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    SubjectRe: Arrays of variable length
    On Mon, Mar 6, 2017 at 2:31 AM, Måns Rullgård <mans@mansr.com> wrote:
    > Henrique de Moraes Holschuh <hmh@hmh.eng.br> writes:
    >
    >> On Sun, 05 Mar 2017, Måns Rullgård wrote:
    >>> Tomas Winkler <tomasw@gmail.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.

    >
    Some parts of the kernel depends on VLA such as ___ON_STACK macros in
    include/crypto/hash.h
    It's actually pretty neat implementation, maybe it's too harsh to
    disable VLA completely.

    Tomas

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