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Re: bool and C23

From: Bruno Haible
Subject: Re: bool and C23
Date: Sun, 18 Sep 2022 19:10:47 +0200

Paul Eggert wrote on 2022-09-10:
> I installed into Gnulib the stdbool patches ...
> After we have more experience with this in Gnulib, we can start thinking 
> about updating Autoconf.

As part of this "gathering experience", I compiled a gnulib testdir with
various compilers on various systems.

No problems seen
  - with GCC,
  - with clang (I tested all versions from clang 3.9 to clang 14),
  - with AIX xlc and xlclang.

But with Sun C++ on Solaris 10 (SOS11, __SUNPRO_CC=0x580 [1]) and
11.3 (solstudio12.2, __SUNPRO_CC=0x5110 [1]) I get a compilation error:

CC -xarch=generic64 -O -DHAVE_CONFIG_H -DEXEEXT=\"\" -I. -I../../gltests -I..   
-I../../gltests/..  -I../gllib -I../../gltests/../gllib 
-I/home/haible/prefix-x86_64/include -D_REENTRANT  -g -c -o test-stdbool-c++.o 
"../../gltests/", line 33: Error: A declaration was expected 
instead of ""error: true is not 1"".
1 Error(s) detected.
*** Error code 1

This compiler defines _BOOL, to indicate that 'bool', 'true', 'false' are
keywords [2]. But in the preprocessor, 'true' evaluates to 0, not 1 !
Although does not clearly state that 'true' and 'false'
must be usable as in preprocessor directives, such a compiler behaviour
will show up as bugs in application programs, when more and more applications
use 'true' and 'false' instead of old-style '1' and '0'. Thus, I think we
should fix this compiler behaviour.

What can the replacement be?

  #define true 1
does not work, because it would be of type 'int' and thus not behave right
w.r.t. overloaded functions, such as
  int func (int);
  int func (bool);

  #define true ((bool) 1)
fixes the type, but is not usable in preprocessor expressions, since casts
are not allowed there.

  #define true (!false)
works! It evaluates to 1 and is of the right type.

Done through the attached patches. The second one should get included into
Autoconf's AC_C_BOOL, when that comes to exist.


2022-09-18  Bruno Haible  <>

        stdbool: Ensure that 'true' can be used in the preprocessor.
        * m4/c-bool.m4 (gl_C_BOOL): With Sun C++, redefine 'true' if it does not
        evaluate to 1 in the preprocessor.

        stdbool-c99: Ensure that 'true' can be used in the preprocessor.
        * lib/ (true): Redefine if it does not evaluate to 1 in the

Attachment: 0001-stdbool-c99-Ensure-that-true-can-be-used-in-the-prep.patch
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Attachment: 0002-stdbool-Ensure-that-true-can-be-used-in-the-preproce.patch
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