@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{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 updatefirst 1|0
If set to 1, update the first written image file again and
again. Default value is 0.
@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.

@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{<normal input/transcoding options>} -movflags isml+frag_keyframe -f ismv http://server/publishingpoint.isml/Streams(Encoder1)
@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 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

@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 ``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 @var
@item cache
Allow caching (only affects M3U8 list files).

@item live
Allow live-friendly file generation.
@end table

Default value is @code{cache}.

@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 @option
@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.

@code{ext} is deprecated in favor or @code{csv}.

@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.
@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 @code{ffmpeg} @var{force_key_frames}
option to force key frames in the input at the specified location, together
with the segment option @var{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 @var{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 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 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 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:

@example
ffmpeg -i ... -c:v libx264 -c:a mp2 -f tee
  "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