Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!rutgers!husc6!wjh12!maynard!campbell
From: campbell@maynard.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: Truncating MS-DOS files
Message-ID: <834@maynard.BSW.COM>
Date: Thu, 19-Feb-87 01:01:39 EST
Article-I.D.: maynard.834
Posted: Thu Feb 19 01:01:39 1987
Date-Received: Fri, 20-Feb-87 07:26:06 EST
References: <1987Feb15.123004.21019@gpu.utcs.toronto.edu> <2596@well.UUCP> <3105@gitpyr.gatech.EDU>
Reply-To: campbell@maynard.UUCP (Larry Campbell)
Organization: The Boston Software Works, Inc.
Lines: 27
Summary: RTFM

In article <3105@gitpyr.gatech.EDU> roy@gitpyr.gatech.EDU (Roy Mongiovi) writes:

>Well, quite some time ago I also heard that CX=0 on a write would truncate
>the file.  When I became curious about it recently, I looked in the IBM
>PC-DOS Technical reference manuals for DOS 2.1 and 3.0 (the only ones I
>had available).  I looked in the function documentation distributed to
>OEM's by Microsoft for MS-DOS 2.0.  I also looked in the book
>"Advanced MS-DOS" by Ray Duncan.
>
>None of these sources mentioned a thing about CX=0 being a legal value or
>not, much less saying that it allows you to truncate a file.

My IBM "DOS Technical Reference", publication no. 6138536 (for DOS 3.10)
documents this behavior, both for function 0x28 (random block write, pg. 6-85)
and function 0x40 (write to a file or device, pg. 6-126).

Also, my Microsoft "MS-DOS Operating System Programmer's Reference Manual",
document no. 8411-200-00 (for DOS 2.01) documents this behavior, both for
function 0x28 (pg. 1-84) and function 0x40 (pg. 1-117).

The latter document is copyright 1981 and 1983 by Microsoft, so this
information has been available for at least four years.
-- 
Larry Campbell                                The Boston Software Works, Inc.
Internet: campbell@maynard.uucp             120 Fulton Street, Boston MA 02109
uucp: {alliant,wjh12}!maynard!campbell              +1 617 367 6846
ARPA: campbell%maynard.uucp@harvisr.harvard.edu      MCI: LCAMPBELL