Re: bool and C23

[Paul Eggert]

On 8/22/22 15:58, Zack Weinberg wrote:
> There isn’t a bool module right now, are you proposing to add one?

Yes.


> I want to get to where _no_ Gnulib module is required for
> bool; AC_C_BOOL by itself should be enough.

Yes, that's the long term goal. But in the shorter term Gnulib-using 
code can't yet require Autoconf 2.72+ so it will need to use a new 
module in Gnulib (maybe it should be called c-bool instead of bool). 
This module will merely say "You should use AC_C_BOOL" and will contain 
an implementation of AC_C_BOOL for use in Autoconf 2.71-. The substitute 
implementation should be a copy of what's in Autoconf 2.72.


> > Sure, but why can't the non-autoheader case define bool, false, and true
> > instead of defining C_BOOL_DEFINITION? That will keep the code simpler
> > for these people.
> I don’t understand what you are suggesting here.

My idea is that for the non-autoheader case, where #defines are 
implemented via -D options given to the compiler, Autoconf arranges to 
pass '-D bool=_Bool -D true=1 -D false=0' to compilers that don't have 
either bool  or stdbool.h. That way, user code need not ever #define 
bool or true or false; non-autoheader user code can say just this:

  #if defined C_NEED_STDBOOL_H && !defined __cpluplus
  # include <stdbool.h>
  #endif

and autoheader user code (the typical case) need not say anything other 
than the usual "#include <config.h>".

Admittedly this is only a minor improvement, as I think pretty much 
everybody who uses Autoconf and C also uses Autoheader and config.h.


> Looking at the existing Gnulib code, though, suggests another possible
> reason why we might need the fallback code to `signed char` (or
> Gnulib’s enumeration): allegedly C99-compliant compilers whose Boolean
> implementation is buggy.  My AC_C_BOOL preserves the existing test for
> a buggy Boolean type, from AC_CHECK_HEADER_STDBOOL, so, if we do as
> Paul suggests and make configure error out when there’s no usable
> Boolean, that will hard reject those compilers as well.  I’m fine with
> that.  Are you fine with that?

That should be OK as far as I know; Bruno may have further thoughts.

One other issue comes to mind: bool bitfield in structs. Although 
Autoconf currently tests that these compile, it doesn't test that bool 
bitfields of width 1 have the same memory layout as unsigned int 
bitfields of width 1. Emacs has this issue and already addresses it 
(look for "bool_bf" in the Emacs source). I don't think Autoconf needs 
to worry about bool bitfield implementation, as (a) the issue comes up 
pretty rarely and (b) neither current Autoconf nor current Gnulib 
addresses or solves the issue. However, the issue might be worth 
documenting in Autoconf.