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• Go / Cgo - How to access a field of a Cstruct - could not make it

  15 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".

  


• avcodec/vc1 : support multi-slice field interlaced pictures with hwaccel

  9 mai 2018, par Jerome Borsboom

  avcodec/vc1 : support multi-slice field interlaced pictures with hwaccel
  When using hardware accelerated decoding for multi-slice field interlaced pictures,
  
  only the first slice was decoded. This patch adds the neccesary looping over the
  
  remaining slices that may exist in field interlaced pictures. Additionally, we align
  
  the calculation of mby_start for the second field with the method given in VC-1 spec.
  Signed-off-by : Jerome Borsboom <jerome.borsboom@carpalis.nl>
  

  • [DH] libavcodec/vc1dec.c

• Remove obsolete maintainers field (same as author).

  26 octobre 2016, par blueimp

  Remove obsolete maintainers field (same as author).