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

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Date:   Sun, 28 Oct 2018 12:46:42 +0100 (CET)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Himanshu Jha <himanshujha199640@...il.com>
cc:     Sasha Levin <sashal@...nel.org>,
        Shayenne da Luz Moura <shayenneluzmoura@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Michael Thayer <michael.thayer@...cle.com>,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        outreachy-kernel@...glegroups.com
Subject: Re: [Outreachy kernel] [RESEND PATCH 2/2] staging: vboxvideo: Use
 unsigned int instead bool



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