What causes "output not opened" in pion/webrtc?

Create/open the destination and check its error before NewWriter

output not opened

Error message

output not opened

What it means

errOutputNotOpened is returned by oggwriter.NewWriter when the provided io.Writer is nil, and by the internal stream-access path when w.stream is nil (e.g. writing before a valid output was attached). It indicates the writer has no destination to serialize Ogg pages into.

Source

Thrown at pkg/media/oggwriter/oggwriter.go:49

	maxOpusPacketSamples               = opusGranuleSampleRate * 120 / 1000
	defaultChannelCount                = 2
	channelMappingFamily0              = 0
	channelMappingFamily1              = 1
	channelMappingFamily2              = 2
	channelMappingFamily255            = 255
	idPageSignature                    = "OpusHead"
	commentPageSignature               = "OpusTags"
	defaultVendor                      = "pion"
	pageHeaderSignature                = "OggS"
	pageHeaderSize                     = 27
	maxOggPageSegments                 = 255
	noGranulePosition                  = ^uint64(0)
	maxUint32Length                    = uint64(1<<32 - 1)
)

var (
	errFileNotOpened        = errors.New("file not opened")
	errOutputNotOpened      = errors.New("output not opened")
	errInvalidNilPacket     = errors.New("invalid nil packet")
	errDuplicateTrackSSRC   = errors.New("duplicate Ogg track SSRC")
	errDuplicateTrackSerial = errors.New("duplicate Ogg track serial")
	errTracksStarted        = errors.New("cannot add Ogg tracks after writing has started")
	errPacketSSRCMismatch   = errors.New("RTP packet SSRC does not match Ogg track SSRC")
	errInvalidOpusPacket    = errors.New("invalid Opus packet")
	errInvalidChannelCount  = errors.New("invalid channel count")
	errInvalidChannelMap    = errors.New("invalid channel mapping")
	errInvalidOpusTags      = errors.New("invalid OpusTags")
)

type pageRewriter interface {
	io.Seeker
	io.WriterAt
}

type writerConfig struct {
	sampleRate     uint32

View on GitHub (pinned to 8c25dc09fa)

Solutions

Create/open the destination and check its error before NewWriter
Never construct Writer directly without a stream; use NewWriter
Verify the file is not closed/nil before each WriteRTP/Close sequence

Example fix

// before
f, _ := os.Create("out.ogg")
w, err := oggwriter.NewWriter(f) // errOutputNotOpened if f nil
// after
f, err := os.Create("out.ogg")
if err != nil {
    return err
}
w, err := oggwriter.NewWriter(f)

Defensive patterns

Strategy: validation

Validate before calling

if out == nil {
    return errors.New("ogg writer requires a non-nil io.Writer")
}
w, err := oggwriter.NewWriter(out)

Type guard

func hasOutputStream(w *oggwriter.Writer) bool {
    return w != nil
}

Try / catch

w, err := oggwriter.NewWriter(out)
if errors.Is(err, oggwriter.ErrOutputNotOpened) {
    return fmt.Errorf("ogg output missing: %w", err)
}

Prevention

Construct writers only via NewWriter with a checked, open destination
Do not close the underlying file before writer.Close()
Avoid zero-value Writer structs; always use the constructor

When it happens

Trigger: NewWriter(nil) (oggwriter.go:391); internal write path encountering w.stream == nil (oggwriter.go:432); asserted by TestOggWriter_NewWriterRequiresOutput.

Common situations: Ignoring the error from os.Create so a nil file reaches NewWriter, constructing a Writer via a struct literal without setting stream, closing the file and continuing to write.

Related errors

AI-assisted analysis of pion/webrtc@8c25dc09fa (2026-09-03). Data as JSON: /api/errors/74805ae72a8ad1dd. Report an issue: GitHub.