svn commit: r358392 - head/share/man/man9

[Mark Millard]

On 2020-Feb-27, at 16:37, John Baldwin <jhb at FreeBSD.org> wrote:

> On 2/27/20 2:45 PM, Mark Millard wrote:
>> John Baldwin jhb at FreeBSD.org wrote on
>> Thu Feb 27 16:55:01 UTC 2020:
>> 
>>> On 2/27/20 7:30 AM, Warner Losh wrote:
>>>> Author: imp
>>>> Date: Thu Feb 27 15:30:13 2020
>>>> New Revision: 358392
>>>> URL: https://svnweb.freebsd.org/changeset/base/358392
>>>> 
>>>> Log:
>>>>  _Static_assert is to be preferred to CTASSERT.
>>>> 
>>>>  Document the existing prefernce that _Static_assert be used in preference to the
>>>>  old CTASSERT we used to use for compile time assertions.
>>> 
>>> Actually, I think what we want to use is static_assert().  The intention in
>>> userland C is that _Static_assert() is an internal keyword and <assert.h>
>>> adds static_assert() as an alias, similar to <stdalign.h> defining alignas,
>>> etc.  I think what we should do for the kernel is have <sys/systm.h> define
>>> map static_assert to _Static_assert and replace existing _Static_assert
>>> usage with the proper spelling.
>>> 
>> 
>> Be warned static_assert is a C++ keyword as of C++11.
>> 
>> c++11 added: static_assert(bool_constexpr,message)
>> c++17 added: static_assert(bool_constexpr)
>> 
>> C11 added _Static_assert(expression,message)
>> C2x gets  _Static_assert(expression)
>> 
>> C11 added "#define static_assert _Static_assert" to <assert.h>
>> 
>> It makes for a bit of a mess in code to be allowed
>> to be processed by both C and C++.
>> 
>> The wording may need to specify enough to tell what to
>> do for such code and the headers may need logic to
>> cause that context to exist across both languages
>> when the header is allowed for both.
> 
> The intent of the C11 changes is to permit equivalent use of static_assert()
> in both languages.  The #define in <assert.h> is guarded to not kick in for
> C++.

Ahh. I had vague memories of past oddities in the area
that I was involved in. So I looked and found (in head
-r3578132 source) :

# more /usr/src/sys/sys/cdefs.h
. . .
#if !__has_extension(c_static_assert)
#if (defined(__cplusplus) && __cplusplus >= 201103L) || \
    __has_extension(cxx_static_assert)
#define _Static_assert(x, y)    static_assert(x, y)
#elif __GNUC_PREREQ__(4,6) && !defined(__cplusplus)
/* Nothing, gcc 4.6 and higher has _Static_assert built-in */
#elif defined(__COUNTER__)
#define _Static_assert(x, y)    __Static_assert(x, __COUNTER__)
#define __Static_assert(x, y)   ___Static_assert(x, y)
#define ___Static_assert(x, y)  typedef char __assert_ ## y[(x) ? 1 : -1] \
                                __unused
#else
#define _Static_assert(x, y)    struct __hack
#endif
#endif
. . .

# more /usr/include/assert.h
. . .
/*
 * Static assertions.  In principle we could define static_assert for
 * C++ older than C++11, but this breaks if _Static_assert is
 * implemented as a macro.
 *
 * C++ template parameters may contain commas, even if not enclosed in
 * parentheses, causing the _Static_assert macro to be invoked with more
 * than two parameters.
 */
#if __ISO_C_VISIBLE >= 2011 && !defined(__cplusplus)
#define static_assert   _Static_assert
#endif
. . .

So one can define a _Static_assert macro, sometimes in terms of
static_assert, and the other sometimes defines a static_assert
macro in terms of _Static_assert.

Messy, but possibly localized.

That prompted a more general grep:

# grep -r "define[     _]*[Ss]tatic_assert" /usr/src/* | more
/usr/src/contrib/llvm-project/libcxx/include/__config:# define static_assert(...) _Static_assert(__VA_ARGS__)
/usr/src/contrib/llvm-project/libunwind/src/config.h:  #define static_assert(__b, __m) \
/usr/src/include/assert.h: * Static assertions.  In principle we could define static_assert for
/usr/src/include/assert.h:#define       static_assert   _Static_assert
/usr/src/sys/sys/cdefs.h:#define        _Static_assert(x, y)    static_assert(x, y)
/usr/src/sys/sys/cdefs.h:#define        _Static_assert(x, y)    __Static_assert(x, __COUNTER__)
/usr/src/sys/sys/cdefs.h:#define        __Static_assert(x, y)   ___Static_assert(x, y)
/usr/src/sys/sys/cdefs.h:#define        ___Static_assert(x, y)  typedef char __assert_ ## y[(x) ? 1 : -1] \
/usr/src/sys/sys/cdefs.h:#define        _Static_assert(x, y)    struct __hack
/usr/src/usr.bin/dtc/util.hh:#define static_assert(x, y) ((void)0)

So not much else.

===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)