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From: franka@mmintl.UUCP (Frank Adams)
Newsgroups: net.unix,net.lang.c
Subject: Re: get size of malloc'd object
Message-ID: <1601@mmintl.UUCP>
Date: Tue, 24-Jun-86 09:56:16 EDT
Article-I.D.: mmintl.1601
Posted: Tue Jun 24 09:56:16 1986
Date-Received: Fri, 27-Jun-86 04:00:51 EDT
References: <165@daisy.UUCP> <334@valid.UUCP> <2206@peora.UUCP> <2081@umcp-cs.UUCP> <2216@peora.UUCP>
Reply-To: franka@mmintl.UUCP (Frank Adams)
Organization: Multimate International, E. Hartford, CT
Lines: 31
Xref: linus net.unix:7703 net.lang.c:8781

In article <2216@peora.UUCP> jer@peora.UUCP writes:
~>determine alignment by difference between consecutive malloc'ed objects

>you're not guaranteed that the two consecutive mallocs will give
>consecutive blocks of memory -- they certainly won't be likely to after
>some other mallocs and frees have been done.  It's hard to see a situation
>where the initial allocations wouldn't be consecutive, but it's not
>guaranteed, and that's sufficient reason to have doubts.

(1) If you are using the "buddy system"[1] for storage allocation, and the
total storage available is not a power of two, initial allocations might
well not be consecutive.

(2) Some version of malloc have header information about the allocated block
immediately before the block.  For such versions, the distance between blocks
always includes at least one such header.

(3) Storage may be allocated and freed in the process of starting the
program, before your code gets control.

(4) Storage for all programs may be allocated from a single heap, so that
other tasks would have done malloc's and free's before (or even between)
yours.

In short, this is completely unreliable.

Frank Adams                           ihnp4!philabs!pwa-b!mmintl!franka
Multimate International    52 Oakland Ave North    E. Hartford, CT 06108

[1] For a description of the buddy system, see Knuth, _The_Art_of_Computer
_Programming,_vol._1,_Fundamental_Algorithms_.