Re: Segments

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).