[libvirt] proposal: allow use of "bool": HACKING: discuss C types

[Daniel P. Berrange]

On Fri, Dec 12, 2008 at 09:58:32AM +0100, Jim Meyering wrote:
> Hello,
> 
> I don't want to make waves, but I do care about certain aspects
> of code quality, so...
> 
> I propose to allow (encourage, even) the use of the standard C99 type,
> bool, in libvirt, but not in public interfaces[1].  There are already
> uses in cgroup.c and xmlrpc.c, as well as those in the gnulib "c-ctype.h"
> header which is included from many of libvirt's .c files.
> 
> The motivation is to make the code as readable as possible.
> When you see the declaration of an "int" variable, member, or function,
> that type gives you no clue whether it is used as a boolean.  If you
> see a few uses that treat it as boolean, that's still no guarantee,
> and it may be non-trivial to ensure that there isn't some non-boolean
> value with a special meaning.
> 
> However, if you see a "bool" variable, you do have that guarantee.

I don't particularly like the idea of using the bool type

 - No system header files use it
 - Library header files which use a boolean type have nearly all defined
   their own custom bool types, not using stdbool.h and there's no guarentee
   that stdbool's idea of 'true' matches the other apps'
 - We don't use it in the public API, or on the wire for the remote 
   protocol, since it has undefined size.
 - The GNULIB bool emulation is unable to provide equivlance between
   C's idea of true (any non-zero value) and the defined 'true'
   constant (whose value is 1)
 - Bitfields are more size efficient than bools.

In a language like python where there's a fundamental builtin type in
the language for 'bool' this it makes sense. In the C world with many
many[1] different definitions of bool, true and false it just feels 
like a world of hurt. The only thing you can ultimately rely on is that
a true value is non-zero.

Daniel

[1] Picking a *tiny* selection of the bool's in my dev box headers

/usr/include/xulrunner-sdk-1.9/stable/jritypes.h:typedef enum JRIBoolean {
/usr/include/xulrunner-sdk-1.9/stable/nptypes.h:    typedef int bool;
/usr/include/xulrunner-sdk-1.9/stable/npapi.h:typedef unsigned char	NPBool;
/usr/include/xulrunner-sdk-1.9/stable/jri_md.h:typedef unsigned char	jbool;
/usr/include/wine/windows/wtypes.idl:typedef long BOOL;
/usr/include/cups/raster.h:typedef enum cups_bool_e		/**** Boolean type ****/
/usr/include/lcms.h:typedef int   LCMSBOOL;
/usr/include/mp4.h:typedef int bool;
/usr/include/tss/tpm.h:typedef BYTE   TPM_BOOL;
/usr/include/tss/platform.h:   typedef int8_t             TSS_BOOL;
/usr/include/tss/platform.h:   typedef  signed char      TSS_BOOL;

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