[FFmpeg-devel] [PATCH 2/5] h264_metadata: Add support for A/53 closed captions

[Mark Thompson]

On 27/03/18 21:38, Michael Niedermayer wrote:
> On Tue, Mar 27, 2018 at 01:31:43AM +0100, Mark Thompson wrote:
>> On 27/03/18 01:20, Michael Niedermayer wrote:
>>> On Sun, Mar 25, 2018 at 06:41:34PM +0100, Mark Thompson wrote:
>>>> Allows insertion (from side data), extraction (to side data), and removal
>>>> of closed captions in SEI messages.
>>>> ---
>>>>  libavcodec/Makefile            |   2 +-
>>>>  libavcodec/h264_metadata_bsf.c | 138 +++++++++++++++++++++++++++++++++++++++++
>>>>  2 files changed, 139 insertions(+), 1 deletion(-)
>>>
>>> This breaks build on mips-linux-gnu-gcc-4.4 (Debian 4.4.5-8) 4.4.5
>>>
>>>
>>> In file included from src/libavcodec/cbs_misc.c:25:
>>> src/libavcodec/cbs_misc.h:73: warning: declaration does not declare anything
>>> src/libavcodec/cbs_misc.h:86: warning: declaration does not declare anything
>>> In file included from src/libavcodec/cbs_misc.c:57:
>>> src/libavcodec/cbs_misc_syntax_template.c: In function ‘cbs_misc_read_a53_atsc_user_data’:
>>> src/libavcodec/cbs_misc_syntax_template.c:102: error: ‘A53ATSCUserData’ has no member named ‘cc_data’
>>> src/libavcodec/cbs_misc_syntax_template.c:104: error: ‘A53ATSCUserData’ has no member named ‘bar_data’
>>> src/libavcodec/cbs_misc_syntax_template.c: In function ‘cbs_misc_read_a53_user_data’:
>>> src/libavcodec/cbs_misc_syntax_template.c:140: error: ‘A53UserData’ has no member named ‘atsc’
>>> src/libavcodec/cbs_misc_syntax_template.c:142: error: ‘A53UserData’ has no member named ‘afd’
>>> In file included from src/libavcodec/cbs_misc.c:82:
>>> src/libavcodec/cbs_misc_syntax_template.c: In function ‘cbs_misc_write_a53_atsc_user_data’:
>>> src/libavcodec/cbs_misc_syntax_template.c:102: error: ‘A53ATSCUserData’ has no member named ‘cc_data’
>>> src/libavcodec/cbs_misc_syntax_template.c:104: error: ‘A53ATSCUserData’ has no member named ‘bar_data’
>>> src/libavcodec/cbs_misc_syntax_template.c: In function ‘cbs_misc_write_a53_user_data’:
>>> src/libavcodec/cbs_misc_syntax_template.c:140: error: ‘A53UserData’ has no member named ‘atsc’
>>> src/libavcodec/cbs_misc_syntax_template.c:142: error: ‘A53UserData’ has no member named ‘afd’
>>> src/libavcodec/cbs_misc.c: In function ‘ff_cbs_read_a53_cc_side_data’:
>>> src/libavcodec/cbs_misc.c:153: error: unknown field ‘atsc’ specified in initializer
>>> src/libavcodec/cbs_misc.c:153: error: extra brace group at end of initializer
>>> src/libavcodec/cbs_misc.c:153: error: (near initialization for ‘(anonymous)’)
>>> src/libavcodec/cbs_misc.c:156: error: extra brace group at end of initializer
>>> src/libavcodec/cbs_misc.c:156: error: (near initialization for ‘(anonymous)’)
>>> src/libavcodec/cbs_misc.c:165: warning: excess elements in struct initializer
>>> src/libavcodec/cbs_misc.c:165: warning: (near initialization for ‘(anonymous)’)
>>> src/libavcodec/cbs_misc.c:167: error: ‘A53UserData’ has no member named ‘atsc’
>>> src/libavcodec/cbs_misc.c: In function ‘ff_cbs_write_a53_cc_side_data’:
>>> src/libavcodec/cbs_misc.c:193: error: ‘A53UserData’ has no member named ‘atsc’
>>> src/libavcodec/cbs_misc.c:196: error: ‘A53UserData’ has no member named ‘atsc’
>>> CC	libavcodec/dpxenc.o
>>> CC	libavcodec/dpx_parser.o
>>> make: *** [libavcodec/cbs_misc.o] Error 1
>>> make: *** Waiting for unfinished jobs....
>>> src/libavcodec/dnxhddec.c:146: warning: ‘dnxhd_decode_init_thread_copy’ defined but not used
>>> src/libavcodec/dnxhdenc.c: In function ‘dnxhd_encode_init’:
>>> src/libavcodec/dnxhdenc.c:518: warning: ‘coded_frame’ is deprecated (declared at src/libavcodec/avcodec.h:2760)
>>> src/libavcodec/dnxhdenc.c:519: warning: ‘coded_frame’ is deprecated (declared at src/libavcodec/avcodec.h:2760)
>>> src/libavcodec/dnxhdenc.c: In function ‘dnxhd_load_picture’:
>>> src/libavcodec/dnxhdenc.c:1272: warning: ‘coded_frame’ is deprecated (declared at src/libavcodec/avcodec.h:2760)
>>> src/libavcodec/dnxhdenc.c: In function ‘dnxhd_encode_picture’:
>>> src/libavcodec/dnxhdenc.c:1337: warning: ‘coded_frame’ is deprecated (declared at src/libavcodec/avcodec.h:2760)
>>> src/libavcodec/dpxenc.c: In function ‘encode_frame’:
>>> src/libavcodec/dpxenc.c:180: warning: ‘need_align’ may be used uninitialized in this function
>>> src/libavcodec/dpxenc.c:180: warning: ‘len’ may be used uninitialized in this function
>>>
>>>
>>> [...]
>>
>> Do you happen to know what set of compilers lack anonymous union support like this?  It's quite tempting to add a configure option to just not build it for them.
> 
> I dont know which compilers, in the sense of names and versions but
> 
> anonymous unions are AFAIK C11, so compilers that support just C99 could
> fail.
> I think we should not drop support for pure C99 compilers yet. Doing that
> would also require better understanding the impact ...

To clarify, I wasn't intending to suggest that we kill pre-C11 support entirely (that would be rather extreme, given we don't even require all of C99).  Rather, I meant adding a configure test something like:

test_cc <<EOF && enable anonymous_union
struct a { union { int b; }; };
int f(void) { struct a x = { .b = 1 }; return x.b; }
EOF

and then new components could depend on anonymous_union in configure to avoid having to add additional nesting depth to structures where it isn't wanted.

If it is only a problem with past compilers then I don't think this would be an unreasonable constraint for a new component.  On the other hand, if current compilers don't support it then maybe not.

Thanks,

- Mark