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From: defaria@hpclapd.HP.COM (Andy DeFaria)
Newsgroups: comp.lang.ada
Subject: Re: C Strings in Ada?
Message-ID: <920025@hpclapd.HP.COM>
Date: 8 Jun 90 16:00:22 GMT
References: <920024@hpclapd.HP.COM>
Organization: Hewlett-Packard Calif. Language Lab
List-Id: <comp.lang.ada>

>/ hpclapd:comp.lang.ada / emery@D74SUN.MITRE.ORG (David Emery) / 10:21 am  Jun  6, 1990 /
>>What is the best way to represent C_Strings in Ada?  
>
>The right solution is to keep all strings in the Ada program as Ada
>strings, and change them to/from C strings at the point of interface
>to C.  (i.e.  within the body of Ada_routine_calling_c).  This
>preserves the Ada abstractions, and makes all the Ada things work just
>fine (e.g. 'IMAGE, which returns an Ada string).  The only other
>alternative (that makes sense to me) is to make C_STRING a private
>type derived from System.address, but you won't be able to do anything
>useful in the debugger with this.
>
>How you do this conversion is very machine dependent, but here's my
>tricks that work on Verdix (Sun 3) and Meridian (IBM PC):
>
>	procedure to_c (str : string) 
>	is
>	  procedure c_routine (addr : system.address);
>          pragma interface (C, c_routine);	-- (void) c_routine(s)
>						-- char * s;
>	  c_string : constant string := str & Ascii.NUL;
>	begin
>	  c_routine(c_string(c_string'first)'address);
>	end to_c;
>
>	function from_c  
>	    return string
>	is
>	    function c_function			
>		return System.address;		
>	    pragma interface (C, c_function);	-- char * c_function
>	    function strlen (addr : System.address)
>		return integer;
>	    pragma interface (C, strlen);	-- the C function of
>						-- the same name
>	    addr_from_c : System.address;
>	 begin
>	    addr_from_c := c_function;
>	    declare
>	      answer : string(1..strlen(addr_from_c);
>	      for answer'address use at addr_from_c;
>	    begin
>	      return answer;
>	    end;
>  	 end from_c;
>
>Hopefully what's going here is obvious, if not let me know and I'll
>send more information.
>
>				dave emery
>				emery@aries.mitre.org
>----------

I kinda agree with you that the "right" way to do this would be to keep Ada
strings as  Ada strings and do the  conversion  in package but you have not
addressed one of my critical concerns: What about  the overhead incurred by
having  to call the  heap manager to allocate a  new string just  to append
ASCII.NUL so  that the C routine I'm   calling  will be  able to handle  it
properly.   I anticipate  that this overhead will have  an severe impact on
performance and Ada already has a bad rap for being a dog.  I don't want to
add to that reputation.

Another, problem that you haven't addressed  is that  of the representation
of a string in a record.  Should it be  and  Ada access to string or should
it be the string itself.  Or should it be a C string (char *)?