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From: franka@mmintl.UUCP (Frank Adams)
Newsgroups: net.unix,net.arch,net.lang.c
Subject: Re: get size of malloc'd object
Message-ID: <1635@mmintl.UUCP>
Date: Wed, 9-Jul-86 03:35:55 EDT
Article-I.D.: mmintl.1635
Posted: Wed Jul  9 03:35:55 1986
Date-Received: Fri, 11-Jul-86 22:06:14 EDT
References: <165@daisy.UUCP> <334@valid.UUCP> <2206@peora.UUCP>
Reply-To: franka@mmintl.UUCP (Frank Adams)
Organization: Multimate International, E. Hartford, CT
Lines: 28
Xref: linus net.unix:7845 net.arch:3350 net.lang.c:8949

In article <2233@peora.UUCP> jer@peora.UUCP writes:
>[Frank Adams]
>> Depends on what you mean by "portable".  Certainly, it is possible to use
>> this number in an implementation-dependent way; but there are natural and
>> non-implementation-dependent ways to use the information.  E.g., if one is
>> allocating character strings and one of them grows, one can try to fit the
>> expanded string into the allocated size, and only reallocate (and reset all
>> the pointers to the data) if it won't fit.
>
>Well, actually it depends on some other factors, just fairly abstract ones
>(and thus debatable).  According to the System V manual, "Malloc returns a
>pointer to a block of at least _size_ bytes suitably aligned for any use."
>
>The problem is that, unless the definition is constrained more, it makes
>it impossible to prove a program that uses the "actual size allocated"
>correct.

So constrain the definition more.  It isn't written in stone.  I dare say
that there is no system in existence that would fail under the assumption
that the actual size allocated fits in a long -- probably none would fail
assuming an int.

Compared to the total task of dealing with the C environment in such a way
that correctness proofs become possible, this is a very tiny issue, easily
dealt with.

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