linux-kernel - Re: Arrays of variable length

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Message-ID: <yw1xh937ckl4.fsf@unicorn.mansr.com>
Date:   Mon, 06 Mar 2017 00:31:03 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:     Tomas Winkler <tomasw@...il.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        sparse@...r.kernel.org
Subject: Re: Arrays of variable length

Henrique de Moraes Holschuh <hmh@....eng.br> writes:

> On Sun, 05 Mar 2017, Måns Rullgård wrote:
>> 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.
>
> Well, if we're going to forbid VLAs in the kernel, IMHO the kernel build
> system should call gcc with -Werror=vla to get that point across early,
> and flush out any offenders.

If it were up to me, that's exactly what I'd do.

-- 
Måns Rullgård