On 2019-07-22, at 14:24:38 +0200, Paul Kocialkowski wrote:
> On Mon 22 Jul 19, 12:12, Jeremy Sowden wrote:
> > On 2019-07-22, at 11:36:51 +0530, Nishka Dasgupta wrote:
> > > Typecast as bool the return value of cedrus_find_format in
> > > cedrus_check_format as the return value of cedrus_check_format is
> > > always treated like a boolean value.
> > >
> > > Signed-off-by: Nishka Dasgupta <nishkadg.linux@xxxxxxxxx>
> > > ---
> > > Changes in v2:
> > > - Add !! to the returned pointer to ensure that the return value
> > > is always either true or false, and never a non-zero value other
> > > than true.
> > >
> > > drivers/staging/media/sunxi/cedrus/cedrus_video.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/staging/media/sunxi/cedrus/cedrus_video.c b/drivers/staging/media/sunxi/cedrus/cedrus_video.c
> > > index e2b530b1a956..b731745f21f8 100644
> > > --- a/drivers/staging/media/sunxi/cedrus/cedrus_video.c
> > > +++ b/drivers/staging/media/sunxi/cedrus/cedrus_video.c
> > > @@ -86,7 +86,7 @@ static struct cedrus_format *cedrus_find_format(u32 pixelformat, u32 directions,
> > > static bool cedrus_check_format(u32 pixelformat, u32 directions,
> > > unsigned int capabilities)
> > > {
> > > - return cedrus_find_format(pixelformat, directions, capabilities);
> > > + return !!(bool)cedrus_find_format(pixelformat, directions, capabilities);
> > > }
> >
> > I think the original was fine. The return value of
> > cedrus_find_format will be automatically converted to bool before
> > being returned from cedrus_check_format since that is the
> > return-type of the function, and the result of converting any
> > non-zero value to bool is 1.
>
> Okay I was a bit unsure about that and wanted to play it on the safe
> side without really looking it up, but that gave me the occasion to
> verify.
>
> From what I could find (from my GNU system's
> /usr/include/unistring/stdbool.h):
>
> Limitations of this substitute, when used in a C89 environment:
>
> - In C99, casts and automatic conversions to '_Bool' or 'bool'
> are performed in such a way that every nonzero value gets
> converted to 'true', and zero gets converted to 'false'.
> This doesn't work with this substitute. With this
> substitute, only the values 0 and 1 give the expected result
> when converted to _Bool' or 'bool'.
>
> So since the kernel is built for C89 (unless I'm mistaken), I don't
> think the compiler provides any guarantee about bool values being
> converted to 1 when they are non-zero.
Ick. I checked the C99 standard and assumed that GCC would use the same
semantics. I've just tested with "gcc-8 -std=gnu89" and it appears to
do the right thing:
[azazel@ulthar:/space/azazel/tmp] $ cat test.c
#include <stdbool.h>
#include <stdio.h>
int
main (void)
{
char *p = "test";
bool b = p;
fprintf (stderr, "%p, %d\n", (void *) p, b);
return 0;
}
[azazel@ulthar:/space/azazel/tmp] $ gcc-8 -O2 -std=gnu89 -Wall -Wextra
test.c -o test
[azazel@ulthar:/space/azazel/tmp] $ ./test
0x55d984e0e004, 1
> As a result, I think it's best to be careful.
Fair enough.
> However, I'm not sure I really see what cocinelle was unhappy about.
> You mentionned single-line functions, but I don't see how that can be
> a problem.
>
> So in the end, I think we should keep the !! and drop the (bool) cast
> if there's no particular warning about it.
>
> What do you think?
Seems sensible.
J.
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