Re: [PATCH 1/1] all: add C99's "bool" define and encourage its use

[Dan Williams <dcbw redhat com>]

On Wed, 2015-11-11 at 13:33 +0100, Thomas Haller wrote:
> ---
>  Let's use bool.
>
>  Comments?

Since glib uses gboolean in quite a few places, would we have problems
with casting it to gboolean if needed?  Casting would be pretty ugly if
we had to do it a bunch.

Also, I don't think we'd save any space because the compiler is going to
pad any field 1-byte field (like bool) out to 4 or 8 bytes anyway,
unless the structure is packed.

Lastly, using bitfields in structs is not generally recommended since
apparently (at least) gcc creates awful code for accesses.  At least
that's the argument in kernel-land for not doing bitfields, and you
never see them there.

Dan

>  Thomas
>
>  include/nm-default.h | 46 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 46 insertions(+)
>
> diff --git a/include/nm-default.h b/include/nm-default.h
> index d5c9d10..dc02763 100644
> --- a/include/nm-default.h
> +++ b/include/nm-default.h
> @@ -70,4 +70,50 @@
>  
>  /*****************************************************************************/
>  
> +/**
> + * The boolean type _Bool is C99 but we mostly stick to C89. However, _Bool is too
> + * convinient to miss and is effectively available in gcc and clang. So, just use it.
> + *
> + * Usually, one would include "stdbool.h" to get the bool define which aliases
> + * _Bool. We provide this define here, because we want to make use of it anywhere.
> + * (also, stdbool.h is again C99).
> + *
> + * Using _Bool has advantages over gboolean:
> + *
> + * - commonly, _Bool is one byte large, instead of gboolean's 4 bytes (because gboolean
> + *   is a typedef for gint). Especially when having boolean fields in a struct, we can
> + *   thereby easily save some space.
> + *
> + * - _Bool type is guaranteed that two true expressions compare equal. E.g. the follwing
> + *   will not work:
> + *        gboolean v1 = 1;
> + *        gboolean v2 = 2;
> + *        g_assert_cmpint (v1, ==, v2); // will fail
> + *   For that, we often to use !! to coerce gboolean values to 0 or 1:
> + *        g_assert_cmpint (!!v2, ==, TRUE);
> + *   With _Bool this will be properly done by the compiler.
> + *
> + * - For structs, we might want to safe even more space and use bitfields:
> + *       struct s1 {
> + *           gboolean v1:1;
> + *       };
> + *   But the problem here is that gboolean is signed, so that
> + *   v1 will be either 0 or -1 (not 1, TRUE). Thus, the following
> + *   doesn't work:
> + *      struct s1 s = { .v1 = TRUE, };
> + *      g_assert_cmpint (s1.v1, ==, TRUE);
> + *   It will however work just fine with bool/_Bool.
> + *
> + * Also, add the defines for true/false. Those are nicely highlighted by the editor
> + * as special types, contrary to glib's TRUE/FALSE.
> + */
> +
> +#ifndef bool
> +#define bool _Bool
> +#define true    1
> +#define false   0
> +#endif
> +
> +/*****************************************************************************/
> +
>  #endif /* __NM_DEFAULT_H__ */