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From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.lang.c
Subject: Re: effect of free()
Message-ID: <29032@news.Think.COM>
Date: 8 Sep 89 16:48:33 GMT
References: <319@cubmol.BIO.COLUMBIA.EDU> <3756@buengc.BU.EDU> <10971@smoke.BRL.MIL> <2054@munnari.oz.au> <247@ssp1.idca.tds.philips.nl>
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Organization: Thinking Machines Corporation, Cambridge MA, USA
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In article <247@ssp1.idca.tds.philips.nl> dolf@idca.tds.PHILIPS.nl (Dolf Grunbauer) writes:
>On the other
>hand there are some people who state that the "if (ptr == 0)" is still
>illegal (which I interpret now as: the compiler did not take care of this
>case and the generated code raises some sort of a trap).

It *could* raise a trap.  The standard doesn't define the behavior, so
anything is permitted.

>What about the case when ptr is already in a register
>(i.e. definition of ptr: register char *ptr) ? Will there be an address trap
>right after the free as some address register now holds an invalid address ?

An implementation that does this would not conform to the standard.
If some hardware does this, then the C implementation would have to
generate code to work around it.  I think such an implementation would
be extremely unlikely, though -- it's likely that the address has to
be loaded into a register in the process of making the system call
that frees the segment, so it would be an extremely bad idea to fault
in this case.

>By the way: what is the effect of the address trap: does the "if (ptr == 0)"
>always evaluate to FALSE or is there a signal (SIGSEGV) ?

As I said, the effect is undefined.  A conforming program should never
try to use a pointer variable if it doesn't know whether it is valid.

>If so: how can I check in my program whether ptr is still valid (after all
>that's why we had the "if (ptr == 0)" in the first place :-) ?

There's no portable way to check whether a pointer is valid.
Even if there were, comparing it to the null pointer probably wouldn't
be the way.

>If "if (ptr == 0)" cases some sort of a trap or is illegal, is the expression
>"if ((long)ptr == 0)" legal, as ptr will now be loaded in a data register
>instead of an address register (assuming: sizeof(cahr *) == sizeof(long)) ?

I doubt that it's valid.  The cast of the pointer variable might be
implemented using address operations.  Also, even if ptr were null,
there's no guarantee that ((long)ptr) == 0) is true; the result of
casting a pointer to an integer is implementation-dependent.

>A final question: how valid is this discussion ? Is there any CPU (commercial
>available) which has this sort of address checking ? 

I answered this in my posting last night.  The Honeywell DPS-88 checks
addresses during loads.

Barry Margolin
Thinking Machines Corp.

barmar@think.com
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