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Message-ID: <aShn5OcYw3EItA3h@intrepid>
Date: Thu, 27 Nov 2025 16:01:56 +0100
From: Markus Wichmann <nullplan@....net>
To: musl@...ts.openwall.com
Cc: Alejandro Colomar <alx@...nel.org>
Subject: Re: [PATCH v2 1/1] include/string.h: Implement QChar wrappers
standardized in C23
Am Wed, Nov 26, 2025 at 04:18:25PM -0500 schrieb Rich Felker:
> Hmm, maybe it doesn't work. It looks like only exactly (void *)0 is a
> null pointer constant. A cast to a qualified void pointer type isn't
> one. I'm not sure if it's fixable.
>
> Rich
There really is no expression that turns void* into char*, or at least
not that I could find. But it is possible to simplify the generic
expression Alejandro is using:
#define _QChar(s) typeof(_Generic(1?(s):(void *)"", void *: (char *)0, const void *: (const char *)0))
#define strstr(s, c) ((_QChar(s))strstr(s, c))
Feel free to reformat as you like.
The controlling expression of the generic is compatible with all pointer
types, and always becomes a void* or const void*. This means it already
has the correct type for the mem* functions and bsearch(), once you
start looking at those. For the str* functions, the only way I see to
get from those to char* is the generic expression.
The idea is to use an expression to query the const qualifier only.
According to the rules for conditional operators, I will get an equally
qualified void pointer only if the third expression is a non-null void
pointer (I would get the original type of s if it were a null pointer
constant). The simplest non-null pointer I can find is a string
literal.
I think it is better to let the const correctness macro only handle the
return value, because it leads to better error messages. With
Alejandro's current attempt, something like
int *p;
strstr(p, "");
will lead to an error message about no case existing for int* in the
generic expression, whereas my solution will leave the original error
message the compiler would generate from initializing a const char *
from an int * in place.
Ciao,
Markus
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