From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!mx02.eternal-september.org!feeder.eternal-september.org!gandalf.srv.welterde.de!news.jacob-sparre.dk!loke.jacob-sparre.dk!pnx.dk!.POSTED!not-for-mail
From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Truncating Direct IO files
Date: Tue, 21 Apr 2015 17:01:58 -0500
Organization: Jacob Sparre Andersen Research & Innovation
Message-ID: <mh6hcn$e3h$1@loke.gir.dk>
References: <8323246f-eb13-49e8-9e0c-5c5ec1002060@googlegroups.com>
NNTP-Posting-Host: rrsoftware.com
X-Trace: loke.gir.dk 1429653719 14449 24.196.82.226 (21 Apr 2015 22:01:59 GMT)
X-Complaints-To: news@jacob-sparre.dk
NNTP-Posting-Date: Tue, 21 Apr 2015 22:01:59 +0000 (UTC)
X-Priority: 3
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 6.00.2900.5931
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.6157
X-RFC2646: Format=Flowed; Original
Xref: news.eternal-september.org comp.lang.ada:25570
Date: 2015-04-21T17:01:58-05:00
List-Id: <comp.lang.ada>

"tonyg" <tonythegair@gmail.com> wrote in message 
news:8323246f-eb13-49e8-9e0c-5c5ec1002060@googlegroups.com...
>
>
> I am recording records to disk. Sometimes I replace thes records with a 
> smaller set of records.
>    I could delete the file each time but would prefer to truncate the 
> file. I can't see a way of doing this, can you?

Ada doesn't provide that, mainly because not all popular operating systems 
provide a truncation mechanism for files, and we don't want implementations 
to have to fall back on the very expensive copy the whole file and replace 
mechanism (obviously, if that is acceptable to you, you can write that 
yourself).

It appears that Ada 95 stream files provide truncation on Close and Reset 
(and thus some implementations do implement that, but all surveyed 
implementations did it wrong), but many implementations don't truncate and 
Ada 2005 and later removed the wording that appeared to require it. So one 
cannot depend on that.

                                           Randy.