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From: mangler@cit-vax.Caltech.Edu (System Mangler)
Newsgroups: comp.unix.wizards
Subject: a creat() hack
Message-ID: <3921@cit-vax.Caltech.Edu>
Date: Mon, 7-Sep-87 22:11:33 EDT
Article-I.D.: cit-vax.3921
Posted: Mon Sep  7 22:11:33 1987
Date-Received: Tue, 8-Sep-87 06:16:31 EDT
Organization: California Institute of Technology
Lines: 20
Keywords: creat truncate versions efficiency

creat() clobbers the file before anything is written.  I wonder if
this behaviour is necessary.

Consider an alternate implementation, which instead of truncating
the file upon creat(), just sets a "deferred truncate" flag.  Upon
a close() or lseek(), the file is truncated at the file pointer
position if the flag is still set, and then the flag is reset.

If the old and new sizes are similar, we've saved ourselves a lot
of block allocation/deallocation.  (Look at 4.3bsd rwhod sometime).

If a long-running program does a creat() and the machine crashes
before anything is written, the original contents of the file are
left untouched, gaining some of the utility of multiple versions.

If you copy a file over itself, it doesn't clobber the file.

Other than tail -f, how much would this break?

Don Speck   speck@vlsi.caltech.edu  {amdahl,rutgers}! die 
and n of