Add once_flag, ONCE_FLAG_INIT and call_once to stdlib.h for C23

C23 adds once_flag, ONCE_FLAG_INIT and call_once to stdlib.h (in C11
they were only in threads.h, in C23 they are in both headers; this
change came from N2840).  Implement this change, with a
bits/types/once_flag.h header for the common type and initializer
definitions.

Note that there's an omnibus bug (bug 33001) that covers more than
just these missing definitions.

This doesn't seem a significant enough feature to be worth mentioning
in NEWS.

ISO C is not concerned with whether functions are in libc or
libpthread, but POSIX links this to what header they are declared in,
so functions declared in stdlib.h are supposed to be in libc.
However, the current edition of POSIX is based on C17; hopefully Hurd
glibc will have completed the merge of libpthread into libc (in
particular, moving call_once) well before a future edition of POSIX
based on C23 (or a later version of ISO C) is released.

Tested for x86_64 and x86.

bits/types/once_flag.h

@@ -0,0 +1,27 @@
+/* Define once_flag and ONCE_FLAG_INIT.
+   Copyright (C) 2018-2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef __once_flag_defined
+#define __once_flag_defined 1
+
+#include <bits/thread-shared-types.h>
+
+typedef __once_flag once_flag;
+#define ONCE_FLAG_INIT __ONCE_FLAG_INIT
+
+#endif

manual/threads.texi

@@ -219,6 +219,9 @@ invoked from several threads.  The completion of the function
 @code{call_once} with the same @code{flag} variable.
 @end deftypefun
 
+These definitions are from C11, where they appear in @file{threads.h}.
+In C23, they appear in @file{stdlib.h} as well as in @file{threads.h}.
+
 
 @node ISO C Mutexes
 @subsection Mutexes

stdlib/Makefile

@@ -37,6 +37,7 @@ headers := \
   bits/time64.h \
   bits/timesize.h \
   bits/types/error_t.h \
+  bits/types/once_flag.h \
   bits/wordsize.h \
   errno.h \
   fmtmsg.h \

stdlib/stdlib.h

@@ -1158,6 +1158,14 @@ extern int getloadavg (double __loadavg[], int __nelem)
 extern int ttyslot (void) __THROW;
 #endif
 
+#if __GLIBC_USE (ISOC23)
+# include <bits/types/once_flag.h>
+
+/* Call function __FUNC exactly once, even if invoked from several threads.
+   All calls must be made with the same __FLAGS object.  */
+extern void call_once (once_flag *__flag, void (*__func)(void));
+#endif
+
 #include <bits/stdlib-float.h>
 
 /* Define some macros helping to catch buffer overflows.  */

sysdeps/pthread/threads.h

@@ -25,6 +25,7 @@
 __BEGIN_DECLS
 
 #include <bits/thread-shared-types.h>
+#include <bits/types/once_flag.h>
 #include <bits/types/struct_timespec.h>
 
 #if (!defined __STDC_VERSION__				\
@@ -58,9 +59,6 @@ enum
   mtx_timed     = 2
 };
 
-typedef __once_flag once_flag;
-#define ONCE_FLAG_INIT __ONCE_FLAG_INIT
-
 typedef union
 {
   char __size[__SIZEOF_PTHREAD_MUTEX_T];