think@opensuse> ffmpeg -formats
File formats:
 D. = Demuxing supported
 .E = Muxing supported
 --
 DE flv             FLV (Flash Video)
 D  aac             raw ADTS AAC (Advanced Audio Coding)
 DE h264            raw H.264 video
 DE hevc            raw HEVC video
  E mp2             MP2 (MPEG audio layer 2)
 DE mp3             MP3 (MPEG audio layer 3)
  E mpeg2video      raw MPEG-2 video
 DE mpegts          MPEG-TS (MPEG-2 Transport Stream)

AVCodec ff_h264_decoder = {
    .name                  = "h264",
    .long_name             = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
    .type                  = AVMEDIA_TYPE_VIDEO,
    .id                    = AV_CODEC_ID_H264,
    ......
};

AVOutputFormat ff_h264_muxer = {
    .name              = "h264",
    .long_name         = NULL_IF_CONFIG_SMALL("raw H.264 video"),
    .extensions        = "h264,264",
    .audio_codec       = AV_CODEC_ID_NONE,
    .video_codec       = AV_CODEC_ID_H264,
    .write_header      = force_one_stream,
    .write_packet      = ff_raw_write_packet,
    .check_bitstream   = h264_check_bitstream,
    .flags             = AVFMT_NOTIMESTAMPS,
};

FF_DEF_RAWVIDEO_DEMUXER(h264, "raw H.264 video", h264_probe, "h26l,h264,264,avc", AV_CODEC_ID_H264)

#define FF_DEF_RAWVIDEO_DEMUXER(shortname, longname, probe, ext, id)\
FF_DEF_RAWVIDEO_DEMUXER2(shortname, longname, probe, ext, id, AVFMT_GENERIC_INDEX)

#define FF_DEF_RAWVIDEO_DEMUXER2(shortname, longname, probe, ext, id, flag)\
FF_RAWVIDEO_DEMUXER_CLASS(shortname)\
AVInputFormat ff_ ## shortname ## _demuxer = {\
    .name           = #shortname,\
    .long_name      = NULL_IF_CONFIG_SMALL(longname),\
    .read_probe     = probe,\
    .read_header    = ff_raw_video_read_header,\
    .read_packet    = ff_raw_read_partial_packet,\
    .extensions     = ext,\
    .flags          = flag,\
    .raw_codec_id   = id,\
    .priv_data_size = sizeof(FFRawVideoDemuxerContext),\
    .priv_class     = &shortname ## _demuxer_class,\
};

AVInputFormat ff_h264_demuxer = {
    .name           = h264,
    .long_name      = "raw H.264 video",
    .read_probe     = h264_probe,
    .read_header    = ff_raw_video_read_header,
    .read_packet    = ff_raw_read_partial_packet,
    .extensions     = "h26l,h264,264,avc",
    .flags          = AVFMT_GENERIC_INDEX,
    .raw_codec_id   = AV_CODEC_ID_H264,
    .priv_data_size = sizeof(FFRawVideoDemuxerContext),
    .priv_class     = &h264_demuxer_class,
};

AVInputFormat ff_mpegts_demuxer = {
    .name           = "mpegts",
    .long_name      = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),
    .priv_data_size = sizeof(MpegTSContext),
    .read_probe     = mpegts_probe,
    .read_header    = mpegts_read_header,
    .read_packet    = mpegts_read_packet,
    .read_close     = mpegts_read_close,
    .read_timestamp = mpegts_get_dts,
    .flags          = AVFMT_SHOW_IDS | AVFMT_TS_DISCONT,
    .priv_class     = &mpegts_class,
};
AVOutputFormat ff_mpegts_muxer = {
    .name           = "mpegts",
    .long_name      = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),
    .mime_type      = "video/MP2T",
    .extensions     = "ts,m2t,m2ts,mts",
    .priv_data_size = sizeof(MpegTSWrite),
    .audio_codec    = AV_CODEC_ID_MP2,
    .video_codec    = AV_CODEC_ID_MPEG2VIDEO,
    .init           = mpegts_init,
    .write_packet   = mpegts_write_packet,
    .write_trailer  = mpegts_write_end,
    .deinit         = mpegts_deinit,
    .check_bitstream = mpegts_check_bitstream,
    .flags          = AVFMT_ALLOW_FLUSH | AVFMT_VARIABLE_FPS | AVFMT_NODIMENSIONS,
    .priv_class     = &mpegts_muxer_class,
};

think@opensuse> ffprobe tnhaoxc.flv 
ffprobe version 4.1 Copyright (c) 2007-2018 the FFmpeg developers
Input #0, flv, from 'tnhaoxc.flv':
  Metadata:
    encoder         : Lavf58.20.100
  Duration: 00:02:13.68, start: 0.000000, bitrate: 838 kb/s
    Stream #0:0: Video: h264 (High), yuv420p(progressive), 784x480, 25 fps, 25 tbr, 1k tbn, 50 tbc
    Stream #0:1: Audio: aac (LC), 44100 Hz, stereo, fltp

ffmpeg -i tnhaoxc.flv -map 0 -c copy tnhaoxc.ts
ffmpeg -i tnhaoxc.flv -map 0 -c copy tnhaoxc.m2t

diff tnhaoxc.ts tnhaoxc.m2t

ffmpeg -i tnhaoxc.flv -map 0 -c copy tnhaoxc.mpegts

ffmpeg version 4.1 Copyright (c) 2000-2018 the FFmpeg developers
Input #0, flv, from 'tnhaoxc.flv':
  Metadata:
    encoder         : Lavf58.20.100
  Duration: 00:02:13.68, start: 0.000000, bitrate: 838 kb/s
    Stream #0:0: Video: h264 (High), yuv420p(progressive), 784x480, 25 fps, 25 tbr, 1k tbn, 50 tbc
    Stream #0:1: Audio: aac (LC), 44100 Hz, stereo, fltp
[NULL @ 0x1d62e80] Unable to find a suitable output format for 'tnhaoxc.mpegts'
tnhaoxc.mpegts: Invalid argument

ffmpeg -i tnhaoxc.flv -map 0 -c copy -f mpegts tnhaoxc.mpegts

diff tnhaoxc.mpegts tnhaoxc.ts

/**
 * Open an input stream and read the header. The codecs are not opened.
 * The stream must be closed with avformat_close_input().
 *
 * @param ps Pointer to user-supplied AVFormatContext (allocated by avformat_alloc_context).
 *           May be a pointer to NULL, in which case an AVFormatContext is allocated by this
 *           function and written into ps.
 *           Note that a user-supplied AVFormatContext will be freed on failure.
 * @param url URL of the stream to open.
 * @param fmt If non-NULL, this parameter forces a specific input format.
 *            Otherwise the format is autodetected.
 * @param options  A dictionary filled with AVFormatContext and demuxer-private options.
 *                 On return this parameter will be destroyed and replaced with a dict containing
 *                 options that were not found. May be NULL.
 *
 * @return 0 on success, a negative AVERROR on failure.
 *
 * @note If you want to use custom IO, preallocate the format context and set its pb field.
 */
int avformat_open_input(AVFormatContext **ps, const char *url, AVInputFormat *fmt, AVDictionary **options);

/**
 * Read packets of a media file to get stream information. This
 * is useful for file formats with no headers such as MPEG. This
 * function also computes the real framerate in case of MPEG-2 repeat
 * frame mode.
 * The logical file position is not changed by this function;
 * examined packets may be buffered for later processing.
 *
 * @param ic media file handle
 * @param options  If non-NULL, an ic.nb_streams long array of pointers to
 *                 dictionaries, where i-th member contains options for
 *                 codec corresponding to i-th stream.
 *                 On return each dictionary will be filled with options that were not found.
 * @return >=0 if OK, AVERROR_xxx on error
 *
 * @note this function isn't guaranteed to open all the codecs, so
 *       options being non-empty at return is a perfectly normal behavior.
 *
 * @todo Let the user decide somehow what information is needed so that
 *       we do not waste time getting stuff the user does not need.
 */
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);

/**
 * Return the next frame of a stream.
 * This function returns what is stored in the file, and does not validate
 * that what is there are valid frames for the decoder. It will split what is
 * stored in the file into frames and return one for each call. It will not
 * omit invalid data between valid frames so as to give the decoder the maximum
 * information possible for decoding.
 *
 * If pkt->buf is NULL, then the packet is valid until the next
 * av_read_frame() or until avformat_close_input(). Otherwise the packet
 * is valid indefinitely. In both cases the packet must be freed with
 * av_packet_unref when it is no longer needed. For video, the packet contains
 * exactly one frame. For audio, it contains an integer number of frames if each
 * frame has a known fixed size (e.g. PCM or ADPCM data). If the audio frames
 * have a variable size (e.g. MPEG audio), then it contains one frame.
 *
 * pkt->pts, pkt->dts and pkt->duration are always set to correct
 * values in AVStream.time_base units (and guessed if the format cannot
 * provide them). pkt->pts can be AV_NOPTS_VALUE if the video format
 * has B-frames, so it is better to rely on pkt->dts if you do not
 * decompress the payload.
 *
 * @return 0 if OK, < 0 on error or end of file
 */
int av_read_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.
 *
 * @param s media file handle
 * @param 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).
 *            <br>
 *            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.
 *            <br>
 *            Packet's @ref AVPacket.stream_index "stream_index" field must be
 *            set to the index of the corresponding stream in @ref
 *            AVFormatContext.streams "s->streams".
 *            <br>
 *            The timestamps (@ref AVPacket.pts "pts", @ref AVPacket.dts "dts")
 *            must be set to correct values in the stream's timebase (unless the
 *            output format is flagged with the AVFMT_NOTIMESTAMPS flag, then
 *            they can be set to AV_NOPTS_VALUE).
 *            The dts for subsequent packets passed to this function must be strictly
 *            increasing when compared in their respective timebases (unless the
 *            output format is flagged with the AVFMT_TS_NONSTRICT, then they
 *            merely have to be nondecreasing).  @ref AVPacket.duration
 *            "duration") should also be set if known.
 * @return < 0 on error, = 0 if OK, 1 if flushed and there is no more data to flush
 *
 * @see av_interleaved_write_frame()
 */
int av_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.
 *
 * Using this function instead of av_write_frame() can give muxers advance
 * knowledge of future packets, improving e.g. the behaviour of the mp4
 * muxer for VFR content in fragmenting mode.
 *
 * @param s media file handle
 * @param pkt The packet containing the data to be written.
 *            <br>
 *            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.
 *            <br>
 *            This parameter can be NULL (at any time, not just at the end), to
 *            flush the interleaving queues.
 *            <br>
 *            Packet's @ref AVPacket.stream_index "stream_index" field must be
 *            set to the index of the corresponding stream in @ref
 *            AVFormatContext.streams "s->streams".
 *            <br>
 *            The timestamps (@ref AVPacket.pts "pts", @ref AVPacket.dts "dts")
 *            must be set to correct values in the stream's timebase (unless the
 *            output format is flagged with the AVFMT_NOTIMESTAMPS flag, then
 *            they can be set to AV_NOPTS_VALUE).
 *            The dts for subsequent packets in one stream must be strictly
 *            increasing (unless the output format is flagged with the
 *            AVFMT_TS_NONSTRICT, then they merely have to be nondecreasing).
 *            @ref AVPacket.duration "duration") should also be set if known.
 *
 * @return 0 on success, a negative AVERROR on error. Libavformat will always
 *         take care of freeing the packet, even if this function fails.
 *
 * @see av_write_frame(), AVFormatContext.max_interleave_delta
 */
int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt);

/**
 * Create and initialize a AVIOContext for accessing the
 * resource indicated by url.
 * @note When the resource indicated by url has been opened in
 * read+write mode, the AVIOContext can be used only for writing.
 *
 * @param s Used to return the pointer to the created AVIOContext.
 * In case of failure the pointed to value is set to NULL.
 * @param url resource to access
 * @param flags flags which control how the resource indicated by url
 * is to be opened
 * @return >= 0 in case of success, a negative value corresponding to an
 * AVERROR code in case of failure
 */
int avio_open(AVIOContext **s, const char *url, int flags);

/**
 * Allocate the stream private data and write the stream header to
 * an output media file.
 *
 * @param 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.
 * @param 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.
 *
 * @return AVSTREAM_INIT_IN_WRITE_HEADER on success if the codec had not already been fully initialized in avformat_init,
 *         AVSTREAM_INIT_IN_INIT_OUTPUT  on success if the codec had already been fully initialized in avformat_init,
 *         negative AVERROR on failure.
 *
 * @see av_opt_find, av_dict_set, avio_open, av_oformat_next, avformat_init_output.
 */
av_warn_unused_result
int avformat_write_header(AVFormatContext *s, AVDictionary **options);

/**
 * 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.
 *
 * @param s media file handle
 * @return 0 if OK, AVERROR_xxx on error
 */
int av_write_trailer(AVFormatContext *s);