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From: arnold@ucsfcgl.UUCP (Ken Arnold%CGL)
Newsgroups: net.unix,net.unix-wizards,net.arch,net.lang.c
Subject: Re: get size of malloc'd object
Message-ID: <9894@ucsfcgl.ucsfcgl.UUCP>
Date: Tue, 24-Jun-86 19:54:58 EDT
Article-I.D.: ucsfcgl.9894
Posted: Tue Jun 24 19:54:58 1986
Date-Received: Sat, 28-Jun-86 03:20:17 EDT
References: <165@daisy.UUCP> <334@valid.UUCP> <2206@peora.UUCP> <2081@umcp-cs.UUCP>
Reply-To: arnold@ucsfcgl.UUCP (Ken Arnold%CGL)
Organization: Computer Graphics Laboratory, UCSF
Lines: 23
Xref: watmath net.unix:8411 net.unix-wizards:18613 net.arch:3588 net.lang.c:9597

In article <2081@umcp-cs.UUCP> chris@umcp-cs.UUCP (Chris Torek) writes:
>In article <2206@peora.UUCP>, jer@peora.UUCP (J. Eric Roskos) writes:
>>... 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.
>>
>
>Unfortunately, it is not necessarily portable.  I can imagine a
>machine where `double' arguments must be aligned on an eight-byte
>boundary, but integers are only four bytes wide.  In this case
>calling the new routine to allocate doubles would result in an
>odd-address-style trap when the returned value is used.

So put the int at the end of the space, after increasing the size
to be allocated by sizeof (int) + enough space to align the int
on a byte which is a multiple of sizeof (int).  Then return the
original malloc()ed pointer.

Can we say "hack"?

		Ken Arnold