[vlc-devel] [PATCH 2/2] thread: fix compilation with clang 7 or older

[Denis Charmet]

Hi,

To settle this matter, I ask for this patch to be put under the 
scrutiny of the TC.

On 2020-02-24 16:45, Marvin Scholz wrote:
> According to the standard, C11's atomic_load_explicit did not allow
> a const qualified first argument, even though it never modifies it.
> Clang versions 7 or older did implement this quite strictly and
> therefore errors when compiling this code:
> 
>   address argument to atomic operation must be a pointer to non-const
>   _Atomic type ('const _Atomic(const void *) *' invalid)
> 
> See https://reviews.llvm.org/D47618 and 
> https://reviews.llvm.org/D47613
> 
> C-standard wise this is fixed in C17, see DR459
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2244.htm#dr_459
> ---
>  src/misc/threads.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/src/misc/threads.c b/src/misc/threads.c
> index ea78e37453..6f5255505f 100644
> --- a/src/misc/threads.c
> +++ b/src/misc/threads.c
> @@ -139,7 +139,14 @@ bool vlc_mutex_held(const vlc_mutex_t *mtx)
>       * Even though other threads may modify the owner field at any 
> time,
>       * they will never make it compare equal to the calling thread.
>       */
> -    return THREAD_SELF == atomic_load_explicit(&mtx->owner,
> +
> +    /* The explicit cast to non-const is needed to workaround a clang
> +     * error with clang 7 or lower, as atomic_load_explicit in C11 
> does not
> +     * allow its first argument to be const-qualified, and clang 
> implmented
> +     * this like that.
> +     * This is fixed in clang version 8 or newer, and in the C17 
> standard.
> +     */
> +    return THREAD_SELF == atomic_load_explicit(&((vlc_mutex_t 
> *)mtx)->owner,
>                                                 memory_order_relaxed);
>  }

Regards,
-- 
Denis Charmet - TypX
Le mauvais esprit est un art de vivre