linux-kernel - Re: Arrays of variable length

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Message-ID: <yw1xlgsjdcjk.fsf@unicorn.mansr.com>
Date:   Sun, 05 Mar 2017 14:27:11 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Tomas Winkler <tomasw@...il.com>
Cc:     "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        sparse@...r.kernel.org
Subject: Re: Arrays of variable length

Tomas Winkler <tomasw@...il.com> writes:

> Sparse complains for arrays declared with variable length
>
> 'warning: Variable length array is used'
>
> Prior to c99 this was not allowed but lgcc (c99) doesn't have problem
> with that  https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html.
> And also Linux kernel compilation with W=1 doesn't complain.
>
> Since sparse is used extensively would like to ask what is the correct
> usage of arrays of variable length
> within Linux Kernel.

Variable-length arrays are a very bad idea.  Don't use them, ever.
If the size has a sane upper bound, just use that value statically.
Otherwise, you have a stack overflow waiting to happen and should be
using some kind of dynamic allocation instead.

Furthermore, use of VLAs generally results in less efficient code.  For
instance, it forces gcc to waste a register for the frame pointer, and
it often prevents inlining.

-- 
Måns Rullgård