[vlc-devel] [PATCH 1/3] threads: generic one-time initializer

[Steve Lhomme]

what's the point of adding #if 0 code ?

On 2020-04-02 16:28, Rémi Denis-Courmont wrote:
> ---
>   include/vlc_threads.h | 20 ++++++++++++++++++++
>   src/misc/threads.c    | 40 ++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 60 insertions(+)
> 
> diff --git a/include/vlc_threads.h b/include/vlc_threads.h
> index 131e319622..b04a3305c1 100644
> --- a/include/vlc_threads.h
> +++ b/include/vlc_threads.h
> @@ -633,6 +633,25 @@ VLC_API void vlc_rwlock_unlock(vlc_rwlock_t *);
>   
>   /** @} */
>   
> +#ifndef __cplusplus
> +#if 0
> +/**
> + * One-time initialization.
> + *
> + * A one-time initialization object must always be initialized assigned to
> + * \ref VLC_STATIC_ONCE before use.
> + */
> +typedef struct
> +{
> +    atomic_uint value;
> +} vlc_once_t;
> +
> +/**
> + * Static initializer for one-time initialization.
> + */
> +#define VLC_STATIC_ONCE { ATOMIC_VAR_INIT(0) }
> +#endif
> +
>   /**
>    * Executes a function one time.
>    *
> @@ -650,6 +669,7 @@ VLC_API void vlc_rwlock_unlock(vlc_rwlock_t *);
>    * \param cb callback to execute (the first time)
>    */
>   VLC_API void vlc_once(vlc_once_t *restrict once, void (*cb)(void));
> +#endif
>   
>   /**
>    * \defgroup threadvar Thread-specific variables
> diff --git a/src/misc/threads.c b/src/misc/threads.c
> index b1eb9888f2..11c46fa730 100644
> --- a/src/misc/threads.c
> +++ b/src/misc/threads.c
> @@ -533,3 +533,43 @@ int vlc_sem_timedwait(vlc_sem_t *sem, vlc_tick_t deadline)
>   
>       return 0;
>   }
> +
> +#if 0
> +enum { VLC_ONCE_UNDONE, VLC_ONCE_DOING, VLC_ONCE_CONTEND, VLC_ONCE_DONE };
> +
> +void vlc_once(vlc_once_t *restrict once, void (*cb)(void))
> +{
> +    unsigned int value = VLC_ONCE_UNDONE;
> +
> +    if (atomic_compare_exchange_strong_explicit(&once->value, &value,
> +                                                VLC_ONCE_DOING,
> +                                                memory_order_acquire,
> +                                                memory_order_acquire)) {
> +        /* First time: run the callback */
> +        cb();
> +
> +        if (atomic_exchange_explicit(&once->value, VLC_ONCE_DONE,
> +                                     memory_order_release) == VLC_ONCE_CONTEND)
> +            /* Notify waiters if any */
> +            vlc_atomic_notify_all(&once->value);
> +
> +        return;
> +    }
> +
> +    assert(value >= VLC_ONCE_DOING);
> +
> +    if (unlikely(value == VLC_ONCE_DOING)
> +     && atomic_compare_exchange_strong_explicit(&once->value, &value,
> +                                                VLC_ONCE_CONTEND,
> +                                                memory_order_acquire,
> +                                                memory_order_acquire))
> +        value = VLC_ONCE_CONTEND;
> +
> +    assert(value >= VLC_ONCE_CONTEND);
> +
> +    while (unlikely(value != VLC_ONCE_DONE)) {
> +        vlc_atomic_wait(&once->value, VLC_ONCE_CONTEND);
> +        value = atomic_load_explicit(&once->value, memory_order_acquire);
> +    }
> +}
> +#endif
> -- 
> 2.26.0
> 
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