[PATCH] stdlib: Do not define once_flag, ONCE_FLAG_INIT for C++

[Jonathan Wakely]

On Monday, 10 November 2025, Florian Weimer <fweimer@redhat.com> wrote:

> The definition of once_flag conflicts with std::once_flag in
> if “using namespace std;” is active.
>
> Updates commit a7ddbf456d97ac8d1aa7afd735e196a1488bd874
> ("Add once_flag, ONCE_FLAG_INIT and call_once to stdlib.h for C23").
>
> Suggested-by: Jonathan Wakely <jwakely@redhat.com>
>
> ---
>  manual/threads.texi | 17 +++++++++++++++--
>  stdlib/stdlib.h     |  4 +++-
>  2 files changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/manual/threads.texi b/manual/threads.texi
> index 1e9be32d66..e0587cc303 100644
> --- a/manual/threads.texi
> +++ b/manual/threads.texi
> @@ -23,8 +23,15 @@ POSIX threads.
>  This section describes the @glibcadj{} ISO C threads implementation.
>  To have a deeper understanding of this API, it is strongly recommended
>  to read ISO/IEC 9899:2011, section 7.26, in which ISO C threads were
> -originally specified.  All types and function prototypes are declared
> -in the header file @file{threads.h}.
> +originally specified.
> +
> +All types and function prototypes are declared in the header file
> +@file{threads.h}.  When compiling for C, some functionality is also
> +available in @file{stdlib.h}.
> +
> +It is recommended that GNU systems use the functionality in
> +@file{pthread.h} instead of @file{thread.h}, as they are more portable
> +between C and C++.
>
>  @menu
>  * ISO C Threads Return Values:: Symbolic constants that represent a
> @@ -203,11 +210,17 @@ called once in the presence of multiple, potentially
> calling threads.
>  @deftp {Data Type} once_flag
>  @standards{C11, threads.h}
>  A complete object type capable of holding a flag used by @code{call_once}.
> +
> +As of C23, @code{once_flags} is also defined in @file{stdlib.h},
>

s/once_flags/once_flag/



> +but only for C, not for C++.
>  @end deftp
>
>  @defvr Macro ONCE_FLAG_INIT
>  @standards{C11, threads.h}
>  This value is used to initialize an object of type @code{once_flag}.
> +
> +As of C23, @code{ONCE_FLAG_INIT} is also defined in @file{stdlib.h},
> +but only for C, not for C++.
>  @end defvr
>
>  @deftypefun void call_once (once_flag *@var{flag}, void (*@var{func})
> (void))
> diff --git a/stdlib/stdlib.h b/stdlib/stdlib.h
> index 8d4d194a46..8416bd4446 100644
> --- a/stdlib/stdlib.h
> +++ b/stdlib/stdlib.h
> @@ -1159,11 +1159,13 @@ extern int ttyslot (void) __THROW;
>  #endif
>
>  #if __GLIBC_USE (ISOC23)
> -# include <bits/types/once_flag.h>
> +# ifndef __cplusplus
> +#  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 /* !__cplusplus */
>
>  /* Return the alignment of P.  */
>  extern size_t memalignment (const void *__p);
>
> base-commit: 3a59076961ec7102e591e1f09a2324265841bbb7
>
>
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