Hi Matthew,
On Thu, Aug 23, 2018 at 07:34:45AM -0700, Matthew Wilcox wrote:
On Wed, Aug 22, 2018 at 04:57:33PM +0100, Brian Starkey wrote:
On Wed, Aug 22, 2018 at 05:11:55PM +0200, Daniel Vetter wrote:
On Wed, Aug 22, 2018 at 4:59 PM, Eric Engestrom eric.engestrom@intel.com wrote:
On Tuesday, 2018-08-21 17:44:17 +0100, Brian Starkey wrote:
On Tue, Aug 21, 2018 at 09:26:39AM -0700, Matthew Wilcox wrote:
Can you turn them into enums? This seems to work ok:
I'm not sure that swapping out explicit 32-bit unsigned integers for enums (unspecified width, signed integers) is necessarily a good idea, it seems like Bad Things could happen.
The C spec says:
"the value of an enumeration constant shall be an integer constant expression that has a value representable as an int"
Which likely gives us 4 bytes to play with on all machines that run Linux, but if drm_fourcc.h is ever going to be some kind of standard reference, making it non-portable seems like a fail.
And even if you do have 4 bytes in an enum, signed integers act differently from unsigned ones, and compilers do love to invoke the UB clause...
I think you're exaggerating how much latitude C compilers have here. Further down in 6.7.2.2, it says:
Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer type. The choice of type is implementation-defined, but shall be capable of representing the values of all the members of the enumeration.
So if we include an integer which isn't representable in a plain int, then the compiler _must_ choose a larger type.
I don't think so... the sentence I pasted says that including a value which isn't representable in a plain int would be illegal, and so the compiler doesn't _have_ to do anything (nasal demons, right?).
It could choose a signed-64-bit type rather than an unsigned-32-bit type, but I can't imagine any compiler being quite so insane.
The paragraph about the implementation choosing a representation is separate from the valid range of values - the compiler can pick whatever storage it likes (smaller or even larger than an int), so long as that storage can fit all the defined values. However, providing a value in an enum definition which is not representable as an int would still be invalid (irrespective of how large the storage is) - it's a separate restriction.
Anyhow, I'm not dying to replace all the current definitions with enums, so if someone else wants to pick that up, be my guest.
Cheers, -Brian