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

[Steve Lhomme]

On 2020-02-10 11:52, Rémi Denis-Courmont wrote:
> Ok. Do we have any relevant platform where unions have high alignment 
> than uint regardless of their members?

No idea. But it's probably safer not too assume too many things.

Also we're assuming the alignment of the structure in C code is the same 
as the structure in C++. Maybe it's not.

> 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)
> 
> 
> -- 
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> 
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