Several standard library functions are specified to return an unqualified
pointer while accepting a pointer to a potentially const-qualified object.
N3020 addresses this behaviour, discarding qualifiers due to incompatible
pointer types, by introducing qualifier-preserving macros for the affected
set of standard library functions.
Add `__qualsel()` helper to `<sys/cdefs.h>`, implemented using the generic
selection, and define qualifier-preserving macros for that set of functions
in `<string.h>`, `<wchar.h>`, and `<stdlib.h>`.
Macros are gated on `_STDC_VERSION__ >= 202311L && !__cplusplus`, therefore
there is no behavioural change for earlier C modes or C++ translation units.
The kernel is likewise unaffected, as it does not include userland headers.
As function-like macros, they are transparent except at a call site where
the address-of operator is applied, the macro is suppressed via `#undef`,
or the identifier appears in parenthesised form; all of which cause the
underlying function designator to be used instead.
Reviewed by: fuz
Approved by: fuz (mentor)
MFC after: 1 month
Pull Request: https://github.com/freebsd/freebsd-src/pull/2288
Faraz Vahedi
d08296c7ab