stddef.h: Update for C23

[minsoochoo0122_proton.me]

Summary

unreachable() is now a macro for __unreachable.
nullptr_t is defined according to C23 draft.
NULL is now a macro for nullptr when -std=c23 or -std=gnu23

[imp]

include/stddef.h

37

yea, hate this construct. I don't really understand the point and in this case, it seems totally wrong.

[minsoochoo0122_proton.me]

include/stddef.h

37

yea, hate this construct. I don't really understand the point and in this case, it seems totally wrong.

C23 N3220 draft

This is a version test macro that tells the current implmented C standard version of a header file.

• 7.1.2 (Standard Headers)
  • 6: Some standard headers define or declare identifiers that had not been present in previous versions of this document. To allow implementations and users to adapt to that situation, they also define a version macro for feature test of the form __STDC_VERSION_ XXXX_H__ which expands to 202311L, where XXXX is the all-caps spelling of the corresponding header <xxxx.h>.

[imp]

include/stddef.h

37

yea, hate this construct. I don't really understand the point and in this case, it seems totally wrong.

C23 N3220 draft

This is a version test macro that tells the current implmented C standard version of a header file.

• 7.1.2 (Standard Headers)
  • 6: Some standard headers define or declare identifiers that had not been present in previous versions of this document. To allow implementations and users to adapt to that situation, they also define a version macro for feature test of the form __STDC_VERSION_ XXXX_H__ which expands to 202311L, where XXXX is the all-caps spelling of the corresponding header <xxxx.h>.

Oh! That's different.
Maybe we could just use this as the guard #define then and eliminate _STDDEF_H_

Then I think this is ready to go.

sys/sys/_null.h

53

A question for the future: Do we need to provide a fallback for old-versions for nullptr_t in the future? This is needed by the standard, so the change stands on its own... more food for thought