[vlc-devel] [PATCH 3/4] threads: store the condition variable value as an atomic_uint with LIBVLC_NEED_CONDVAR

[Rémi Denis-Courmont]

Ok. Do we have any relevant platform where unions have high alignment than uint regardless of their members?

Le 10 février 2020 11:02:21 GMT+02:00, Steve Lhomme <robux4 at ycbcr.xyz> a écrit :
>We can include stdatomic.h in the headers, we do it in many places. But
>we
>can't use it with C++ because the std:atomic_int storage is likely
>different
>and can be initialized/released the same way.
>
>We change the static assert in the core to make sure the type in the
>union that
>will be used for storage by C++ modules is correct.
>---
> include/vlc_threads.h | 12 +++++++++++-
> src/misc/threads.c    |  8 ++++----
> 2 files changed, 15 insertions(+), 5 deletions(-)
>
>diff --git a/include/vlc_threads.h b/include/vlc_threads.h
>index f30f0699e37..1a05e57b82c 100644
>--- a/include/vlc_threads.h
>+++ b/include/vlc_threads.h
>@@ -351,9 +351,19 @@ typedef struct vlc_timer *vlc_timer_t;
> #endif
> 
> #ifdef LIBVLC_NEED_CONDVAR
>+#ifndef __cplusplus
>+#include <stdatomic.h>
>+#endif
>+
>+
> typedef struct
> {
>-    unsigned value;
>+    union {
>+#ifndef __cplusplus
>+        atomic_uint value;
>+#endif
>+        int        cpp_value;
>+    };
> } vlc_cond_t;
> # define VLC_STATIC_COND { 0 }
> #endif
>diff --git a/src/misc/threads.c b/src/misc/threads.c
>index 18855e85bd4..304ae972199 100644
>--- a/src/misc/threads.c
>+++ b/src/misc/threads.c
>@@ -210,12 +210,12 @@ void (vlc_tick_sleep)(vlc_tick_t delay)
> 
> static inline atomic_uint *vlc_cond_value(vlc_cond_t *cond)
> {
>-    /* XXX: ugly but avoids including stdatomic.h in vlc_threads.h */
>-    static_assert (sizeof (cond->value) <= sizeof (atomic_uint),
>+    /* Don't use C++ atomic types in vlc_threads.h for the C atomic
>storage */
>+    static_assert (sizeof (cond->cpp_value) <= sizeof (cond->value),
>                    "Size mismatch!");
>-    static_assert ((alignof (cond->value) % alignof (atomic_uint)) ==
>0,
>+    static_assert ((alignof (cond->cpp_value) % alignof (cond->value))
>== 0,
>                    "Alignment mismatch");
>-    return (atomic_uint *)&cond->value;
>+    return &cond->value;
> }
> 
> void vlc_cond_init(vlc_cond_t *cond)
>-- 
>2.17.1
>
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