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Message-ID: <20251126013439.GV1827@brightrain.aerifal.cx>
Date: Tue, 25 Nov 2025 20:34:39 -0500
From: Rich Felker <dalias@...c.org>
To: Alejandro Colomar <alx@...nel.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH v2 1/1] include/string.h: Implement QChar wrappers
standardized in C23
On Tue, Nov 25, 2025 at 10:49:29PM +0100, Alejandro Colomar wrote:
> I used compound literals instead of casts because they're less
> dangerous. They only allow conversions that would be legal implicitly.
>
> The register storage-class specifier is used because compound literals
> are lvalues. register makes sure one can't take the address of them.
> That is, it rejects:
>
> &strchr(s, c);
>
> Signed-off-by: Alejandro Colomar <alx@...nel.org>
> ---
> include/string.h | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/include/string.h b/include/string.h
> index 83e2b946..c492698a 100644
> --- a/include/string.h
> +++ b/include/string.h
> @@ -24,6 +24,16 @@ extern "C" {
>
> #include <bits/alltypes.h>
>
> +#define __QCharof(s) typeof \
> +( \
> + _Generic(s, \
> + const char *: (const char){}, \
> + const void *: (const char){}, \
> + char *: (char){}, \
> + void *: (char){} \
> + ) \
> +)
> +
> void *memcpy (void *__restrict, const void *__restrict, size_t);
> void *memmove (void *, const void *, size_t);
> void *memset (void *, int, size_t);
> @@ -55,6 +65,13 @@ size_t strlen (const char *);
>
> char *strerror (int);
>
> +#if __STDC_VERSION__ >= 202311L
> +# define strchr(s, ...) ((register __QCharof(s) *){ strchr(s, __VA_ARGS__)})
> +# define strrchr(s, ...) ((register __QCharof(s) *){strrchr(s, __VA_ARGS__)})
> +# define strpbrk(s, ...) ((register __QCharof(s) *){strpbrk(s, __VA_ARGS__)})
> +# define strstr(s, ...) ((register __QCharof(s) *){ strstr(s, __VA_ARGS__)})
> +#endif
> +
> #if defined(_BSD_SOURCE) || defined(_GNU_SOURCE)
> #include <strings.h>
> #endif
> @@ -95,6 +112,10 @@ char *strchrnul(const char *, int);
> char *strcasestr(const char *, const char *);
> void *memrchr(const void *, int, size_t);
> void *mempcpy(void *, const void *, size_t);
> +# if __STDC_VERSION__ >= 202311L
> +# define strchrnul(s, ...) ((register __QCharof(s) *){ strchrnul(s, __VA_ARGS__)})
> +# define strcasestr(s, ...) ((register __QCharof(s) *){strcasestr(s, __VA_ARGS__)})
> +# endif
> #endif
>
> #ifdef __cplusplus
> --
> 2.51.0
Could you explain a bit about your motivations for doing it this way?
Why are compound literals needed at all instead of just a value cast?
Also, can't __Qcharof just be defined as something like
typeof(1?(s):"") without any fancy _Generic machinery?
Rich
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