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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: CRC in Ada?
Date: 1997/03/04
Message-ID: <dewar.857536653@merv>#1/1
X-Deja-AN: 223207911
References: <1997Mar2.220652@nova.wright.edu>
 	<331bf6ce.2832564@news.logica.co.uk> <oslo83b70l.fsf@ted.vigra.com>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-03-04T00:00:00+00:00
List-Id: <comp.lang.ada>


David Brown:

<<Well, I tried this.  I wrote a small program to read files and compute
CRC16's on them.  What I found was disheartening.  The CRC code didn't
take very long.  However, the Stream_IO read of the data was very
slow.  I traced it down to doing a call to the C read function for
each byte>>

  Are you using stream attributes to read into a string. This is a mistake
  that I have seen a number of people make. Why a mistake (if you are 
  worrying about efficiency?) Because Ada semantics require element by
  element processing of arrays (and for a string, the element is a
  character). 

  Instead use Read and Write directly to read and/or write buffers
  of stream elements.

<<I'm using GNAT 3.09 on Linux (pentium).  Is there anything about the
specification for Stream_IO (or Text_IO for that matter) that is
causing the GNAT runtime to turn off buffering.  This is a real
performance hit, and I have had to revert to calling the C routines
directly when streams would be adequate, if they just weren't so
inefficient.>>

  As for buffering, I am not sure what the default is, but you can use
  setvbuf to control buffering (read the section in the GNAT manual about
  getting at the underlying streams).