Re: [FFmpeg-devel] [PATCH 07/25] fftools/ffmpeg: remove OutputStream.stream_copy

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Michael Niedermayer:
> On Wed, Aug 03, 2022 at 03:58:26PM +0200, Anton Khirnov wrote:
>> There are currently three possible modes for an output stream:
>> 1) The stream is produced by encoding output from some filtergraph. This
>>    is true when ost->enc_ctx != NULL, or equivalently when
>>    ost->encoding_needed != 0.
>> 2) The stream is produced by copying some input stream's packets. This
>>    is true when ost->enc_ctx == NULL && ost->source_index >= 0.
>> 3) The stream is produced by attaching some file directly. This is true
>>    when ost->enc_ctx == NULL && ost->source_index < 0.
>>
>> OutputStream.stream_copy is currently used to identify case 2), and
>> sometimes to confusingly (or even incorrectly) identify case 1). Remove
>> it, replacing its usage with checking enc_ctx/source_index values.
>> ---
>>  fftools/ffmpeg.c     | 23 +++++++++--------------
>>  fftools/ffmpeg.h     |  1 -
>>  fftools/ffmpeg_opt.c | 33 ++++++++++++---------------------
>>  3 files changed, 21 insertions(+), 36 deletions(-)
> 
> seems to break build with shared libs:
> 
> CC	fftools/ffmpeg.o
> src/fftools/ffmpeg.c:405:32: warning: suggest braces around initialization of subobject [-Wmissing-braces]
>     struct sigaction action = {0};
>                                ^
>                                {}
> src/fftools/ffmpeg.c:1612:37: error: address argument to atomic operation must be a pointer to non-const _Atomic type ('const atomic_uint_least64_t *' (aka 'const _Atomic(uint_least64_t) *') invalid)
>             uint64_t frame_number = atomic_load(&ost->packets_written);
>                                     ^           ~~~~~~~~~~~~~~~~~~~~~
> /usr/lib/llvm-6.0/lib/clang/6.0.0/include/stdatomic.h:134:29: note: expanded from macro 'atomic_load'
> #define atomic_load(object) __c11_atomic_load(object, __ATOMIC_SEQ_CST)
>                             ^                 ~~~~~~
> 1 warning and 1 error generated.
> src/ffbuild/common.mak:81: recipe for target 'fftools/ffmpeg.o' failed
> make: *** [fftools/ffmpeg.o] Error 1
> 
> [...]
> 

This has nothing to do with shared libs. It is just that C11 defines
atomic_load in such a way that it does not allow a pointer to const
atomic; presumably because atomic types might be emulated via mutexes in
which case even a read involves a write.

- Andreas
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