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.
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?
-Ed
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