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• CGO : How to access a C pointer array from Golang

  24 avril 2018, par nevernew

  I’m writing an app for the windows platform using FFmpeg and it’s golang wrapper goav, but I’m having trouble understanding how to use the C pointers to gain access to an array.

  I’m trying to get the streams stored in the AVFormatContext class to use in go, and eventually add frames to a texture in OpenGl to make a video player with cool transitions.

  I think understanding how to cast and access the C data will make coding this a lot easier.

  I’ve stripped out all the relevant parts of the C code, the wrapper and my code, shown below :

  C code - libavformat/avformat.h

  typedef struct AVFormatContext { 
  
      unsigned int nb_streams; 
  
      AVStream **streams; 
  
  }

  Golang goav wrapper

  package avutil
  
  
  
  //#cgo pkg-config: libavformat
  
  //#include <libavformat></libavformat>avformat.h>
  
  import "C"
  
  import (
  
      "unsafe"
  
  )
  
  
  
  type Context C.struct_AVFormatContext; 
  
  
  
  func (ctxt *Context) StreamsGet(i uintptr) *Stream {
  
      streams := (**Stream)(unsafe.Pointer(ctxt.streams));
  
      // I think this is where it's going wrong, I'm brand new to this stuff 
  
      return (*Stream)(unsafe.Pointer(uintptr(unsafe.Pointer(streams)) + i*unsafe.Sizeof(*streams)));
  
  }

  My Golang code

  package main
  
  
  
  import "github.com/giorgisio/goav/avformat"
  
  
  
  func main() {
  
      ctx := &avformat.Context{} // the actual function to initiate this does an mallocz for the streams
  
  
  
      stream := ctx.StreamsGet(0)
  
  
  
      //do stuff with stream...
  
  }

  In C it looks like I just have to do just streams[i], but that wont work in go, so I added a function to the wrapper using the technique from my question here.
  However I’m not getting the data ; It looks like I’m getting a pointer to somewhere random in memory. So, how can I access these elements form golang ? Any resources would be helpful too ; I’m going to be investing a fair bit of time into this.

• How to access a C pointer array from Golang

  24 avril 2018, par nevernew

  I’m writing an app for the windows platform using FFmpeg and it’s golang wrapper goav, but I’m having trouble understanding how to use the C pointers to gain access to an array.

  I’m trying to get the streams stored in the AVFormatContext class to use in go, and eventually add frames to a texture in OpenGl to make a video player with cool transitions.

  I think understanding how to cast and access the C data will make coding this a lot easier.

  I’ve stripped out all the relevant parts of the C code, the wrapper and my code, shown below :

  C code - libavformat/avformat.h

  typedef struct AVFormatContext { 
  
      unsigned int nb_streams; 
  
      AVStream **streams; 
  
  }

  Golang goav wrapper

  package avutil
  
  
  
  //#cgo pkg-config: libavformat
  
  //#include <libavformat></libavformat>avformat.h>
  
  import "C"
  
  import (
  
      "unsafe"
  
  )
  
  
  
  type Context C.struct_AVFormatContext; 
  
  
  
  func (ctxt *Context) StreamsGet(i uintptr) *Stream {
  
      streams := (**Stream)(unsafe.Pointer(ctxt.streams));
  
      // I think this is where it's going wrong, I'm brand new to this stuff 
  
      return (*Stream)(unsafe.Pointer(uintptr(unsafe.Pointer(streams)) + i*unsafe.Sizeof(*streams)));
  
  }

  My Golang code

  package main
  
  
  
  import "github.com/giorgisio/goav/avformat"
  
  
  
  func main() {
  
      ctx := &avformat.Context{} // the actual function to initiate this does an mallocz for the streams
  
  
  
      stream := ctx.StreamsGet(0)
  
  
  
      //do stuff with stream...
  
  }

  In C it looks like I just have to do just streams[i], but that wont work in go, so I added a function to the wrapper using the technique from my question here.
  However I’m not getting the data ; It looks like I’m getting a pointer to somewhere random in memory. So, how can I access these elements form golang ? Any resources would be helpful too ; I’m going to be investing a fair bit of time into this.

• PowerShell progress bar won't display

  23 juillet 2014, par Brett

  I’m trying to write my first PowerShell GUI. Basically I’m trying to run a ffmpeg command which is fine and works, but I cannot get the progress bar to run. (I’m brand new to this.) Here is my attempt.

  cd C:\Users\brett\Documents\convert
  
  $cmd = 'ffmpeg.exe'
  
  $arg0 = '-i'
  
  $arg1 = 'MASH_01.ts'
  
  $arg2 = '-c:v'
  
  $arg3 = '-c:a'
  
  $arg4 = 'MASH_01.mp4'
  
  $cf = 'copy'
  
  $isFile = 'MASH_01.mp4'
  
  if (-not(Test-Path -Path $isFile) -eq $false) {
  
    echo "Shit Go"
  
    del $isFile
  
  
  
        & $cmd $arg0 $arg1 $arg2 $cf $arg3 $cf $arg4
  
      for ($i = 1; $i -le 100; $i++) {
  
      Start-Sleep -m 100
  
      Write-Progress -Activity 'Progress Of The Coversion' -Status "$i Percent Complete" -PercentComplete $i;
  
      }
  
  } else {
  
  & $cmd $arg0 $arg1 $arg2 $cf $arg3 $cf $arg4
  
      for ($i = 1; $i -le 100; $i++) {
  
      Start-Sleep -m 100
  
      Write-Progress -Activity 'Progress Of The Coversion' -Status "$i Percent Complete" -PercentComplete $i;
  
      }
  
  }

  Update : There is no error output I can see, but the progress bar runs after the file has been processed, not during.

  here is my latest attempt.. but now i get ffmpeg saying "m" is not a valid switch

  cd C:\Users\brett\Documents\convert
  
  $oldVideo = Get-ChildItem -Include @("*.ts")
  
  Write-Host -ForegroundColor Green -Object $ArgumentList;
  
  # Pause the script until user hits enter
  
  $isFile = 'MASH_01.mp4'
  
  if( -not(Test-Path -Path $isFile) -eq $false) {
  
    echo "Shit Go"
  
    del $isFile
  
    }a
  
    $tool = ffmpeg.exe
  
  $ArgumentList = '`-i'+' '+'MASH_01.ts'+' '+'-c:v'+' '+'copy'+' '+'-c:a'+' '+'copy'+' '+'MASH_01.mp4';
  
  Invoke-Expression $tool $ArgumentList
  
  for($i = 1; $i -le 100; $i++){
  
      ffmpeg $ArgumentList -m 100
  
  
  
      Write-Progress -Activity 'Progress Of The Coversion' -Status "$i Percent Complete" -PercentComplete $i 
  
     `-SecondsRemaining $a -CurrentOperation
  
     "$i% complete" `
  
  
  
      }