Muxing

Muxers take encoded data in the form of AVPackets and write it into files or other output bytestreams in the specified container format. More...

Data Structures
struct	AVOutputFormat

Functions
int	avformat_write_header (AVFormatContext *s, AVDictionary **options)
	Allocate the stream private data and write the stream header to an output media file. More...
int	av_write_frame (AVFormatContext *s, AVPacket *pkt)
	Write a packet to an output media file. More...
int	av_interleaved_write_frame (AVFormatContext *s, AVPacket *pkt)
	Write a packet to an output media file ensuring correct interleaving. More...
int	av_write_uncoded_frame (AVFormatContext *s, int stream_index, AVFrame *frame)
	Write a uncoded frame to an output media file. More...
int	av_interleaved_write_uncoded_frame (AVFormatContext *s, int stream_index, AVFrame *frame)
	Write a uncoded frame to an output media file. More...
int	av_write_uncoded_frame_query (AVFormatContext *s, int stream_index)
	Test whether a muxer supports uncoded frame. More...
int	av_write_trailer (AVFormatContext *s)
	Write the stream trailer to an output media file and free the file private data. More...
AVOutputFormat *	av_guess_format (const char *short_name, const char *filename, const char *mime_type)
	Return the output format in the list of registered output formats which best matches the provided parameters, or return NULL if there is no match. More...
enum AVCodecID	av_guess_codec (AVOutputFormat *fmt, const char *short_name, const char *filename, const char *mime_type, enum AVMediaType type)
	Guess the codec ID based upon muxer and filename. More...
int	av_get_output_timestamp (struct AVFormatContext *s, int stream, int64_t *dts, int64_t *wall)
	Get timing information for the data currently output. More...

Detailed Description

Muxers take encoded data in the form of AVPackets and write it into files or other output bytestreams in the specified container format.

The main API functions for muxing are avformat_write_header() for writing the file header, av_write_frame() / av_interleaved_write_frame() for writing the packets and av_write_trailer() for finalizing the file.

At the beginning of the muxing process, the caller must first call avformat_alloc_context() to create a muxing context. The caller then sets up the muxer by filling the various fields in this context:

• The oformat field must be set to select the muxer that will be used.
• Unless the format is of the AVFMT_NOFILE type, the pb field must be set to an opened IO context, either returned from avio_open2() or a custom one.
• Unless the format is of the AVFMT_NOSTREAMS type, at least one stream must be created with the avformat_new_stream() function. The caller should fill the stream codec context information, such as the codec type, id and other parameters (e.g. width / height, the pixel or sample format, etc.) as known. The stream timebase should be set to the timebase that the caller desires to use for this stream (note that the timebase actually used by the muxer can be different, as will be described later).
• It is advised to manually initialize only the relevant fields in AVCodecContext, rather than using avcodec_copy_context() during remuxing: there is no guarantee that the codec context values remain valid for both input and output format contexts.
• The caller may fill in additional information, such as global or per-stream metadata, chapters, programs, etc. as described in the AVFormatContext documentation. Whether such information will actually be stored in the output depends on what the container format and the muxer support.

When the muxing context is fully set up, the caller must call avformat_write_header() to initialize the muxer internals and write the file header. Whether anything actually is written to the IO context at this step depends on the muxer, but this function must always be called. Any muxer private options must be passed in the options parameter to this function.

The data is then sent to the muxer by repeatedly calling av_write_frame() or av_interleaved_write_frame() (consult those functions' documentation for discussion on the difference between them; only one of them may be used with a single muxing context, they should not be mixed). Do note that the timing information on the packets sent to the muxer must be in the corresponding AVStream's timebase. That timebase is set by the muxer (in the avformat_write_header() step) and may be different from the timebase requested by the caller.

Once all the data has been written, the caller must call av_write_trailer() to flush any buffered packets and finalize the output file, then close the IO context (if any) and finally free the muxing context with avformat_free_context().

Function Documentation

int avformat_write_header	(	AVFormatContext *	s,
		AVDictionary **	options
	)

Allocate the stream private data and write the stream header to an output media file.

Parameters

s	Media file handle, must be allocated with avformat_alloc_context(). Its oformat field must be set to the desired output format; Its pb field must be set to an already opened AVIOContext.
options	An AVDictionary filled with AVFormatContext and muxer-private options. On return this parameter will be destroyed and replaced with a dict containing options that were not found. May be NULL.

Returns

0 on success, negative AVERROR on failure.

See Also

av_opt_find, av_dict_set, avio_open, av_oformat_next.

Examples:

muxing.c, remuxing.c, transcode_aac.c, and transcoding.c.

Definition at line 447 of file mux.c.

Referenced by build_feed_streams(), dash_write_header(), ff_rtp_chain_mux_open(), hds_write_header(), hls_start(), hls_write_header(), http_prepare_data(), ism_write_header(), main(), mpegts_write_header(), open_output_file(), open_slave(), rtp_mpegts_write_header(), rtp_new_av_stream(), seg_write_header(), segment_start(), write_output_file_header(), and write_packet().

int av_write_frame	(	AVFormatContext *	s,
		AVPacket *	pkt
	)

Write a packet to an output media file.

This function passes the packet directly to the muxer, without any buffering or reordering. The caller is responsible for correctly interleaving the packets if the format requires it. Callers that want libavformat to handle the interleaving should call av_interleaved_write_frame() instead of this function.

Parameters

s

media file handle

pkt

The packet containing the data to be written. Note that unlike av_interleaved_write_frame(), this function does not take ownership of the packet passed to it (though some muxers may make an internal reference to the input packet). This parameter can be NULL (at any time, not just at the end), in order to immediately flush data buffered within the muxer, for muxers that buffer up data internally before writing it to the output. Packet's stream_index field must be set to the index of the corresponding stream in s->streams. It is very strongly recommended that timing information (pts, dts, duration) is set to correct values.

Returns

< 0 on error, = 0 if OK, 1 if flushed and there is no more data to flush

See Also

av_interleaved_write_frame()

Examples:

transcode_aac.c.

Definition at line 691 of file mux.c.

Referenced by av_write_uncoded_frame_internal(), dash_flush(), encode_audio_frame(), ff_write_chained(), hls_write_packet(), http_prepare_data(), ism_flush(), mpegts_write_packet_internal(), rtp_mpegts_write_packet(), seg_write_header(), and segment_end().

int av_interleaved_write_frame	(	AVFormatContext *	s,
		AVPacket *	pkt
	)

Write a packet to an output media file ensuring correct interleaving.

This function will buffer the packets internally as needed to make sure the packets in the output file are properly interleaved in the order of increasing dts. Callers doing their own interleaving should call av_write_frame() instead of this function.

Parameters

s

media file handle

pkt

The packet containing the data to be written. If the packet is reference-counted, this function will take ownership of this reference and unreference it later when it sees fit. The caller must not access the data through this reference after this function returns. If the packet is not reference-counted, libavformat will make a copy. This parameter can be NULL (at any time, not just at the end), to flush the interleaving queues. Packet's stream_index field must be set to the index of the corresponding stream in s->streams. It is very strongly recommended that timing information (pts, dts, duration) is set to correct values.

Returns

0 on success, a negative AVERROR on error. Libavformat will always take care of freeing the packet, even if this function fails.

See Also

av_write_frame(), AVFormatContext.max_interleave_delta

Examples:

muxing.c, remuxing.c, and transcoding.c.

Definition at line 932 of file mux.c.

Referenced by av_write_uncoded_frame_internal(), encode_write_frame(), ff_write_chained(), main(), tee_write_packet(), write_frame(), write_packet(), and write_video_frame().

int av_write_uncoded_frame	(	AVFormatContext *	s,
		int	stream_index,
		AVFrame *	frame
	)

Write a uncoded frame to an output media file.

The frame must be correctly interleaved according to the container specification; if not, then av_interleaved_write_frame() must be used.

See av_interleaved_write_frame() for details.

Definition at line 1102 of file mux.c.

int av_interleaved_write_uncoded_frame	(	AVFormatContext *	s,
		int	stream_index,
		AVFrame *	frame
	)

Write a uncoded frame to an output media file.

If the muxer supports it, this function makes it possible to write an AVFrame structure directly, without encoding it into a packet. It is mostly useful for devices and similar special muxers that use raw video or PCM data and will not serialize it into a byte stream.

To test whether it is possible to use it with a given muxer and stream, use av_write_uncoded_frame_query().

The caller gives up ownership of the frame and must not access it afterwards.

Returns

>=0 for success, a negative code on error

Definition at line 1108 of file mux.c.

Referenced by main().

int av_write_uncoded_frame_query	(	AVFormatContext *	s,
		int	stream_index
	)

Test whether a muxer supports uncoded frame.

Returns

>=0 if an uncoded frame can be written to that muxer and stream, <0 if not

Definition at line 1114 of file mux.c.

Referenced by main().

int av_write_trailer

(

AVFormatContext *

s

)

Write the stream trailer to an output media file and free the file private data.

May only be called after a successful call to avformat_write_header.

Parameters

s	media file handle

Returns

0 if OK, AVERROR_xxx on error

Examples:

muxing.c, remuxing.c, transcode_aac.c, and transcoding.c.

Definition at line 986 of file mux.c.

Referenced by close_connection(), dash_free(), ff_mov_close_hinting(), ff_rtsp_undo_setup(), hds_free(), hls_write_trailer(), http_prepare_data(), ism_free(), main(), rtp_mpegts_write_close(), sap_write_close(), seg_write_trailer(), segment_end(), tee_write_trailer(), transcode(), write_output_file_trailer(), and write_packet().

AVOutputFormat* av_guess_format	(	const char *	short_name,
		const char *	filename,
		const char *	mime_type
	)

Return the output format in the list of registered output formats which best matches the provided parameters, or return NULL if there is no match.

Parameters

short_name	if non-NULL checks if short_name matches with the names of the registered formats
filename	if non-NULL checks if filename terminates with the extensions of the registered formats
mime_type	if non-NULL checks if mime_type matches with the MIME type of the registered formats

Examples:

transcode_aac.c.

Definition at line 94 of file format.c.

Referenced by av_guess_codec(), avformat_alloc_output_context2(), dash_write_header(), ff_rtp_chain_mux_open(), ffserver_guess_format(), ffserver_parse_config_feed(), hds_write_header(), hls_write_header(), ism_write_header(), mpegts_write_header(), open_output_file(), prepare_sdp_description(), rtp_mpegts_write_header(), rtp_new_av_stream(), seg_write_header(), show_help_muxer(), and webm_chunk_write_header().

enum AVCodecID av_guess_codec	(	AVOutputFormat *	fmt,
		const char *	short_name,
		const char *	filename,
		const char *	mime_type,
		enum AVMediaType	type
	)

Guess the codec ID based upon muxer and filename.

Definition at line 129 of file format.c.

Referenced by choose_encoder(), and open_output_file().

int av_get_output_timestamp	(	struct AVFormatContext *	s,
		int	stream,
		int64_t *	dts,
		int64_t *	wall
	)

Get timing information for the data currently output.

The exact meaning of "currently output" depends on the format. It is mostly relevant for devices that have an internal buffer and/or work in real time.

Parameters

	s	media file handle
	stream	stream in the media file
[out]	dts	DTS of the last packet output for the stream, in stream time_base units
[out]	wall	absolute time when that packet whas output, in microsecond

Returns

0 if OK, AVERROR(ENOSYS) if the format does not support it Note: some formats or devices may not allow to measure dts and wall atomically.

Definition at line 1032 of file mux.c.