On Fri, Feb 17, 2017 at 01:13:51PM +0000, Edward Cree wrote:
> On 17/02/17 12:37, Luc Van Oostenryck wrote:
> > In fact this warning is correct, by design.
> > Your not supposed to cast a bitwise type, even zero.
> But you _are_ allowed to initialise a bitwise typed variable with zero...
> This is inconsistent; if a value is implicitly convertible to a type, it
> should also be legal to cast it explicitly to that type. Such is the
> case with all types in standard C.
> So it's my opinion that the design is wrong.
> > Here is a few examples that doesn't give a warning:
> > mask[1] = 0;
> > mask[2] = ~msk;
> > mask[3] = ~(__force __be32) 0;
> > mask[4] = ~({ __be32 msk = 0; msk; });
> >
> > The canonical way to do what you need is via the (__force ...) cast.
> > In fact, in the driver you're talking about, just a few lines above
> > this fill_mask() function, there is a few masks already defined that
> > do exactly this, like IP4_ADDR_FULL_MASK.
> Yes, and I'd actually like to be able to remove __force from those casts as
> well; __forced casts are a very big hammer that imho should be confined to
> things like architecture-specific code. They should not be needed in
> drivers; they certainly should not be needed for a conversion that can
> occur implicitly.
Well, it's a bit the same for all casts: you should not abuse them but theys
have their uses.
> You mention that ({ __be32 msk = 0; msk; }) works. But surely that is a
> strictly less forceful cast than (__be32)0, since it is using an implicit
> conversion, and if that doesn't give a warning, then neither should the
> explicit cast.
>
> So can you explain to me why 0 should be treated specially by _everything
> except_ the cast operator?
It was a design decision, justified (I think) by:
- 0 is already magic in C (can be used to initialize integer *and* pointers)
- since 0 is a constant that make sense for all sizes, have the same
representation in big endian and little endian, ... it can be used for
all bitwise types.
I think the answer to your question could be: "since it's best to avoid casts,
especially unnneded ones for 0 zero you don't need a cast, it's good to warn
about it".
Luc
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