Re: Patch: CPU RNG framework with stub x86 implementation

[christos%astron.com@localhost (Christos Zoulas)]

In article <20160112133034.GH18788%pony.stderr.spb.ru@localhost>,
Valery Ushakov  <uwe%stderr.spb.ru@localhost> wrote:
>On Mon, Jan 11, 2016 at 21:59:20 -0500, Thor Lancelot Simon wrote:
>
>> On Mon, Jan 11, 2016 at 07:58:09PM -0500, Andrew Cagney wrote:
>> > On 10 January 2016 at 21:08, Thor Lancelot Simon <tls%panix.com@localhost> wrote:
>> > > As requested - here's just "cpu_rng" itself for review.  I believe this
>> > > version addresses all comments received so far except that I _like_ the
>> > > construct "size_t cnt = <constant>; type_t foo[x]" so I've retained
>> > > that.
>> > 
>> > According to that ever reliable wikipedia :-)
>> > 
>> > + size_t cnt = 2 * RND_ENTROPY_THRESHOLD / sizeof(cpu_rng_t);
>> > + cpu_rng_t buf[cnt];
>> > 
>> > variable-length arrays were added in C99, but subsequently, in C11,
>> > were relegated to a conditional feature which implementations are not
>> > required to support.
>> 
>> That's not a variable-length array.
>
>It definitely looks like one to me.
>
>    6.7.5.2  Array declarators
>
>    [#4] ... If the size is an integer constant expression and the
>    element type has a known constant size, the array type is not a
>    variable length array type; otherwise, the array type is a
>    variable length array type.
>
>and
>
>    6.6  Constant expressions
>
>    [#6] An integer constant expression shall have integer type and
>    shall only have operands that are integer constants, enumeration
>    constants, character constants, sizeof expressions whose results
>    are integer constants, and floating constants that are the
>    immediate operands of casts.  Cast operators in an integer
>    constant expression shall only convert arithmetic types to integer
>    types, except as part of an operand to the sizeof operator.
>
>And gcc -ansi -pedantic complains about
>
>    size_t cnt = 128;
>    int vla[cnt];
>
>    warning: ISO C90 forbids variable length array ?vla? [-Wvla]
>
>Constifying cnt doesn't change that since 'cnt' is still not an
>constant expression.

Looking at the assembly code produced, gcc seems to always (on x86)
move the stack by the right (constant) amount to allocate space for
the array, but as you say to guarantee that this does not create a
variable length array once should use a #define :-)

christos