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From: thomas@utah-gr.UUCP (Spencer W. Thomas)
Newsgroups: net.unix,net.unix-wizards,net.arch,net.lang.c
Subject: Re: get size of malloc'd object
Message-ID: <1742@utah-gr.UUCP>
Date: Fri, 13-Jun-86 09:21:33 EDT
Article-I.D.: utah-gr.1742
Posted: Fri Jun 13 09:21:33 1986
Date-Received: Tue, 17-Jun-86 07:47:15 EDT
References: <165@daisy.UUCP>
Reply-To: thomas@utah-gr.UUCP (Spencer W. Thomas)
Distribution: net
Organization: University of Utah CS Dept
Lines: 21
Xref: watmath net.unix:8147 net.unix-wizards:18397 net.arch:3436 net.lang.c:9416

In article <165@daisy.UUCP> misha@daisy.UUCP (Mike Umansky) writes:
>	I have a need in my application to find the size of a
>	previously malloc'd object (maybe an array or structure).

We had this same need, and for many years satisfied it by writing a
macro SIZE(ptr) that `knew' how malloc saved the block size (usually in
the word preceding ptr).  Finally, with the 4.3 release, and in the
interests of easier portability (we were distributing our own malloc
with our code), we went to a different scheme:  We now do storage
allocation via two routines `new' and `dispose'.  The function `new'
asks malloc for the size requested plus one word, puts the size in the
extra word, and returns a pointer to the next word (obviously, it has to
worry about alignment, too, so it may not be quite this simple). The
function `dispose' readjusts the pointer passed to it and calls free.  A
third routine, `size', can then easily look at the word preceding the
pointer passed to it and return the size of the block.  This scheme has
the advantage of being blissfully independent of the underlying storage
allocation management.

-- 
=Spencer   ({ihnp4,decvax}!utah-cs!thomas, thomas@utah-cs.ARPA)