Repackaging C types causes type cast error in Go

Question

I'm trying to repackage some go code that integrates with an existing C library.

The following works perfectly.

File 1:

package avcodec

type Codec C.struct_AVCodec

File 2:

package avformat

//#cgo pkg-config: libavformat libavcodec
//#include <libavformat/avformat.h>
//#include <libavcodec/avcodec.h>
import "C" 
import (    
"unsafe" 
) 

type Codec C.struct_AVCodec

func (s *FormatContext) AvFormatGetVideoCodec() *Codec {    
  result := C.av_format_get_video_codec((*C.struct_AVFormatContext)(s))
  return (*Codec)(result) // <- This works. Codec is defined in this package.
}

If I try to reference or move Codec from File 2 into a separate package (eg. File 1) I get the error:

cannot convert (func literal)((*C.struct_AVFormatContext)(s)) (type *C.struct_AVCodec) to type *Codec

For example, this fails:

package avformat

//#cgo pkg-config: libavformat libavcodec
//#include <libavformat/avformat.h>
//#include <libavcodec/avcodec.h>
import "C" 
import (    
"avcodec"
"unsafe" 
) 

func (s *FormatContext) AvFormatGetVideoCodec() *avcodec.Codec {    
 result := C.av_format_get_video_codec((*C.struct_AVFormatContext)(s))      
 return (*avcodec.Codec)(result) // <- This fails. Codec defined in avcodec.
}

This also fails:

package avformat

//#cgo pkg-config: libavformat libavcodec
//#include <libavformat/avformat.h>
//#include <libavcodec/avcodec.h>    
import "C" 
import ( 
"avcodec"   
"unsafe" 
) 

type Codec avcodec.Codec

func (s *FormatContext) AvFormatGetVideoCodec() *Codec {    
  result := C.av_format_get_video_codec((*C.struct_AVFormatContext)(s))
  return (*Codec)(result) // <- This also fails. Codec is based on avcodec.Codec.
}

I'd like to:

1. Understand why it fails, and
2. Identify how I can change the packaging so that the function uses the Codec type that is defined in the avcodec package.

Thanks in advance.

Answer 1

This was failing due to how Go represents types.

For instance, given:

//Ex1
package avformat 
//.. deleted for simplicity

type Codec C.struct_AVCodec

...and

//Ex2
package avcode 
//.. deleted for simplicity

type Codec C.struct_AVCodec

In the code above, the C.struct_AVCodec in ex1 is different from the C.struct_AVCodec in ex2, even though the same are lexically the same.

Specifically, the fully qualified type in ex1 is avformat._Ctype_struct_AVCodec, while ex2 is avcodec._Ctype_struct_AVCodec

This explains why the functions in the package avformat that were trying to cast anything from an external type (in this case from package avcodec) to the local C.struct_AVCodec were failing.

Solution

To get this to work, I relied on type assertions.

package avformat

func (s *FormatContext) AvformatNewStream(c avcodec.ICodec) *Stream {
  v, _ := c.(*C.struct_AVCodec)
  return (*Stream)(C.avformat_new_stream((*C.struct_AVFormatContext)(s), (*C.struct_AVCodec)(v)))
}

...and

package avcodec

type ICodec interface{}