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From: wcs@ho95e.UUCP (#Bill_Stewart)
Newsgroups: net.unix-wizards,net.sources.bugs
Subject: Re: What happens during an unlink(2) - really malloc()
Message-ID: <662@ho95e.UUCP>
Date: Wed, 30-Apr-86 16:51:49 EDT
Article-I.D.: ho95e.662
Posted: Wed Apr 30 16:51:49 1986
Date-Received: Fri, 2-May-86 23:55:49 EDT
References: <947@kitty.UUCP> <403@ukecc.UUCP> <979@kitty.UUCP> <422@ukecc.UUCP>
Reply-To: wcs@ho95e.UUCP (Bill Stewart 1-201-949-0705 ihnp4!ho95c!wcs HO 2G202)
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 23
Keywords: ``Relink'' April Fool
Xref: watmath net.unix-wizards:17877 net.sources.bugs:815
Summary: Please write portable code!

In article <422@ukecc.UUCP> edward@ukecc.UUCP (Edward C. Bennett) writes:
>
>>.....[stuff about what happens to blocks when you unlink the file]...
>        When the old data blocks are placed in the free list, their contents
>are unimportant. Zeroing the blocks would be a waste of time since whatever
>is there will be overwritten when the blocks are allocated to another file.
>This situation is identical to the malloc()/free() process. i.e. Space
>that has been malloc()ed will have data written into it. When this space
>is free()ed, the data in it is not altered and is useable until the next
>malloc().

Please don't depend on this behaviour!  Under the older mallocs (including
System V and 4.1BSD), you could depend on it, but systems with newer and
fancier mallocs don't promise it.  In particular, System V Release 2 offers
two malloc routines.  The default is the old malloc, but the new -lmalloc
library provides routines that offer better performance under some kinds of
input, and the man page says you can't trust freed-up memory after a free()
or realloc().

In general, the safety of using freed space depends on the implementation of
the data structures chaining space together.
-- 
# Bill Stewart, AT&T Bell Labs 2G-202, Holmdel NJ 1-201-949-0705 ihnp4!ho95c!wcs