Re: Large strings in ADA

[tmoran@bix.com]

> >      Msg : String(1 .. 2_500_000);  -- 2.5 MB
>
> The above is the current solution but it doesn't work as it uses to much
> memory.
  It seems to me 2.5MB is the smallest possible amount of memory
needed to hold 2.5MB of bytes.

> The C-program reallocates the buffer only if the string added to
> it exceeds its current size and if it reallocates it grows with a power
> of 2. So if the buffer was 8 bytes big and you added 9 bytes the buffer
> would become 32 bytes big.
  So when the buffer is 2,097,152 (2^21) bytes long, and you add
a string on your way to 2,500,000, the C program will for a moment
have a 2^21 old buffer allocated, plus a 2^22 new buffer, for a
total memory requirement of 6,291,456 bytes as the high point.  So
the C approach must be able to allocate 2.5 *times* as much memory
as the simple Ada declaration.  Why?  If it's because your system
allows a large heap, but only a small (less than 2.5MB) area for
a Msg : String(1 .. 2_500_000); declaration, then how about
  type Msgs is array(1 .. 2_500_000) of aliased Character;
  type Ptr_To_Msgs is access all Character;
  P_Msg : Ptr_To_Msgs := new Msgs;
and then use the buffer at P_Msg instead of Msg.
  Note that the "double the size and reallocate" technique requires
copying the old buffer each time.  It will copy nearly 2^22=4M of
old bytes, in addition to the 2.5Mb of new bytes.  Even on a fast
machine, that's a chunk.