stdnoreturn.h (GNU Gnulib)

POSIX specification:
Not in POSIX yet, but we expect it will be. ISO C11 (latest free draft http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf) sections 7.23.

Gnulib module: stdnoreturn

Portability problems fixed by Gnulib:

This header file is missing on many platforms: FreeBSD 6.4, NetBSD 7.1, OpenBSD 6.7, Minix 3.3.0, AIX 7.1, HP-UX 11.31, IRIX 6.5, Solaris 11.3, Cygwin 2.9.0, mingw, MSVC 14, Android 9.0.
This file conflicts with some system header files, such as <stdlib.h> and <process.h>, on some platforms: MSVC/clang.

Portability problems not fixed by Gnulib:

<stdnoreturn.h> and the noreturn macro are obsolescent in C23.
<stdnoreturn.h> cannot be #included in C++ mode on some platforms: FreeBSD 13.1.
<stdnoreturn.h> should be #included before ‘_Noreturn’ is used.
You cannot assume that _Noreturn is a reserved word; it might be a macro.
When the macro lint is defined, standard headers define _Noreturn (and therefore noreturn) to be a macro that expands to the empty token sequence on some platforms: Cygwin 2.5.1, FreeBSD 10.3.
On Cygwin 1.7.30 and MSVC 14, noreturn expands to the empty token sequence, to avoid problems with standard headers that use noreturn in combination with __attribute__ or __declspec. Although the resulting code operates correctly, the compiler is not informed whether noreturn functions do not return, so it may generate incorrect warnings at compile-time, or code that is slightly less optimized. This problem does not occur with _Noreturn.
Circa 2012 bleeding-edge GCC with -Werror=old-style-declaration requires _Noreturn or noreturn before the returned type in a declaration, and therefore rejects valid but unusually-worded declarations such as void _Noreturn foo (void);.