Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!cbosgd!ihnp4!mhuxn!mhuxr!ulysses!allegra!mit-eddie!genrad!decvax!decwrl!amdcad!amd!pesnta!wjvax!brett
From: brett@wjvax.UUCP
Newsgroups: net.unix,net.unix-wizards,net.arch,net.lang.c
Subject: Re: get size of malloc'd object
Message-ID: <718@wjvax.wjvax.UUCP>
Date: Fri, 20-Jun-86 11:19:25 EDT
Article-I.D.: wjvax.718
Posted: Fri Jun 20 11:19:25 1986
Date-Received: Mon, 23-Jun-86 01:14:19 EDT
References: <165@daisy.UUCP> <334@valid.UUCP> <2206@peora.UUCP>
Reply-To: brett@wjvax.UUCP (Brett Galloway)
Organization: Watkins-Johnson Co., San Jose, Calif.
Lines: 32
Xref: watmath net.unix:8331 net.unix-wizards:18555 net.arch:3540 net.lang.c:9537

In article <2206@peora.UUCP> jer@peora.UUCP (J. Eric Roskos) writes:
>
>	It is really hard to come up which general purpose algorithmic 
>	solutions to problems without having even a glimmer of what
>	environment you are talking about ...
>
>Sure it is.  Just write your own routine to call malloc; allocate a
>sizeof(int) worth of extra space, then store the size of the thing you
>malloc'ed in the int at the front of the allocated block, advance the
>pointer past the place where your stored the size, and return that as
>the pointer to the block you allocated.  The size of the object is then
>found in the word preceeding the location pointed to by the object pointer.
>
>Also provide a routine to deallocate the block by backing the pointer up
>before calling free().
>
>This approach is portable, simple, and easy to understand.
>Also it doesn't require any assumptions about what kind of objects are
>being allocated.

In fact, this assumes that the resulting pointer

	(malloc(len+sizeof(int))+sizeof(int))

is still well-aligned.  This is only true if sizeof(int) is the most coarse
alignment boundary on the machine.  If, for example, structures require more
coarse alignment, attempting to cast the above pointer to a structure pointer
will fail.

-------------
Brett Galloway
{pesnta,twg,ios,qubix,turtlevax,tymix,vecpyr,certes,isi}!wjvax!brett