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SubjectRE: [PATCH] incorrect use of sizeof() in ioctl definitions
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> Andries Brouwer [mailto:aebr@win.tue.nl] wrote:
> On Tue, Sep 30, 2003 at 11:25:56PM +0100, Matthew Wilcox wrote:
> > On Tue, Sep 30, 2003 at 02:08:05PM -0700, Andrew Morton wrote:
> > > Arun Sharma <arun.sharma@intel.com> wrote:
> > > >
> > > > Some drivers seem to use macros such as _IOR/_IOW in a way that
ends
> up
> > > > calling the sizeof() operator twice. For eg:
> > > >
> > > > -#define FBIO_ATY128_GET_MIRROR _IOR('@', 1, sizeof(__u32*))
> > > > +#define FBIO_ATY128_GET_MIRROR _IOR('@', 1, __u32*)
>
> But this changes the define. You want
>
> #define FBIO_ATY128_GET_MIRROR _IOR_BAD('@', 1, __u32*)

Maybe I got something wrong, but could someone please help me to
understand why introduce _IOR_BAD here? Thanks first! :)

IMO, the birth of new ioctl definition convention considers
"size" of argument as a part, so we should conform to this rule.
_IOR_BAD is no different as old one <_IOR('@', 1, sizeof(__u32*))>,
which expands as sizeof(sizeof(__32*)) and actually assume temp result
of internal sizeof as the argument. Of course this didn't reflect the
true point and we should change the definition. :)

Also I'm confused about the modification about using size_t to
replace sizeof(). Take MATROXFB_SET_OUTPUT_MODE for example:

(old) #define MATROXFB_SET_OUTPUT_MODE
_IOW('n',0xFA,sizeof(struct matroxioc_output_mode))
(now) #define MATROXFB_SET_OUTPUT_MODE _IOW('n',0xFA,size_t)

The size of matroxioc_output_mode is 8 bytes on all platforms,
however size_t will be calculated as 4 bytes on 32bit arch and 8 bytes
on 64bit arch. So this is also like using sizeof(), which imposes the
difference between 32bit ioctl number and 64bit ioctl number. However in
standard manner, I mean:
#define MATROXFB_SET_OUTPUT_MODE _IOW('n',0xFA,struct
matroxioc_output_mode)
The value should be identical on all platforms, which save our
efforts to do useless conversion when running 32bit apps on 64bit
platform.

The most important is: to use sizeof() or size_t here both
messed the ioctl definition, which violate the initial motivation of
_IO**, is it?

> Something else we should do is to change all occurrences of 'size'
> here into 'argtype'. All this nonsense came because of the bad choice
> of identifier.
Agree. The inaccurate name here confused us... :)
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