[vlc-devel] [PATCH v2] thread: fix compilation with clang 7 or older

[Thomas Guillem]

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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