convert mp3 to pcm with golang

mp3_to_pcm.go

package ffmpeg

import (
	"errors"
	"fmt"
	"io"
	"unsafe"

	"github.com/giorgisio/goav/avcodec"
	"github.com/giorgisio/goav/avformat"
	"github.com/giorgisio/goav/avutil"
)

func init() {
	avformat.AvRegisterAll()
}

// Mp3TransformPcm depend ffmpeg
func Mp3TransformPcm(in string, w io.Writer) error {
	inCtx := avformat.AvformatAllocContext()
	if avformat.AvformatOpenInput(&inCtx, in, nil, nil) < 0 {
		return errors.New("open file error")
	}
	defer inCtx.AvformatCloseInput()
	if inCtx.AvformatFindStreamInfo(nil) < 0 {
		return errors.New("couldn't find stream information")
	}
	var audioStreamIndex = -1
	for i := 0; i < len(inCtx.Streams()); i++ {
		if inCtx.Streams()[i].CodecParameters().AvCodecGetType() == avformat.AVMEDIA_TYPE_AUDIO {
			fmt.Println("find audio index", i)
			audioStreamIndex = i
		}
	}
	codec := avcodec.AvcodecFindDecoder(avcodec.CodecId(inCtx.AudioCodecId()))
	codecCtx := codec.AvcodecAllocContext3()
	defer codecCtx.AvcodecClose()
	pkt := avcodec.AvPacketAlloc()
	defer pkt.AvPacketUnref()
	if pkt == nil {
		return errors.New("packet alloc error")
	}
	defer pkt.AvFreePacket()
	utilFrame := avutil.AvFrameAlloc()
	if utilFrame == nil {
		return errors.New("frame alloc error")
	}
	defer avutil.AvFrameFree(utilFrame)
	var gotName int
	var index int
	for inCtx.AvReadFrame(pkt) == 0 {
		if pkt.StreamIndex() == audioStreamIndex {
			if codecCtx.AvcodecDecodeAudio4((*avcodec.Frame)(unsafe.Pointer(utilFrame)), &gotName, pkt) < 0 {
				return errors.New("codec decode audio4 error")
			}
			if gotName > 0 {
				if _, err := w.Write(getFramBytes(utilFrame)); err != nil {
					return err
				}
				index++
			}
		}
	}
	return nil
}

func getFramBytes(f *avutil.Frame) []byte {
	data := avutil.Data(f)
	var bs = make([]byte, 0, len(data))
	for i := 0; i < len(data); i++ {
		bs = append(bs, *data[i])
	}
	return bs
}

mp3_to_pcm_test.go

package ffmpeg

import (
	"os"
	"testing"
)

func TestMp3TransformPcm(t *testing.T) {
	out, err := os.Create("1.pcm")
	if err != nil {
		t.Fatal(err)
	}
	defer out.Close()
	if err := Mp3TransformPcm("1.mp3", out); err != nil {
		t.Fatal(err)
	}
}

Hi @kaiiak,
Thank you for the snippet. However, I find line 53 to be throwing error. Error stack is below:

fatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x1 addr=0xa8 pc=0x5c0ab9a]

runtime stack:
runtime.throw(0x410693f, 0x2a)
        /usr/local/Cellar/go/1.12.9/libexec/src/runtime/panic.go:617 +0x72
runtime.sigpanic()
        /usr/local/Cellar/go/1.12.9/libexec/src/runtime/signal_unix.go:374 +0x4a9

goroutine 1 [syscall]:
runtime.cgocall(0x40b1090, 0xc000048d88, 0x40b29f0)
        /usr/local/Cellar/go/1.12.9/libexec/src/runtime/cgocall.go:128 +0x5b fp=0xc000048d58 sp=0xc000048d20 pc=0x400564b
github.com/giorgisio/goav/avcodec._Cfunc_avcodec_decode_audio4(0x9012800, 0x8c32080, 0xc00001c0a8, 0x8c28900, 0x0)
        _cgo_gotypes.go:2173 +0x4d fp=0xc000048d88 sp=0xc000048d58 pc=0x409ed8d
github.com/giorgisio/goav/avcodec.(*Context).AvcodecDecodeAudio4.func1(0x9012800, 0x8c32080, 0xc00001c0a8, 0x8c28900, 0xc000000000)
        /Users/Oluwashola/go/pkg/mod/github.com/giorgisio/goav@v0.1.0/avcodec/context.go:100 +0xce fp=0xc000048dc0 sp=0xc000048d88 pc=0x409f1fe
github.com/giorgisio/goav/avcodec.(*Context).AvcodecDecodeAudio4(0x9012800, 0x8c32080, 0xc00001c0a8, 0x8c28900, 0x2)
        /Users/Oluwashola/go/pkg/mod/github.com/giorgisio/goav@v0.1.0/avcodec/context.go:100 +0x49 fp=0xc000048df8 sp=0xc000048dc0 pc=0x409ef99
bitbucket.org/gbedu/core/features/track/recommendation.Mp3TransformPcm(0x41029e8, 0x14, 0x0, 0x0, 0x0, 0x0)

What's your ffmpeg version?