[vlc-devel] [PATCH v2] thread: fix compilation with clang 7 or older
Thomas Guillem
thomas at gllm.fr
Mon Mar 2 09:59:40 CET 2020
On Mon, Mar 2, 2020, at 09:28, Steve Lhomme wrote:
> On 2020-03-02 8:53, Marvin Scholz wrote:
> > C11's atomic_load_explicit did not allow a const qualified first
> > argument, even though it never modifies it (see DR459).
> > 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
> > ---
> > src/misc/threads.c | 12 +++++++++++-
> > 1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/src/misc/threads.c b/src/misc/threads.c
> > index ea78e37453..db90a27240 100644
> > --- a/src/misc/threads.c
> > +++ b/src/misc/threads.c
> > @@ -135,11 +135,21 @@ static _Thread_local char thread_self[1];
> >
> > bool vlc_mutex_held(const vlc_mutex_t *mtx)
> > {
> > +#if defined(__clang__) && !defined(__apple_build_version__) && (__clang_major__ < 8)
>
> The Apple 7- versions don't have this issue ?
>
> IMO it's OK not to add the distinction for this compiler flavor.
> Otherwsise we'd have to check which version from which vendor has the issue.
>
> LGTM otherwise.
Thanks for the patch. Ok for me too.
>
> > + /* The explicit cast to non-const is needed to workaround a clang
> > + * error with clang 7 or lower, as atomic_load_explicit in C11 did not
> > + * allow its first argument to be const-qualified (see DR459)
> > + */
> > + vlc_mutex_t *tmp_mtx = (vlc_mutex_t *)mtx;
> > +#else
> > + const vlc_mutex_t *tmp_mtx = mtx;
> > +#endif
> > +
> > /* This comparison is thread-safe:
> > * 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,
> > + return THREAD_SELF == atomic_load_explicit(&tmp_mtx->owner,
> > memory_order_relaxed);
> > }
> >
> > --
> > 2.21.1 (Apple Git-122.3)
> >
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