Path: utzoo!attcan!uunet!cs.utexas.edu!sun-barr!olivea!tymix!cirrusl!sunstorm!dhesi
From: dhesi%cirrusl@oliveb.ATC.olivetti.com (Rahul Dhesi)
Newsgroups: comp.lang.c
Subject: Re: How do I SHORTEN a file without rewriting it?
Keywords: truncation
Message-ID: <2652@cirrusl.UUCP>
Date: 5 Nov 90 07:13:36 GMT
References: <1162@bilver.UUCP> <747@csource.oz.au> <9505@jarthur.Claremont.EDU>
Sender: news@cirrusl.UUCP
Organization: Cirrus Logic Inc.
Lines: 33

In <9505@jarthur.Claremont.EDU> dfoster@jarthur.Claremont.EDU (Derek R.
Foster) writes:

>>>Is there a way to shorten a file...

>>Write zero bytes at that position.

>If this works, it isn't documented in the Microsoft C manuals I have.
>[chsize()] seems to be the only way of doing this from within
>Microsoft C...

A write of zero bytes using the MS-DOS "write to file handle" system
call will indeed truncate the file at the current seek position.
User programs could well contain statements of the sort:

     count = read(fin, buf, count);
     if (count != -1)
          (void) write(fout, buf, count);

A zero-byte write could truncate the file connected with fout.  If we
had seeked to near the beginning of a long file and were trying to
overwrite some of the data without losing the rest, truncation would be
disastrous.

So it's at least an even bet that your C library carefully avoids
passing zero-byte writes on to MS-DOS.

The chsize() function, on the other hand, most likely simply does a
seek followed by zero-byte write.
--
Rahul Dhesi <dhesi%cirrusl@oliveb.ATC.olivetti.com>
UUCP:  oliveb!cirrusl!dhesi
A pointer is not an address.  It is a way of finding an address. -- me