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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Access to array slices?
References: <5ad0dd8a.0301141717.2f1a9685@posting.google.com>
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Date: Wed, 15 Jan 2003 03:13:02 GMT
Xref: archiver1.google.com comp.lang.ada:33025
Date: 2003-01-15T03:13:02+00:00
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> What I was able to figure out so far is that a fat pointer is two
An Ada Unconstrained string has two pieces of information - the characters
and the length.  Your C strings are missing the second.  So you can either
copy them (up to nul) and put them into correct size Ada Strings, or you
can forget about String lengths and handle them as C would.

 type C_Char_Ptrs is access all Interfaces.C.Char;

 type C_String_Ptrs is access all Interfaces.C.Char_Array(Interfaces.C.Size_T);
 -- a pointer to a fixed size (albeit huge) char_array

 function Convert is new Ada.Unchecked_Conversion
   (Source=>C_Char_Ptrs, Target=>C_String_Ptrs);

 Big_Collection_Of_C_Strings : Interfaces.C.Char_Array(...)
 Chopped : array(1 .. n) of C_String_Ptrs;
 Count : Natural := 0;
 si : Interfaces.C.Size_T := Big_Collection_Of_C_Strings'first;
begin
 loop
   Count := Count+1;
   Chopped(Count) := Convert(Big_Collection_Of_C_String(si'unchecked_access));
   -- now scan si up to the start of the next logical string
   -- exit if no more
 end loop;
Of course Ada will think that Chopped(i).all is an extremely long string,
so it's up to you to keep your indexes in range.  If you call your
processing routines with slices
  process(Chopped(i)(0 .. size(i)));
then the processing routines will believe there are index range limits.
If you can process everything sequentially, you can skip most of the
above and just
  loop
    first := determine first index of this part
    last := determine last index of this part
    process(Big_Collection_Of_C_Strings(first .. last));
  end loop;