Re: C Strings in Ada?

[jb@rti.rti.org (Jeff Bartlett)]

In article <920025@hpclapd.HP.COM>, defaria@hpclapd.HP.COM (Andy DeFaria) writes:
> >/ 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?  
> >
> >	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;

> ... 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. ...

A good compiler should grow the stack by (str'length + 1 + string_descriptor)
then copy 'str' and the NUL into the area.  It would also fill in the
new string_descriptor for 'c_string' from the information in 'str's descriptor
and the additional byte.  (A smart compiler may not even allocate space for
the new descriptor, keeping the information in registers).

Constant strings are a good 'trick'. Such as:

    ...
	str : string(1..10);
	N   : natural;
    begin
	...

	-- a subroutineless call like above
	declare
           s : constant string := str & ascii.nul;
        begin
           c_routine( s(s'first)'address );
        end;

	-- remove the leading space supplied by 'image.
	declare
	   s : constant string := integer'image(N);
        begin
           text_io.put_line("Activity on PE" & s(s'first+1..s'last) );
        end;

	-- figuring out how much of a buffer to overwrite
        declare
           s : constant string := function_that_returns_a_string(j);
        begin
           line(line'first..line'first+s'length-1) := s;  -- result used twice
        end;

> 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 *)?

It depends on what operations need to be performed on it.  I'll leave this for
someone else.  Don't forget that you can:

	type s_ptr is access string;
        p : s_ptr;
    begin
        ...
	-- this will call the function, allocate space for the result, and
        -- initialize the area with the result.
        p := new string'( ada_function_that_returns_a_string );
        ...
	text_io.put_line("answer was '" & p.all & "'");


If you are paranoid about the Ada run-time heap manager forgetting to free
the space pointed to by 'p', you can instantate a new UNCHECKED_DEALLOCATION.

Hope this helps,

Jeff Bartlett
Research Engineer, Center for Digital Systems Research
Research Triangle Institute, RTP NC.
jb@rti.rti.org     mcnc!rti!jb     rti!jb@mcnc.org     (919)-541-6945

PS: The code above was only compiled with a chemical computer. ;-)