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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: alloca()-support
Date: Sun, 06 Oct 2019 06:16:49 -0700
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Keith Thompson <kst-u@mib.org> writes:

> Soviet_Mario <SovietMario@CCCP.MIR> writes:
>
>> On 30/09/2019 18:58, Bonita Montero wrote:
>>
>>>>>> Since when (if at any time) was variable size automatic
>>>>>> array supported ?
>>>>>
>>>>> Since C99.
>>>>
>>>> for curiosity, this feature has one or more of the
>>>> limitations I feared ... or even none of them ?
>>>
>>> Yes, you could run out of stack-space, maybe hitting a
>>> guard-page
>>> in the best case so that you wouldn't corrupt any other data.
>>
>> uhm, I didn't mean THAT kind of risk (unavoidable), but the
>> possible difficulties in defining variables beyond the point
>> of declaration of the variable-size array.
>>
>> I can't figure out how the compiler would reference them in
>> standard efficient ways (use of registers).
>>
>> Anyway, the difference with alloca () would be in a safer
>> management of possible collisions and running out of stack
>> space ? (compared with the plain declarations) ?
>
> No.  Neither VLAs nor alloca() provide any protection against
> running out of space.  Both have undefined behavior if there isn't
> enough memory for the allocation.

The function alloca() is extra-linguistic, and is not -- nor can
it be -- written in C.  From that point of view it has undefined
behavior no matter what argument value is given.

Declaring a VLA that needs more memory than is available is not
undefined behavior, any more than declaring a fixed-size array of
the same size is undefined behavior.  Both can blow up at
runtime, but neither one is undefined behavior.