDU  (Pat Rogers)
Subject: Re: Passing procedures as parameters to procedures.
Message-ID: <1993May4.153926.11547@sei.cmu.edu>
List-Id: <comp.lang.ada>

Well, it isn't that you cannot do it at all, it just isn't portable (which
it should be, of course, thus 9X), and restrictions do exist.  In particular,
the code that follows only works (on my compiler) for library unit level
parameterless procedures.

Certainly, on some implementations this approach won't work at all, due to 
restrictions regarding 'Address for subprograms (at least one always returns
"address zero").  These restrictions are not universal by any means. 

The following is for Systems Designers 680X0 compilers:

 

with system;
package call_backs is
  subtype parameterless_procedure is system.address;
  procedure sign_in( caller : in parameterless_procedure );
  procedure make_calls;
end call_backs;


with call_by_address;
package body call_backs is

  callers : array(1..255) of parameterless_procedure;

  next : integer range 1 .. 255 := 1;

  procedure sign_in( caller : in parameterless_procedure ) is
  begin
    callers(next) := caller;
    next := next + 1;
  end sign_in;

  procedure make_calls is
  begin
    for K in 1 .. Next-1 loop
      call_by_address( callers(K) );
    end loop;
  end make_calls;

end call_backs;


All well and good, you say, but what is this "call_by_address" procedure?
It uses package Machine_Code to do a JSR to the desired routine, and uses
an implementation-defined mechanism (the pragma) to associate the parameter
(callee) with the register (A0) used in the JSR.  Other implementations 
would use a different format for the JSR and for the parameter association 
mechanism...


with System;
with Machine_Code; 

procedure call_by_address( callee : in system.address ) is
  use Machine_Code;
begin
  EA_INST'( Mode => A0_Indirect, Opcode => JSR );
end call_by_address;

pragma Inline( Call_By_Address );

pragma Call_Sequence_Procedure( unit => call_by_address,
                                parameter_types => (system.address),
                                mechanism => ( Value (A0) )
                              );