Re: [Outreachy kernel] [RESEND PATCH 2/2] staging: vboxvideo: Use unsigned int instead bool

[Shayenne Moura]

Hi,

> On Sun, 28 Oct 2018, Himanshu Jha wrote:
> 
> > On Sun, Oct 28, 2018 at 09:47:15AM +0100, Julia Lawall wrote:
> > > > The "possible alignement issues" in CHECK report is difficult to figure
> > > > out by just doing a glance analysis. :)
> > > >
> > > > Linus also suggested to use bool as the base type i.e., `bool x:1` but
> > > > again sizeof(_Bool) is implementation defined ranging from 1-4 bytes.
> > >
> > > If bool x:1 has the size of bool, then wouldn't int x:1 have the size of
> > > int?  But my little experiments suggest that the size is the smallest that
> > > fits the requested bits and alignment chosen by the compiler, regardless of
> > > the type.
> >
> > Yes, correct!
> > And we can't use sizeof on bitfields *directly*, nor reference it using a
> > pointer.
> >
> > It can be applied only when these bitfields are wrapped in a structure.
> >
> > Testing:
> >
> > #include <stdio.h>
> > #include <stdbool.h>
> >
> > struct S {
> > bool a:1;
> > bool b:1;
> > bool c:1;
> > bool d:1;
> > };
> >
> > int main(void)
> > {
> >     printf("%zu\n", sizeof(struct S));
> > }
> >
> > Output: 1
> >
> > If I change all bool to unsigned int, output is: *4*.
> >
> > So, conclusion is compiler doesn't squeeze the size less than
> > native size of the datatype i.e., if we changed all members to
> > unsigned int:1,
> > total width = 4 bits
> > padding = 4 bits
> >
> > Therefore, total size should have been = 1 byte!
> > But since sizeof(unsigned int) == 4, it can't be squeezed to
> > less than it.
> 
> This conclusion does not seem to be correct, if you try the following
> program.  I get 4 for everything, meaning that the four unsigned int bits
> are getting squeezed into one byte when it is convenient.
> 
> #include <stdio.h>
> #include <stdbool.h>
> 
> struct S1 {
> bool a:1;
> bool b:1;
> bool c:1;
> bool d:1;
> char a1;
> char a2;
> char a3;
> };
> 
> struct S2 {
> unsigned int a:1;
> unsigned int b:1;
> unsigned int c:1;
> unsigned int d:1;
> char a1;
> char a2;
> char a3;
> };
> 
> int main(void)
> {
>     printf("%zu\n", sizeof(struct S1));
>     printf("%zu\n", sizeof(struct S2));
>     printf("%zu\n", sizeof(unsigned int));
> }
> 
> > Well, int x:1 can either have 0..1 or -1..0 range due implementation
> > defined behavior as I said in the previous reply.
> >
> > If you really want to consider negative values, then make it explicit
> > using `signed int x:1` which make range guaranteed to be -1..0
> 
> The code wants booleans, not negative values.
> 
> julia

Thank you all for the discussion!

However, I think I do not understand the conclusion.

It means that the best way is to use only boolean instead of use unsigned
int with bitfield? I mean specifically in the case of my patch, where there 
are some boolean variables are mixed with other variables types. 

Best,

Shayenne