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• Go / Cgo - How to access a field of a Cstruct ?

  17 août 2021, par ChrisG

  I develope an application in Go for transcode an audio file from one format to another one :

  


  I use the goav library that use Cgo to bind the FFmpeg C-libs :
https://github.com/giorgisio/goav/

  


  ---

  


  The goav library ; package avformat has a typedef that cast the original FFmpeg lib C-Struct AVOutputFormat :

  


  type ( 
   OutputFormat               C.struct_AVOutputFormat
)


  


  In my code i have a variable called outputF of the type OutputFormat that is a C.struct_AVOutputFormat.

  


  The C real AVOutputFormat struct has fields :

  


  name, long_name, mime_type, extensions, audio_codec, video_codec, subtitle_codec,..


  


  and many fields more.

  


  See : https://ffmpeg.org/doxygen/2.6/structAVOutputFormat.html

  


  ---

  


  I verified the situation by fmt.Println(outputF) and reached :

  


  {0x7ffff7f23383 0x7ffff7f23907 0x7ffff7f13c33 0x7ffff7f23383 86017 61 0 128 <nil> 0x7ffff7f8cfa0 <nil> 3344 0x7ffff7e3ec10 0x7ffff7e3f410 0x7ffff7e3ecc0 <nil> 0x7ffff7e3dfc0 <nil> <nil> <nil> <nil> <nil> <nil> 0 0x7ffff7e3e070 0x7ffff7e3e020 <nil>}&#xA;</nil></nil></nil></nil></nil></nil></nil></nil></nil></nil>

  


  The audio codec field is on position 5 and contains 86017

  


  I verified the field name by using the package reflect :

  


  val := reflect.Indirect(reflect.ValueOf(outputF))
fmt.Println(val)
fmt.Println("Fieldname: ", val.Type().Field(4).Name)

Output:
Fieldname:  audio_codec


  


  ---

  


  I try to access the field audio_codec of the original AVOutputFormat using :

  


  fmt.Println(outputF.audio_codec)
ERROR: outputF.audio_codec undefined (cannot refer to unexported field or method audio_codec)


fmt.Println(outputF._audio_codec)
ERROR: outputF._audio_codec undefined (type *avformat.OutputFormat has no field or method _audio_codec)


  


  


  As i read in the Cgo documentation :
Within the Go file, C's struct field names that are keywords in Go can be accessed by prefixing them with an underscore : if x points at a C struct with a field named "type", x._type accesses the field. C struct fields that cannot be expressed in Go, such as bit fields or misaligned data, are omitted in the Go struct, replaced by appropriate padding to reach the next field or the end of the struct.

  


  


  But I have no idea what im doing wrong.

  


  Edit :
Okay for sure no underscore is required as audio_codec is not a keyword in Go. This i understood for now. But still there is the question why im not able to access the CStruct field "audio_codec".

  


• FFmpeg HLS streaming - Why the program date time of older segment changing when re-streaming after discontinuity

  9 novembre 2020, par Suriya Krish

  Following is the ffmpeg command to stream the data.

  


  ffmpeg -re -f avfoundation 
-framerate 30 -i 0 
-codec:v h264_videotoolbox -x264-params keyint=120:scenecut=0 
-codec:a copy -f hls 
-hls_list_size 60 
-hls_time 10 
-hls_flags delete_segments 
-hls_flags +append_list 
-hls_flags +discont_start 
-hls_flags +program_date_time 
-strftime 1 
-strftime_mkdir 1 
-hls_segment_filename 
'%Y%m%dT%H%M%S%z.ts' playlist.m3u8


  


  Following is the playlist.m3u8 content which generated for few seconds.

  


  #EXTM3U
#EXT-X-VERSION:3
#EXT-X-TARGETDURATION:10
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-DISCONTINUITY
#EXT-X-DISCONTINUITY
#EXTINF:10.000000,
#EXT-X-PROGRAM-DATE-TIME:2020-11-09T22:07:53.000+0530
20201109T220753+0530.ts
#EXTINF:10.000000,
#EXT-X-PROGRAM-DATE-TIME:2020-11-09T22:08:03.000+0530
20201109T220803+0530.ts
#EXT-X-ENDLIST


  


  When the re-stream happens(in case of restarting the streaming service), the program date time of older segments(before #EXT-X-DISCONTINUITY) also get changed like below, which is unexpected.

  


  #EXTM3U
#EXT-X-VERSION:3
#EXT-X-TARGETDURATION:10
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-DISCONTINUITY
#EXT-X-DISCONTINUITY
#EXTINF:10.000000,
#EXT-X-PROGRAM-DATE-TIME:2020-11-09T22:12:50.633+0530
20201109T220753+0530.ts
#EXTINF:10.000000,
#EXT-X-PROGRAM-DATE-TIME:2020-11-09T22:13:00.633+0530
20201109T220803+0530.ts
#EXT-X-DISCONTINUITY
#EXTINF:10.000000,
#EXT-X-PROGRAM-DATE-TIME:2020-11-09T22:13:12.000+0530
20201109T221312+0530.ts
#EXTINF:5.400000,
#EXT-X-PROGRAM-DATE-TIME:2020-11-09T22:13:22.000+0530
20201109T221322+0530.ts
#EXT-X-ENDLIST


  


  Why the program date time of older segment should change ? Should we add any arguments in ffmpeg command to handle this ?

  


• how too download this link with ffmpeg

  27 décembre 2020, par Behrad kabiri

  i want download video with this command in ffmpeg :

  


  ffmpeg -i https://msx.namava.ir/hvod/2020/12/e9936676-eea2-44bd-b17b-dc5e58fd000d/v_854.mp4/index-v1-a1.m3u8?x=EgvsRR7zZLXNoaz49v4D05BwcjHwPaNSOtkB9y98QGOIB_EyAhV_hc44Z4McLT_FTwzBWxpaK1WlNgSBkLd415LU1jRGz0yFNhciw_SDSwgj6OlAQCARiRIb9UU8XMh_dMXKr4KncBfgwnYLMOlUn6zfSa3ltot63vKiuw0CUpU1 -vcodec copy output.mp4

  


  but recive this eror :

  


  [https @ 00000242dbcfeec0] HTTP error 420
https://msx.namava.ir/hvod/2020/12/e9936676-eea2-44bd-b17b-dc5e58fd000d/v_854.mp4/index-v1-a1.m3u8?x=EgvsRR7zZLXNoaz49v4D05BwcjHwPaNSOtkB9y98QGOIB_EyAhV_hc44Z4McLT_FTwzBWxpaK1WlNgSBkLd415LU1jRGz0yFNhciw_SDSwgj6OlAQCARiRIb9UU8XMh_dMXKr4KncBfgwnYLMOlUn6zfSa3ltot63vKiuw0CUpU1 : Server returned 4XX Client Error, but not one of 400,1,3,4

  


  please help me