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From: guy@sun.uucp (Guy Harris)
Newsgroups: net.unix-wizards
Subject: Re: truncating a file opened via open()
Message-ID: <2315@sun.uucp>
Date: Sun, 16-Jun-85 23:39:31 EDT
Article-I.D.: sun.2315
Posted: Sun Jun 16 23:39:31 1985
Date-Received: Wed, 19-Jun-85 03:37:15 EDT
References: <340@cmu-cs-edu1.ARPA> <786@turtlevax.UUCP> <217@luke.UUCP>
Organization: Sun Microsystems, Inc.
Lines: 33

> In article <786@turtlevax.UUCP> ken@turtlevax.UUCP (Ken Turkowski) writes:
> >In article <340@cmu-cs-edu1.ARPA> hua@cmu-cs-edu1.ARPA (Ernest Hua) writes:
> >>Does anyone have any idea how to truncate a file at the current point in
> >>writing if it is opened by open()?
> >
> >size = lseek(fd, 0L, 1);	/* tell(fd) */
> >ftruncate(fd, size);		/* int fd, size; */
> >
> >It's yet another undocumented feature on 4.2.

Undocumented?  Hardly.  See TRUNCATE(2) in the UNIX Programmer's Manual.

> OK, how about System III?  That's my problem.  Someone out there posted
> a program that uses ftruncate, and we don't have it.  Solutions are
> welcome!

There are precisely two solutions for non-4.2BSD systems:

1) implement (f)truncate on your system.  Any implementation on a non-4.2BSD
version of UNIX will probably move to any other non-4.2BSD version with
little difficulty.

2) Copy the entire file, up to the point of truncation, to a temporary file,
then copy it back to the original file.

The "f77" support library uses solution 2), except under 4.2BSD where it
uses solution 1); I believe "truncate" was introduced for the benefit of
"f77".

A comparision of solutions 1) and 2) may indicate why the Berkeley people
added "truncate".  Solution 2) is slow and ugly.

	Guy Harris