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    SubjectRe: [Outreachy kernel] [RESEND PATCH 2/2] staging: vboxvideo: Use unsigned int instead bool
    On 10/30, Julia Lawall wrote:
    >
    > On Tue, 30 Oct 2018, Shayenne Moura wrote:
    >
    > > 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.
    >
    > To my recollection, your code had a bool with larger types on either side.
    > In that case, I think bool is fine. The compiler it likely to align those
    > larger typed values such that the field with the bool type will get more
    > than one byte no matter what type you use. If there are several fields
    > with very small types adjacent, there might be some benefit to thinking
    > about what the type should be.
    >
    > julia

    Got it! Thank you!

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