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Message-ID: <g2kgwwwliakevnpovnqtx3hyvnkwlpqvnjuzjj5ittjdsyubqs@i57staomvmlq>
Date: Wed, 26 Nov 2025 16:36:38 +0100
From: Alejandro Colomar <alx@...nel.org>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH v2 1/1] include/string.h: Implement QChar wrappers
standardized in C23
Hi Rich,
On Wed, Nov 26, 2025 at 10:02:22AM -0500, Rich Felker wrote:
> On Wed, Nov 26, 2025 at 03:56:21PM +0100, Alejandro Colomar wrote:
[...]
> It doesn't have to be functionally breaking common code in the wild to
> be breaking the *contract*. You'd not allowed to change them because
> the specification fixes the signatures in stone.
Agree. Given how few functions use QChar in the standard library, and
how stable the standard library is, typos would be unlikely and would be
obvious enough that they wouldn't pass any review.
> > > > > Also, can't __Qcharof just be defined as something like
> > > > > typeof(1?(s):"") without any fancy _Generic machinery?
> > > >
> > > > This wouldn't accept void*, and these functions should accept void*
> > > > arguments.
> > >
> > > I was thinking the ternary of char* and void* would produce char*, but
> > > indeed I'm always wrong about that.
> >
> > You weren't totally wrong. There's special case where that's true: NULL
> >
> > alx@...uan:~/tmp$ cat typeof.c
> > #include <stdio.h>
> > int
> > main(void)
> > {
> > const char *cc;
> > const void *cv;
> > char *c;
> > void *v;
> >
> > _Generic(typeof(1?cc:""), const char *: 0);
> > _Generic(typeof(1?cv:""), const char *: 0);
> > _Generic(typeof(1? c:""), char *: 0);
> > _Generic(typeof(1? v:""), char *: 0);
> >
> > _Generic(typeof(1?(void *) 0:""), const char *: 0);
> > _Generic(typeof(1?(const void *)0:""), char *: 0);
> > }
> > alx@...uan:~/tmp$ gcc typeof.c
> > typeof.c: In function ‘main’:
> > typeof.c:11:18: error: ‘_Generic’ selector of type ‘const void *’ is not compatible with any association
> > 11 | _Generic(typeof(1?cv:""), const char *: 0);
> > | ^~~~~~
> > typeof.c:13:18: error: ‘_Generic’ selector of type ‘void *’ is not compatible with any association
> > 13 | _Generic(typeof(1? v:""), char *: 0);
> > | ^~~~~~
> > typeof.c:15:18: error: ‘_Generic’ selector of type ‘char *’ is not compatible with any association
> > 15 | _Generic(typeof(1?(void *) 0:""), const char *: 0);
> > | ^~~~~~
> > typeof.c:16:18: error: ‘_Generic’ selector of type ‘const void *’ is not compatible with any association
> > 16 | _Generic(typeof(1?(const void *)0:""), char *: 0);
> > | ^~~~~~
> >
> > As you can see, typeof(1?(void*)0:"") produces a char*. Any other void*
> > doesn't. This means that the type of NULL is a magic type different
> > from void*, but which is compatible with void*. This is slightly
> > different from nullptr_t in that _Generic(3) is able to distinguish
> > nullptr_t form void*, but not the type of NULL from void*, but it's
> > pretty similar.
>
> typeof(1?"":(typeof(s))0) maybe? I think it works!
Almost! It's closer. :)
alx@...uan:~/tmp$ cat typeof.c
#include <stdio.h>
#if defined TRY1
# define QCharptrof(s) typeof(1?s:"")
#elif defined TRY2
# define QCharptrof(s) typeof(1?"":(typeof(s))0)
#else
# define QCharptrof(s) typeof(QCharof(s) *)
# define QCharof(s) typeof \
( \
_Generic(s, \
const char *: (const char){}, \
const void *: (const char){}, \
char *: (char){}, \
void *: (char){} \
) \
)
#endif
int
main(void)
{
const char *cc;
const void *cv;
char *c;
void *v;
_Generic(QCharptrof( cc), const char *: 0);
_Generic(QCharptrof( cv), const char *: 0);
_Generic(QCharptrof( c), char *: 0);
_Generic(QCharptrof( v), char *: 0);
_Generic(QCharptrof(NULL), char *: 0);
}
alx@...uan:~/tmp$ gcc typeof.c
alx@...uan:~/tmp$ gcc -D TRY1 typeof.c
typeof.c: In function ‘main’:
typeof.c:4:25: error: ‘_Generic’ selector of type ‘const void *’ is not compatible with any association
4 | # define QCharptrof(s) typeof(1?s:"")
| ^~~~~~
typeof.c:31:18: note: in expansion of macro ‘QCharptrof’
31 | _Generic(QCharptrof( cv), const char *: 0);
| ^~~~~~~~~~
typeof.c:4:25: error: ‘_Generic’ selector of type ‘void *’ is not compatible with any association
4 | # define QCharptrof(s) typeof(1?s:"")
| ^~~~~~
typeof.c:33:18: note: in expansion of macro ‘QCharptrof’
33 | _Generic(QCharptrof( v), char *: 0);
| ^~~~~~~~~~
alx@...uan:~/tmp$ gcc -D TRY2 typeof.c
typeof.c: In function ‘main’:
typeof.c:7:25: error: ‘_Generic’ selector of type ‘const void *’ is not compatible with any association
7 | # define QCharptrof(s) typeof(1?"":(typeof(s))0)
| ^~~~~~
typeof.c:31:18: note: in expansion of macro ‘QCharptrof’
31 | _Generic(QCharptrof( cv), const char *: 0);
| ^~~~~~~~~~
Have a lovely day!
Alex
--
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).
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