@chapter Muxers @c man begin MUXERS Muxers are configured elements in FFmpeg which allow writing multimedia streams to a particular type of file. When you configure your FFmpeg build, all the supported muxers are enabled by default. You can list all available muxers using the configure option @code{--list-muxers}. You can disable all the muxers with the configure option @code{--disable-muxers} and selectively enable / disable single muxers with the options @code{--enable-muxer=@var{MUXER}} / @code{--disable-muxer=@var{MUXER}}. The option @code{-formats} of the ff* tools will display the list of enabled muxers. A description of some of the currently available muxers follows. @anchor{aiff} @section aiff Audio Interchange File Format muxer. It accepts the following options: @table @option @item write_id3v2 Enable ID3v2 tags writing when set to 1. Default is 0 (disabled). @item id3v2_version Select ID3v2 version to write. Currently only version 3 and 4 (aka. ID3v2.3 and ID3v2.4) are supported. The default is version 4. @end table @anchor{crc} @section crc CRC (Cyclic Redundancy Check) testing format. This muxer computes and prints the Adler-32 CRC of all the input audio and video frames. By default audio frames are converted to signed 16-bit raw audio and video frames to raw video before computing the CRC. The output of the muxer consists of a single line of the form: CRC=0x@var{CRC}, where @var{CRC} is a hexadecimal number 0-padded to 8 digits containing the CRC for all the decoded input frames. For example to compute the CRC of the input, and store it in the file @file{out.crc}: @example ffmpeg -i INPUT -f crc out.crc @end example You can print the CRC to stdout with the command: @example ffmpeg -i INPUT -f crc - @end example You can select the output format of each frame with @command{ffmpeg} by specifying the audio and video codec and format. For example to compute the CRC of the input audio converted to PCM unsigned 8-bit and the input video converted to MPEG-2 video, use the command: @example ffmpeg -i INPUT -c:a pcm_u8 -c:v mpeg2video -f crc - @end example See also the @ref{framecrc} muxer. @anchor{framecrc} @section framecrc Per-packet CRC (Cyclic Redundancy Check) testing format. This muxer computes and prints the Adler-32 CRC for each audio and video packet. By default audio frames are converted to signed 16-bit raw audio and video frames to raw video before computing the CRC. The output of the muxer consists of a line for each audio and video packet of the form: @example @var{stream_index}, @var{packet_dts}, @var{packet_pts}, @var{packet_duration}, @var{packet_size}, 0x@var{CRC} @end example @var{CRC} is a hexadecimal number 0-padded to 8 digits containing the CRC of the packet. For example to compute the CRC of the audio and video frames in @file{INPUT}, converted to raw audio and video packets, and store it in the file @file{out.crc}: @example ffmpeg -i INPUT -f framecrc out.crc @end example To print the information to stdout, use the command: @example ffmpeg -i INPUT -f framecrc - @end example With @command{ffmpeg}, you can select the output format to which the audio and video frames are encoded before computing the CRC for each packet by specifying the audio and video codec. For example, to compute the CRC of each decoded input audio frame converted to PCM unsigned 8-bit and of each decoded input video frame converted to MPEG-2 video, use the command: @example ffmpeg -i INPUT -c:a pcm_u8 -c:v mpeg2video -f framecrc - @end example See also the @ref{crc} muxer. @anchor{framemd5} @section framemd5 Per-packet MD5 testing format. This muxer computes and prints the MD5 hash for each audio and video packet. By default audio frames are converted to signed 16-bit raw audio and video frames to raw video before computing the hash. The output of the muxer consists of a line for each audio and video packet of the form: @example @var{stream_index}, @var{packet_dts}, @var{packet_pts}, @var{packet_duration}, @var{packet_size}, @var{MD5} @end example @var{MD5} is a hexadecimal number representing the computed MD5 hash for the packet. For example to compute the MD5 of the audio and video frames in @file{INPUT}, converted to raw audio and video packets, and store it in the file @file{out.md5}: @example ffmpeg -i INPUT -f framemd5 out.md5 @end example To print the information to stdout, use the command: @example ffmpeg -i INPUT -f framemd5 - @end example See also the @ref{md5} muxer. @anchor{hls} @section hls Apple HTTP Live Streaming muxer that segments MPEG-TS according to the HTTP Live Streaming specification. It creates a playlist file and numbered segment files. The output filename specifies the playlist filename; the segment filenames receive the same basename as the playlist, a sequential number and a .ts extension. @example ffmpeg -i in.nut out.m3u8 @end example @table @option @item -hls_time @var{seconds} Set the segment length in seconds. @item -hls_list_size @var{size} Set the maximum number of playlist entries. @item -hls_wrap @var{wrap} Set the number after which index wraps. @item -start_number @var{number} Start the sequence from @var{number}. @end table @anchor{ico} @section ico ICO file muxer. Microsoft's icon file format (ICO) has some strict limitations that should be noted: @itemize @item Size cannot exceed 256 pixels in any dimension @item Only BMP and PNG images can be stored @item If a BMP image is used, it must be one of the following pixel formats: @example BMP Bit Depth FFmpeg Pixel Format 1bit pal8 4bit pal8 8bit pal8 16bit rgb555le 24bit bgr24 32bit bgra @end example @item If a BMP image is used, it must use the BITMAPINFOHEADER DIB header @item If a PNG image is used, it must use the rgba pixel format @end itemize @anchor{image2} @section image2 Image file muxer. The image file muxer writes video frames to image files. The output filenames are specified by a pattern, which can be used to produce sequentially numbered series of files. The pattern may contain the string "%d" or "%0@var{N}d", this string specifies the position of the characters representing a numbering in the filenames. If the form "%0@var{N}d" is used, the string representing the number in each filename is 0-padded to @var{N} digits. The literal character '%' can be specified in the pattern with the string "%%". If the pattern contains "%d" or "%0@var{N}d", the first filename of the file list specified will contain the number 1, all the following numbers will be sequential. The pattern may contain a suffix which is used to automatically determine the format of the image files to write. For example the pattern "img-%03d.bmp" will specify a sequence of filenames of the form @file{img-001.bmp}, @file{img-002.bmp}, ..., @file{img-010.bmp}, etc. The pattern "img%%-%d.jpg" will specify a sequence of filenames of the form @file{img%-1.jpg}, @file{img%-2.jpg}, ..., @file{img%-10.jpg}, etc. The following example shows how to use @command{ffmpeg} for creating a sequence of files @file{img-001.jpeg}, @file{img-002.jpeg}, ..., taking one image every second from the input video: @example ffmpeg -i in.avi -vsync 1 -r 1 -f image2 'img-%03d.jpeg' @end example Note that with @command{ffmpeg}, if the format is not specified with the @code{-f} option and the output filename specifies an image file format, the image2 muxer is automatically selected, so the previous command can be written as: @example ffmpeg -i in.avi -vsync 1 -r 1 'img-%03d.jpeg' @end example Note also that the pattern must not necessarily contain "%d" or "%0@var{N}d", for example to create a single image file @file{img.jpeg} from the input video you can employ the command: @example ffmpeg -i in.avi -f image2 -frames:v 1 img.jpeg @end example @table @option @item start_number @var{number} Start the sequence from @var{number}. Default value is 1. Must be a positive number. @item -update @var{number} If @var{number} is nonzero, the filename will always be interpreted as just a filename, not a pattern, and this file will be continuously overwritten with new images. @end table The image muxer supports the .Y.U.V image file format. This format is special in that that each image frame consists of three files, for each of the YUV420P components. To read or write this image file format, specify the name of the '.Y' file. The muxer will automatically open the '.U' and '.V' files as required. @section matroska Matroska container muxer. This muxer implements the matroska and webm container specs. The recognized metadata settings in this muxer are: @table @option @item title=@var{title name} Name provided to a single track @end table @table @option @item language=@var{language name} Specifies the language of the track in the Matroska languages form @end table @table @option @item stereo_mode=@var{mode} Stereo 3D video layout of two views in a single video track @table @option @item mono video is not stereo @item left_right Both views are arranged side by side, Left-eye view is on the left @item bottom_top Both views are arranged in top-bottom orientation, Left-eye view is at bottom @item top_bottom Both views are arranged in top-bottom orientation, Left-eye view is on top @item checkerboard_rl Each view is arranged in a checkerboard interleaved pattern, Left-eye view being first @item checkerboard_lr Each view is arranged in a checkerboard interleaved pattern, Right-eye view being first @item row_interleaved_rl Each view is constituted by a row based interleaving, Right-eye view is first row @item row_interleaved_lr Each view is constituted by a row based interleaving, Left-eye view is first row @item col_interleaved_rl Both views are arranged in a column based interleaving manner, Right-eye view is first column @item col_interleaved_lr Both views are arranged in a column based interleaving manner, Left-eye view is first column @item anaglyph_cyan_red All frames are in anaglyph format viewable through red-cyan filters @item right_left Both views are arranged side by side, Right-eye view is on the left @item anaglyph_green_magenta All frames are in anaglyph format viewable through green-magenta filters @item block_lr Both eyes laced in one Block, Left-eye view is first @item block_rl Both eyes laced in one Block, Right-eye view is first @end table @end table For example a 3D WebM clip can be created using the following command line: @example ffmpeg -i sample_left_right_clip.mpg -an -c:v libvpx -metadata stereo_mode=left_right -y stereo_clip.webm @end example This muxer supports the following options: @table @option @item reserve_index_space By default, this muxer writes the index for seeking (called cues in Matroska terms) at the end of the file, because it cannot know in advance how much space to leave for the index at the beginning of the file. However for some use cases -- e.g. streaming where seeking is possible but slow -- it is useful to put the index at the beginning of the file. If this option is set to a non-zero value, the muxer will reserve a given amount of space in the file header and then try to write the cues there when the muxing finishes. If the available space does not suffice, muxing will fail. A safe size for most use cases should be about 50kB per hour of video. Note that cues are only written if the output is seekable and this option will have no effect if it is not. @end table @anchor{md5} @section md5 MD5 testing format. This muxer computes and prints the MD5 hash of all the input audio and video frames. By default audio frames are converted to signed 16-bit raw audio and video frames to raw video before computing the hash. The output of the muxer consists of a single line of the form: MD5=@var{MD5}, where @var{MD5} is a hexadecimal number representing the computed MD5 hash. For example to compute the MD5 hash of the input converted to raw audio and video, and store it in the file @file{out.md5}: @example ffmpeg -i INPUT -f md5 out.md5 @end example You can print the MD5 to stdout with the command: @example ffmpeg -i INPUT -f md5 - @end example See also the @ref{framemd5} muxer. @section MOV/MP4/ISMV The mov/mp4/ismv muxer supports fragmentation. Normally, a MOV/MP4 file has all the metadata about all packets stored in one location (written at the end of the file, it can be moved to the start for better playback by adding @var{faststart} to the @var{movflags}, or using the @command{qt-faststart} tool). A fragmented file consists of a number of fragments, where packets and metadata about these packets are stored together. Writing a fragmented file has the advantage that the file is decodable even if the writing is interrupted (while a normal MOV/MP4 is undecodable if it is not properly finished), and it requires less memory when writing very long files (since writing normal MOV/MP4 files stores info about every single packet in memory until the file is closed). The downside is that it is less compatible with other applications. Fragmentation is enabled by setting one of the AVOptions that define how to cut the file into fragments: @table @option @item -moov_size @var{bytes} Reserves space for the moov atom at the beginning of the file instead of placing the moov atom at the end. If the space reserved is insufficient, muxing will fail. @item -movflags frag_keyframe Start a new fragment at each video keyframe. @item -frag_duration @var{duration} Create fragments that are @var{duration} microseconds long. @item -frag_size @var{size} Create fragments that contain up to @var{size} bytes of payload data. @item -movflags frag_custom Allow the caller to manually choose when to cut fragments, by calling @code{av_write_frame(ctx, NULL)} to write a fragment with the packets written so far. (This is only useful with other applications integrating libavformat, not from @command{ffmpeg}.) @item -min_frag_duration @var{duration} Don't create fragments that are shorter than @var{duration} microseconds long. @end table If more than one condition is specified, fragments are cut when one of the specified conditions is fulfilled. The exception to this is @code{-min_frag_duration}, which has to be fulfilled for any of the other conditions to apply. Additionally, the way the output file is written can be adjusted through a few other options: @table @option @item -movflags empty_moov Write an initial moov atom directly at the start of the file, without describing any samples in it. Generally, an mdat/moov pair is written at the start of the file, as a normal MOV/MP4 file, containing only a short portion of the file. With this option set, there is no initial mdat atom, and the moov atom only describes the tracks but has a zero duration. Files written with this option set do not work in QuickTime. This option is implicitly set when writing ismv (Smooth Streaming) files. @item -movflags separate_moof Write a separate moof (movie fragment) atom for each track. Normally, packets for all tracks are written in a moof atom (which is slightly more efficient), but with this option set, the muxer writes one moof/mdat pair for each track, making it easier to separate tracks. This option is implicitly set when writing ismv (Smooth Streaming) files. @item -movflags faststart Run a second pass moving the moov atom on top of the file. This operation can take a while, and will not work in various situations such as fragmented output, thus it is not enabled by default. @item -movflags rtphint Add RTP hinting tracks to the output file. @end table Smooth Streaming content can be pushed in real time to a publishing point on IIS with this muxer. Example: @example ffmpeg -re @var{} -movflags isml+frag_keyframe -f ismv http://server/publishingpoint.isml/Streams(Encoder1) @end example @section mp3 The MP3 muxer writes a raw MP3 stream with an ID3v2 header at the beginning and optionally an ID3v1 tag at the end. ID3v2.3 and ID3v2.4 are supported, the @code{id3v2_version} option controls which one is used. The legacy ID3v1 tag is not written by default, but may be enabled with the @code{write_id3v1} option. For seekable output the muxer also writes a Xing frame at the beginning, which contains the number of frames in the file. It is useful for computing duration of VBR files. The muxer supports writing ID3v2 attached pictures (APIC frames). The pictures are supplied to the muxer in form of a video stream with a single packet. There can be any number of those streams, each will correspond to a single APIC frame. The stream metadata tags @var{title} and @var{comment} map to APIC @var{description} and @var{picture type} respectively. See @url{http://id3.org/id3v2.4.0-frames} for allowed picture types. Note that the APIC frames must be written at the beginning, so the muxer will buffer the audio frames until it gets all the pictures. It is therefore advised to provide the pictures as soon as possible to avoid excessive buffering. Examples: Write an mp3 with an ID3v2.3 header and an ID3v1 footer: @example ffmpeg -i INPUT -id3v2_version 3 -write_id3v1 1 out.mp3 @end example To attach a picture to an mp3 file select both the audio and the picture stream with @code{map}: @example ffmpeg -i input.mp3 -i cover.png -c copy -map 0 -map 1 -metadata:s:v title="Album cover" -metadata:s:v comment="Cover (Front)" out.mp3 @end example @section mpegts MPEG transport stream muxer. This muxer implements ISO 13818-1 and part of ETSI EN 300 468. The muxer options are: @table @option @item -mpegts_original_network_id @var{number} Set the original_network_id (default 0x0001). This is unique identifier of a network in DVB. Its main use is in the unique identification of a service through the path Original_Network_ID, Transport_Stream_ID. @item -mpegts_transport_stream_id @var{number} Set the transport_stream_id (default 0x0001). This identifies a transponder in DVB. @item -mpegts_service_id @var{number} Set the service_id (default 0x0001) also known as program in DVB. @item -mpegts_pmt_start_pid @var{number} Set the first PID for PMT (default 0x1000, max 0x1f00). @item -mpegts_start_pid @var{number} Set the first PID for data packets (default 0x0100, max 0x0f00). @end table The recognized metadata settings in mpegts muxer are @code{service_provider} and @code{service_name}. If they are not set the default for @code{service_provider} is "FFmpeg" and the default for @code{service_name} is "Service01". @example ffmpeg -i file.mpg -c copy \ -mpegts_original_network_id 0x1122 \ -mpegts_transport_stream_id 0x3344 \ -mpegts_service_id 0x5566 \ -mpegts_pmt_start_pid 0x1500 \ -mpegts_start_pid 0x150 \ -metadata service_provider="Some provider" \ -metadata service_name="Some Channel" \ -y out.ts @end example @section null Null muxer. This muxer does not generate any output file, it is mainly useful for testing or benchmarking purposes. For example to benchmark decoding with @command{ffmpeg} you can use the command: @example ffmpeg -benchmark -i INPUT -f null out.null @end example Note that the above command does not read or write the @file{out.null} file, but specifying the output file is required by the @command{ffmpeg} syntax. Alternatively you can write the command as: @example ffmpeg -benchmark -i INPUT -f null - @end example @section ogg Ogg container muxer. @table @option @item -page_duration @var{duration} Preferred page duration, in microseconds. The muxer will attempt to create pages that are approximately @var{duration} microseconds long. This allows the user to compromise between seek granularity and container overhead. The default is 1 second. A value of 0 will fill all segments, making pages as large as possible. A value of 1 will effectively use 1 packet-per-page in most situations, giving a small seek granularity at the cost of additional container overhead. @end table @section segment, stream_segment, ssegment Basic stream segmenter. The segmenter muxer outputs streams to a number of separate files of nearly fixed duration. Output filename pattern can be set in a fashion similar to @ref{image2}. @code{stream_segment} is a variant of the muxer used to write to streaming output formats, i.e. which do not require global headers, and is recommended for outputting e.g. to MPEG transport stream segments. @code{ssegment} is a shorter alias for @code{stream_segment}. Every segment starts with a keyframe of the selected reference stream, which is set through the @option{reference_stream} option. Note that if you want accurate splitting for a video file, you need to make the input key frames correspond to the exact splitting times expected by the segmenter, or the segment muxer will start the new segment with the key frame found next after the specified start time. The segment muxer works best with a single constant frame rate video. Optionally it can generate a list of the created segments, by setting the option @var{segment_list}. The list type is specified by the @var{segment_list_type} option. The segment muxer supports the following options: @table @option @item reference_stream @var{specifier} Set the reference stream, as specified by the string @var{specifier}. If @var{specifier} is set to @code{auto}, the reference is choosen automatically. Otherwise it must be a stream specifier (see the ``Stream specifiers'' chapter in the ffmpeg manual) which specifies the reference stream. The default value is @code{auto}. @item segment_format @var{format} Override the inner container format, by default it is guessed by the filename extension. @item segment_list @var{name} Generate also a listfile named @var{name}. If not specified no listfile is generated. @item segment_list_flags @var{flags} Set flags affecting the segment list generation. It currently supports the following flags: @table @samp @item cache Allow caching (only affects M3U8 list files). @item live Allow live-friendly file generation. @end table Default value is @code{samp}. @item segment_list_size @var{size} Update the list file so that it contains at most the last @var{size} segments. If 0 the list file will contain all the segments. Default value is 0. @item segment_list_type @var{type} Specify the format for the segment list file. The following values are recognized: @table @samp @item flat Generate a flat list for the created segments, one segment per line. @item csv, ext Generate a list for the created segments, one segment per line, each line matching the format (comma-separated values): @example @var{segment_filename},@var{segment_start_time},@var{segment_end_time} @end example @var{segment_filename} is the name of the output file generated by the muxer according to the provided pattern. CSV escaping (according to RFC4180) is applied if required. @var{segment_start_time} and @var{segment_end_time} specify the segment start and end time expressed in seconds. A list file with the suffix @code{".csv"} or @code{".ext"} will auto-select this format. @samp{ext} is deprecated in favor or @samp{csv}. @item ffconcat Generate an ffconcat file for the created segments. The resulting file can be read using the FFmpeg @ref{concat} demuxer. A list file with the suffix @code{".ffcat"} or @code{".ffconcat"} will auto-select this format. @item m3u8 Generate an extended M3U8 file, version 3, compliant with @url{http://tools.ietf.org/id/draft-pantos-http-live-streaming}. A list file with the suffix @code{".m3u8"} will auto-select this format. @end table If not specified the type is guessed from the list file name suffix. @item segment_time @var{time} Set segment duration to @var{time}, the value must be a duration specification. Default value is "2". See also the @option{segment_times} option. Note that splitting may not be accurate, unless you force the reference stream key-frames at the given time. See the introductory notice and the examples below. @item segment_time_delta @var{delta} Specify the accuracy time when selecting the start time for a segment, expressed as a duration specification. Default value is "0". When delta is specified a key-frame will start a new segment if its PTS satisfies the relation: @example PTS >= start_time - time_delta @end example This option is useful when splitting video content, which is always split at GOP boundaries, in case a key frame is found just before the specified split time. In particular may be used in combination with the @file{ffmpeg} option @var{force_key_frames}. The key frame times specified by @var{force_key_frames} may not be set accurately because of rounding issues, with the consequence that a key frame time may result set just before the specified time. For constant frame rate videos a value of 1/2*@var{frame_rate} should address the worst case mismatch between the specified time and the time set by @var{force_key_frames}. @item segment_times @var{times} Specify a list of split points. @var{times} contains a list of comma separated duration specifications, in increasing order. See also the @option{segment_time} option. @item segment_frames @var{frames} Specify a list of split video frame numbers. @var{frames} contains a list of comma separated integer numbers, in increasing order. This option specifies to start a new segment whenever a reference stream key frame is found and the sequential number (starting from 0) of the frame is greater or equal to the next value in the list. @item segment_wrap @var{limit} Wrap around segment index once it reaches @var{limit}. @item segment_start_number @var{number} Set the sequence number of the first segment. Defaults to @code{0}. @item reset_timestamps @var{1|0} Reset timestamps at the begin of each segment, so that each segment will start with near-zero timestamps. It is meant to ease the playback of the generated segments. May not work with some combinations of muxers/codecs. It is set to @code{0} by default. @item initial_offset @var{offset} Specify timestamp offset to apply to the output packet timestamps. The argument must be a time duration specification, and defaults to 0. @end table @subsection Examples @itemize @item To remux the content of file @file{in.mkv} to a list of segments @file{out-000.nut}, @file{out-001.nut}, etc., and write the list of generated segments to @file{out.list}: @example ffmpeg -i in.mkv -codec copy -map 0 -f segment -segment_list out.list out%03d.nut @end example @item As the example above, but segment the input file according to the split points specified by the @var{segment_times} option: @example ffmpeg -i in.mkv -codec copy -map 0 -f segment -segment_list out.csv -segment_times 1,2,3,5,8,13,21 out%03d.nut @end example @item As the example above, but use the @command{ffmpeg} @option{force_key_frames} option to force key frames in the input at the specified location, together with the segment option @option{segment_time_delta} to account for possible roundings operated when setting key frame times. @example ffmpeg -i in.mkv -force_key_frames 1,2,3,5,8,13,21 -codec:v mpeg4 -codec:a pcm_s16le -map 0 \ -f segment -segment_list out.csv -segment_times 1,2,3,5,8,13,21 -segment_time_delta 0.05 out%03d.nut @end example In order to force key frames on the input file, transcoding is required. @item Segment the input file by splitting the input file according to the frame numbers sequence specified with the @option{segment_frames} option: @example ffmpeg -i in.mkv -codec copy -map 0 -f segment -segment_list out.csv -segment_frames 100,200,300,500,800 out%03d.nut @end example @item To convert the @file{in.mkv} to TS segments using the @code{libx264} and @code{libfaac} encoders: @example ffmpeg -i in.mkv -map 0 -codec:v libx264 -codec:a libfaac -f ssegment -segment_list out.list out%03d.ts @end example @item Segment the input file, and create an M3U8 live playlist (can be used as live HLS source): @example ffmpeg -re -i in.mkv -codec copy -map 0 -f segment -segment_list playlist.m3u8 \ -segment_list_flags +live -segment_time 10 out%03d.mkv @end example @end itemize @section tee The tee muxer can be used to write the same data to several files or any other kind of muxer. It can be used, for example, to both stream a video to the network and save it to disk at the same time. It is different from specifying several outputs to the @command{ffmpeg} command-line tool because the audio and video data will be encoded only once with the tee muxer; encoding can be a very expensive process. It is not useful when using the libavformat API directly because it is then possible to feed the same packets to several muxers directly. The slave outputs are specified in the file name given to the muxer, separated by '|'. If any of the slave name contains the '|' separator, leading or trailing spaces or any special character, it must be escaped (see the ``Quoting and escaping'' section in the ffmpeg-utils manual). Options can be specified for each slave by prepending them as a list of @var{key}=@var{value} pairs separated by ':', between square brackets. If the options values contain a special character or the ':' separator, they must be escaped; note that this is a second level escaping. Example: encode something and both archive it in a WebM file and stream it as MPEG-TS over UDP (the streams need to be explicitly mapped): @example ffmpeg -i ... -c:v libx264 -c:a mp2 -f tee -map 0:v -map 0:a "archive-20121107.mkv|[f=mpegts]udp://10.0.1.255:1234/" @end example Note: some codecs may need different options depending on the output format; the auto-detection of this can not work with the tee muxer. The main example is the @option{global_header} flag. @c man end MUXERS