Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!rutgers!seismo!nbires!hao!gaia!zhahai
From: zhahai@gaia.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: Truncating MS-DOS files
Message-ID: <286@gaia.UUCP>
Date: Tue, 17-Feb-87 13:52:31 EST
Article-I.D.: gaia.286
Posted: Tue Feb 17 13:52:31 1987
Date-Received: Thu, 19-Feb-87 06:12:18 EST
References: <1987Feb15.123004.21019@gpu.utcs.toronto.edu>
Reply-To: zhahai@gaia.UUCP (Zhahai Stewart)
Organization: Gaia Corp., Boulder, CO
Lines: 22
Summary: 0 byte write truncates, documentation pages

In article <1987Feb15.123004.21019@gpu.utcs.toronto.edu> lewan@gpu.utcs.toronto.edu (S. Lewandowsky) writes:
>How can I truncate a file using DOS?  For example, I may open a
>file, write to it and read from it extensively, and as a result its size
>--when I use the close file handle call--would be about 120K or greater.
>However, I only want to save the first 10K of that file, so I'd like to
>place the file pointer to whereever I want the file to end, and then 
>truncate it so that the information beyond the pointer is discarded
>and the file size is reduced to 10K.
>There is a truncate procedure in Turbo Pascal, so there must be a way

Writing 0 bytes (ie: CX = 0 when doing the write) will do this.  I think
it works for all DOS 2.X and above.  Due to MS DOS's 2 parents (CP/M and UN*X)
there are two sets of file i/o functions (one based on each).  For function
28 (hex) "Random Block Write" this is documented pn page 5-28 of the DOS 2.1
tech ref manual, and page 6-85 for the DOS 3.1 tech ref manual.  For function
40 (hex) it is described on page 6-126 of the DOS 3.1 tech ref, but is not
mentioned in the older DOS 2.1 tech ref manual.  Others can tell us if these
functions are documented differently for DOS 2.0, 3.0, 3.2, etc - these are
the only tech ref manuals I have.  I hope this helps.   ~z~
-- 
Zhahai Stewart
{hao | nbires}!gaia!zhahai