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• CGO : How do I write to a file in Golang using a pointer to the C data ?

  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 write the frame data, pointed to by a uint8 pointer from C, to a .ppm file in golang.

  Once I have this done, for proof of concept that FFmpeg is doing what I expect it to, I want to set the frames to a texture in OpenGl to make a video player with cool transitions ; any pointers to do that nice and efficiently would be so very helpful ! I’m guessing I need to write some shader code to draw the ppm as a texture...

  I’m starting to understanding how to cast the pointers between C and Go types, but how can I access the data and write it in Go with the same result as C ? In C I just have to set the pointer offset for the data and state how much of it to write :

  for (y = 0; y < height; y++) {
  
      fwrite(pFrame->data[0]+y*pFrame->linesize[0], 1, width*3, pFile);
  
  }

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

  C code - libavutil/frame.h

  #include 
  
  
  
  typedef struct AVFrame {
  
  #define AV_NUM_DATA_POINTERS 8
  
      uint8_t *data[AV_NUM_DATA_POINTERS];
  
      int linesize[AV_NUM_DATA_POINTERS];
  
  }

  Golang goav wrapper

  package avutil
  
  
  
  /*
  
      #cgo pkg-config: libavutil
  
      #include <libavutil></libavutil>frame.h>
  
      #include 
  
  */
  
  import "C"
  
  import (
  
      "unsafe"
  
  )
  
  
  
  type Frame C.struct_AVFrame
  
  
  
  func Data(f *Frame) *uint8 {
  
      return (*uint8)(unsafe.Pointer((*C.uint8_t)(unsafe.Pointer(&amp;f.data))))
  
  }
  
  func Linesize(f *Frame) int {
  
      return int(*(*C.int)(unsafe.Pointer(&amp;f.linesize)))
  
  }

  My Golang code

  package main
  
  
  
  import "github.com/giorgisio/goav/avutil"
  
  
  
  func saveFrame(videoFrame *avutil.Frame, width int, height int, iFrame int) {
  
      var szFilename string
  
      var y int
  
      var file *os.File
  
      var err error
  
  
  
      szFilename = ""
  
  
  
      // Open file
  
      szFilename = fmt.Sprintf("frame%d.ppm", iFrame)
  
  
  
      if file, err = os.Open(szFilename); err != nil {
  
          log.Println("Error Reading")
  
      }
  
  
  
      // Write header
  
      fh := []byte(fmt.Sprintf("P6\n%d %d\n255\n", width, height))
  
      file.Write(fh)
  
      var b byte = 0
  
      // Write pixel data
  
      for y = 0; y < height; y++ {
  
          d := avutil.Data(videoFrame) // d should be a pointer to the first byte of data
  
          l := avutil.Linesize(videoFrame)
  
  
  
          // I'm basically lost trying to figure out how to write this to a file
  
          data := make([]byte, width*3)
  
  
  
          addr := int(*d) + y*l // figure out the address
  
  
  
          for i := 0; i < l; i++ {
  
              // This is where I'm having the problem, I get an "invalid
  
              // memory address or nil pointer dereference" error
  
              byteArrayPtr := (*byte)(unsafe.Pointer(uintptr(addr) + uintptr(i)*unsafe.Sizeof(b)))
  
              data = append(data, *byteArrayPtr)
  
              fmt.Println(*byteArrayPtr)
  
          }
  
          file.Write(data)
  
      }
  
  
  
      file.Close()
  
  }

  So, how can I write to a file using a pointer to the data, like you can do in C ?

• avcodec/agm : Check for too many too short codes in make_new_tree()

  21 avril 2019, par Michael Niedermayer

  avcodec/agm : Check for too many too short codes in make_new_tree()
  Fixes : SEGV on unknown address
  
  Fixes : 14198/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AGM_fuzzer-5723579234123776
  Found-by : continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
  
  Reviewed-by : Paul B Mahol <onemda@gmail.com>
  
  Signed-off-by : Michael Niedermayer <michael@niedermayer.cc>
  

  • [DH] libavcodec/agm.c

• avcodec/libdav1d : free the Dav1dData packet on dav1d_send_data() failure

  1er février 2022, par James Almer

  avcodec/libdav1d : free the Dav1dData packet on dav1d_send_data() failure
  We still own it on failure, and there's no point trying to feed it again.
  This should address the issue reported in dav1d #383 and part of VLC #26259.
  Signed-off-by : James Almer <jamrial@gmail.com>
  
  Reviewed-by : Ronald S. Bultje <rsbultje@gmail.com>
  

  • [DH] libavcodec/libdav1d.c