2023-03-12：mp3音频解码为pcm，代码用go语言编写，调用moonfdd/ffmpeg-go库。

go run ./examples/a15.audio_decode_mp32pcm/main.go

package main

import (
  "fmt"
  "os"
  "os/exec"
  "unsafe"

  "github.com/moonfdd/ffmpeg-go/ffcommon"
  "github.com/moonfdd/ffmpeg-go/libavcodec"
  "github.com/moonfdd/ffmpeg-go/libavformat"
  "github.com/moonfdd/ffmpeg-go/libavutil"
)

func main() {
  os.Setenv("Path", os.Getenv("Path")+";./lib")
  ffcommon.SetAvutilPath("./lib/avutil-56.dll")
  ffcommon.SetAvcodecPath("./lib/avcodec-58.dll")
  ffcommon.SetAvdevicePath("./lib/avdevice-58.dll")
  ffcommon.SetAvfilterPath("./lib/avfilter-56.dll")
  ffcommon.SetAvformatPath("./lib/avformat-58.dll")
  ffcommon.SetAvpostprocPath("./lib/postproc-55.dll")
  ffcommon.SetAvswresamplePath("./lib/swresample-3.dll")
  ffcommon.SetAvswscalePath("./lib/swscale-5.dll")

  genDir := "./out"
  _, err := os.Stat(genDir)
  if err != nil {
    if os.IsNotExist(err) {
      os.Mkdir(genDir, 0777) //  Everyone can read write and execute
    }
  }

  inVFileName := "./out/test.mp3"
  outFileName := "./out/test.pcm"

  // ./lib/ffmpeg -i ./resources/big_buck_bunny.mp4 -acodec libmp3lame -vn ./out/test.mp3
  //是否存在mp3文件
  _, err = os.Stat(inVFileName)
  if err != nil {
    if os.IsNotExist(err) {
      fmt.Println("create mp3 file")
      exec.Command("./lib/ffmpeg", "-i", "./resources/big_buck_bunny.mp4", "-acodec", "libmp3lame", "-vn", inVFileName, "-y").CombinedOutput()
    }
  }

  os.Remove(outFileName)
  f, err := os.OpenFile(outFileName, os.O_CREATE|os.O_RDWR, 0777)
  if err != nil {
    fmt.Println("open file failed,err:", err)
    return
  }

  fmtCtx := libavformat.AvformatAllocContext()
  var codecCtx *libavcodec.AVCodecContext
  pkt := libavcodec.AvPacketAlloc()
  frame := libavutil.AvFrameAlloc()

  aStreamIndex := -1

  for {
    if libavformat.AvformatOpenInput(&fmtCtx, inVFileName, nil, nil) < 0 {
      fmt.Printf("Cannot open input file.\n")
      break
    }

    if fmtCtx.AvformatFindStreamInfo(nil) < 0 {
      fmt.Printf("Cannot find stream info in input file.\n")
      break
    }

    fmtCtx.AvDumpFormat(0, inVFileName, 0)

    //查找视频流在文件中的位置
    for i := uint32(0); i < fmtCtx.NbStreams; i++ {
      if fmtCtx.GetStream(i).Codecpar.CodecType == libavutil.AVMEDIA_TYPE_AUDIO {
        aStreamIndex = int(i)
        break
      }
    }

    if aStreamIndex == -1 {
      fmt.Printf("Cannot find audio stream.\n")
      return
    }

    aCodecPara := fmtCtx.GetStream(uint32(aStreamIndex)).Codecpar
    codec := libavcodec.AvcodecFindDecoder(aCodecPara.CodecId)
    if codec == nil {
      fmt.Printf("Cannot find any codec for audio.\n")
      return
    }

    codecCtx = codec.AvcodecAllocContext3()

    if codecCtx.AvcodecParametersToContext(aCodecPara) < 0 {
      fmt.Printf("Cannot alloc codec context.\n")
      return
    }

    codecCtx.PktTimebase = fmtCtx.GetStream(uint32(aStreamIndex)).TimeBase

    if codecCtx.AvcodecOpen2(codec, nil) < 0 {
      fmt.Printf("Cannot open audio codec.\n")
      return
    }

    for (fmtCtx.AvReadFrame(pkt)) >= 0 {
      if pkt.StreamIndex == uint32(aStreamIndex) {
        if codecCtx.AvcodecSendPacket(pkt) >= 0 {
          for codecCtx.AvcodecReceiveFrame(frame) >= 0 {
            /*
               Planar（平面），其数据格式排列方式为 (特别记住，该处是以点nb_samples采样点来交错，不是以字节交错）:
               LLLLLLRRRRRRLLLLLLRRRRRRLLLLLLRRRRRRL...（每个LLLLLLRRRRRR为一个音频帧）
               而不带P的数据格式（即交错排列）排列方式为：
               LRLRLRLRLRLRLRLRLRLRLRLRLRLRLRLRLRLRL...（每个LR为一个音频样本）
            */
            if libavutil.AvSampleFmtIsPlanar(codecCtx.SampleFmt) != 0 {
              numBytes := libavutil.AvGetBytesPerSample(codecCtx.SampleFmt)
              //pcm播放时是LRLRLR格式，所以要交错保存数据
              bytes := []byte{}
              for i := int32(0); i < frame.NbSamples; i++ {
                for ch := int32(0); ch < codecCtx.Channels; ch++ {
                  ptr := uintptr(unsafe.Pointer(uintptr(unsafe.Pointer(frame.Data[ch])) + uintptr(numBytes*i)))
                  for k := int32(0); k < numBytes; k++ {
                    bytes = append(bytes, *(*byte)(unsafe.Pointer(ptr)))
                    ptr++
                  }

                }
              }
              f.Write(bytes)
            }
          }
        }
      }
      pkt.AvPacketUnref()
    }

    break
  }

  libavutil.AvFrameFree(&frame)
  libavcodec.AvPacketFree(&pkt)
  codecCtx.AvcodecClose()
  libavcodec.AvcodecFreeContext(&codecCtx)
  fmtCtx.AvformatFreeContext()
  f.Close()

  fmt.Println("-----------------------------------------")
  // ./lib/ffplay -ar 22050 -ac 2 -f f32le -i ./out/test.pcm
  _, err = exec.Command("./lib/ffplay.exe", "-ar", "22050", "-ac", "2", "-f", "f32le", "-i", "./out/test.pcm").Output()
  if err != nil {
    fmt.Println("play err = ", err)
  }
}