Re: C23 thoughts and opinions

David Brown <david.brown@hesbynett.no> writes:

On 23/05/2024 18:40, Keith Thompson wrote:

Michael S <already5chosen@yahoo.com> writes:
[...]

Removed

[...]

7) static_assert is not provided as a macro defined in <assert.h>
(becomes a keyword)
8) thread_local is not provided as a macro defined in <threads.h>
(becomes a keyword)
>

[...]

7) bad. Breaks existing code for weak reason
8) bad. Breaks existing code for weak reason

In pre-C23, _Static_assert and _Thread_local are keywords, and
static_assert and thread_local are macros that expand to those keywords.
In C23, _Static_assert, _Thread_local, static_assert, and
thread_local
are all keywords.  Code that simply uses the old ugly keywords would not
break.
Code that does something like "#ifdef static_assert".  I suppose the
headers could have retained the old macro definitions.
     #define static_assert static_assert
     #define thread_local thread_local
 

>
The sort of code that could theoretically break is when you have
definitions like this:
>
#define STATIC_ASSERT_NAME_(line)       STATIC_ASSERT_NAME2_(line)
#define STATIC_ASSERT_NAME2_(line)      assertion_failed_at_line_##line
#define static_assert(claim, warning) \
    typedef struct { \
        char STATIC_ASSERT_NAME_(__COUNTER__) [(claim) ? 2 : -2]; \
    } STATIC_ASSERT_NAME_(__COUNTER__)
>
That works in any C version, until C23, almost as well as
_static_assert.  I used this when C11 support was rare in the tools I
used.

You mean _Static_assert.

While using #define for a C keyword is undefined behaviour, in
practice I think you'd have a hard time finding code and a compiler
that used such a macro and which did not work just as well in C23
mode.
>
(I don't know if anyone is in the habit of declaring macros named
"thread_local".)

"static_assert" is already a macro defined in <assert.h> starting in
C11.  The above code is valid in pre-C23, but will break in C11 and C17
if it includes <assert.h> directly or indirectly.  You can fix it by
adding "#undef static_assert" or by picking a different name, or by
making your macro definition conditional on __STDC_VERSION__ >= 202311L.

--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */