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Reply-To: Brian.Inglis@SystematicSw.ab.ca
Subject: Re: malloc(0) crashing with SIGABRT
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References: <366918d8-b505-45be-dc28-303579f17341@gmail.com> <78e19eb7-956e-cd05-a076-e56ce347bbbe@gmail.com> <749ecff2-e384-bbfe-c961-481157c73052@SystematicSw.ab.ca> <7ee0dd42337eff1b2a173d06d1cc5990@mail.kylheku.com>
From: Brian Inglis <Brian.Inglis@SystematicSw.ab.ca>
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Date: Thu, 12 Sep 2019 11:38:42 -0600
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On 2019-09-11 23:18, Kaz Kylheku wrote:
> On 2019-09-11 20:59, Brian Inglis wrote:
>> On 2019-09-09 11:13, Petr Skočík wrote:
>>> There's been a twitter discussion on how different POSIX platforms
>>> handle malloc(0): https://twitter.com/sortiecat/status/1170697927804817412 .
>>>
>>> As for Cygwin, the answer appears to be "not well", but this should be
>>> easy to fix.
>>
>> POSIX SUS V4 2018 says:
>>
>> "RETURN VALUE
>>
>> Upon successful completion with size not equal to 0, malloc() shall return a
>> pointer to the allocated space. If size is 0, either:
>>
>>     A null pointer shall be returned [CX] [Option Start]  and errno may be set to
>> an implementation-defined value, [Option End] or
>>
>>     A pointer to the allocated space shall be returned. The application shall
>> ensure that the pointer is not used to access an object.
>>
>> Otherwise, it shall return a null pointer [CX] [Option Start]  and set errno to
>> indicate the error. [Option End]"
>>
>> The second option could be implemented by a pointer to an unmapped page, or a
>> reference to an inaccessible mmap-ed area length zero.
> 
> That's easy: the null pointer, plus some small offset that observes alignment,
> like 16.

It's more a question of what the NULL pointer maps to: I liked systems mapping
NULL pointers to inaccessible pages; and compilers that allow bss to be filled
with bits: carelessness got caught fast!

> (Alignment is important even if the memory isn't accessed, because
> nonportable programs depend on it for other reasons, like being able to use
> the least significant few bits of a pointer for tagging.)

[Keeping tag bits or a byte elsewhere is less overhead than the instructions
required to sanitize tainted pointers before use, assuming all the code
remembers to do so, and those programs deserve what they get! Blargh!]

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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