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From: Richard Harnden <richard.nospam@gmail.invalid>
Newsgroups: comp.lang.c
Subject: Re: So You Think You Can Const?
Date: Sun, 12 Jan 2025 16:54:17 +0000
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On 12/01/2025 11:22, Keith Thompson wrote:
> Michael S <already5chosen@yahoo.com> writes:
> [...]
>> Tried to read the old discussion. Didn't understand much.
>> But it strengthened my opinion: it's ridiculous. Standard would be way
>> better with this nonsense removed.
>> The function below should be guaranteed by standard to return 42.
>> int foo(void)
>> {
>>    char* a = malloc(100);
>>    if (!a) return 42;
>>    char* b = a + 42;
>>    free(a);
>>    return b - a;
>> }
> 
> How exactly would that be useful?
> 
> The intent, I think, is to allow for implementations that perform
> checking when loading a pointer value.  It's possible that there
> currently are no such implementations, but even so, how would
> reading a free()d pointer value be useful?
> 
> I'd say that passing a pointer value to free() means you no longer care
> about it.
> 

What if, after the free(a), you have ...

char *b = malloc(100);

.... and the old address of 'a' gets reused.

Could you then carry on using 'a'?

You'd be relying on chance, and it'd be terribly bad form ... but would 
it be legal?