Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!hp4nl!ctisbv!pim
From: pim@cti-software.nl (Pim Zandbergen)
Newsgroups: comp.unix.questions
Subject: F_FREESP argument to fcntl() (was: Re: sparse files)
Message-ID: <1650@ctisbv.cti-software.nl>
Date: 10 Dec 89 16:19:52 GMT
References: <21581@adm.BRL.MIL> <235@dg.dg.com> <2700@auspex.auspex.com>
Organization: CTI Software BV, The Hague, the Netherlands
Lines: 27

guy@auspex.auspex.com (Guy Harris) writes:

>>Once an area of a file is written,
>>it cannot be returned to its previous sparse state.

>Not in general, anyway.  At least the first version of AIX for the RT PC
>claimed, in its documentation, that it had an "fclear()" call to punch
>holes in files; I think this may show up in future releases of other
>UNIXes as well.

I think the *undocumented* F_FREESP argument to fcntl()
in lots of versions of SYSVR3 can do this too.
Take a look in /usr/include/sys/fcntl.h to find out if yours has it.

Some time ago somebody posted a SYSV implementation of
ftruncate(). It used F_FREESP to chop off the end of a file.
But I see no reason why it could not be used to punch 
a hole in the middle of a file.

Can anybody confirm this? This could be the base of a handy
tool able to free LOTS of disk space.

-- 
Pim Zandbergen                           domain : pim@cti-software.nl
CTI Software BV                          uucp   : uunet!mcsun!hp4nl!ctisbv!pim
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