Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Thomas Koenig <tkoenig@netcologne.de>
Newsgroups: comp.arch
Subject: Re: Segments
Date: Fri, 17 Jan 2025 16:42:17 -0000 (UTC)
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David Brown <david.brown@hesbynett.no> schrieb:
> On 16/01/2025 17:46, Waldek Hebisch wrote:
>> David Brown <david.brown@hesbynett.no> wrote:
>>> On 16/01/2025 13:35, Michael S wrote:
>>>> On Thu, 16 Jan 2025 12:36:45 +0100
>>>> David Brown <david.brown@hesbynett.no> wrote:
>>>>
>>>>> On 15/01/2025 21:59, Thomas Koenig wrote:
>>>>>> Michael S <already5chosen@yahoo.com> schrieb:
>>>>>>> On Wed, 15 Jan 2025 18:00:34 -0000 (UTC)
>>>>>>> Thomas Koenig <tkoenig@netcologne.de> wrote:
>>>>>>>    
>> 
>>>> As you can guess, in kernel drivers VLA are unwelcome.
>>>
>>> I can imagine that they are - but I really don't understand why.  I've
>>> never understood why people think there is something "dangerous" about
>>> VLAs, or why they think using heap allocations is somehow "safer".
>> 
>> VLA normally allocate on the stack.  Which at first glance look
>> great.  But once one realize how small are stacks in modern
>> systems (compared to whole memory), this no longer looks good.
>> Basically, to use VLA one needs rather small bound on maximal
>> size of array.
>
> Sure.
>
>> Given such bound always allocating maximal
>> size is simpler.  Without _small_ bound on size heap is
>> safer, as it is desined to handle also big allocations.
>
> You don't allocate anything in a VLA without knowing the bounds and 
> being sure it is appropriate to put on the stack.

In general, that is a hard thing to know - there is no standard
way to enquire the size of the stack, how much you have already
used, how deep you are going to recurse, or how much stack
a function will use.

These may be points that you are looking at for your embedded work,
but the average programmer does not.

An example, Fortran-specific:  Fortran 2018 made all procedures
recursive by default.  This means that some Fortran codes will start
crashing because of stack overruns when this is implemented :-(

> You don't allocate 
> anything on the heap without knowing the bounds and being sure it is 
> appropriate.  There's no fundamental difference - it's just the cut-off 
> point that is different.

What would you recommend as a limt?  (See fmax-stack-var-size=N
in gfortran).