On Sun, 25 Oct 2015 14:29:59 +0100
Marek Vasut <ma...@denx.de> wrote:
> On Sunday, October 25, 2015 at 02:22:53 PM, Albert ARIBAUD wrote:
> > Hello Ian,
>
> Hi!
>
> > On Sun, 25 Oct 2015 12:40:45 +0000, Ian Campbell
> >
> > <ijc+ub...@hellion.org.uk> wrote:
> > > On Sun, 2015-10-25 at 12:46 +0100, Albert ARIBAUD wrote:
> > > > > > +static u8 last_int_usb;
> > > > > > +
> > > > > > +bool dfu_usb_get_reset(void)
> > > > > > +{
> > > > > > + return !!(last_int_usb & MUSB_INTR_RESET);
> > > > >
> > > > > The !! is not needed.
> > > >
> > > > Except if you want to be sure that you return 0 or 1 rather than 0 or
> > > > (1 << something).
> > >
> > > Doesn't the bool return type already cause that to happen? (from the
> > > PoV of the caller at least)
> >
> > When all is said and done, a C bool is a C int, and anyway C does not
> > perform value conversion (except for size and possibly sign extension)
> > on type casts.
> >
> > So no, types, bool or otherwise, do not cause any implicit '!!' to
> > happen.
> >
> > What happens is, wherever C expects a boolean value ('if', 'while'...)
> > it considers 0 to be false and anything else to be true. But that's
> > independent of the value's alleged type.
>
> Which is the case here -- one is not supposed to test boolean type for
> any particular value.
Sure, this works fine as long as everyone has exactly the same idea
about how this is supposed to work. Please consider the following code:
if (one_boolean_variable != another_boolean_variable) {
/* Sanity check failed, features X and Y must be either
both enabled or both disabled at the same time */
}
The author of this hypothetical code may claim that a boolean
variable must be always 0 or 1. And both of you will have a long
and entertaining discussion as a result.
One more example:
#include <stdbool.h>
#include <stdio.h>
bool foo(void)
{
return 123;
}
int main(void)
{
printf("%d\n", (int)foo());
return 0;
}
Guess what is printed after compiling and executing this code? Then
replace "#include <stdbool.h>" with "typedef int bool;" and try it
again. With the GCC compiler, the former prints "1" and the latter
prints "123".
This stuff is a potential source of non-obvious bugs. Using "!!" is
always safe, but may be in many cases redundant.
--
Best regards,
Siarhei Siamashka
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