Rich,

on Thu, 18 May 2023 09:29:05 -0400 you (Rich Felker <dalias@libc.org>)
wrote:

> Of course call_once is exempt from any such requirements

it was, but isn't anymore. In C23, now we have

    Completion of an effective call to the `call_once` function
    synchronizes with all subsequent calls to the `call_once` function
    with the same value of `flag`.

POSIX (for which the ISO 9945 instantiation is currently at NB ballot)
also has updated all of this

      The pthread_once() and call_once() functions shall synchronize
      memory for the first successful call in each thread for a given
      pthread_once_t or once_flag object, respectively. If the
      init_routine called by pthread_once() or call_once() is a
      cancellation point and is canceled, a successful call to
      pthread_once() for the same pthread_once_t object or to
      call_once() for the same once_flag object, made from a
      cancellation cleanup handler shall also synchronize memory.

If I understand this correctly, the C11 interfaces become mandatory if
the platform supports threads.


Thanks
Jₑₙₛ

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