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File system layout

There’s no dedicated include directory. Each include file is located in the same directory its accompanying cpp file is located in as well.

Directories in alphabetical order:

Strings & encoding

Internally all strings are encoded in UTF-8. Strings must be re-coded during input/output.


All timestamps and durations are stored as int64_t variables with nanosecond precision.

Outputting messages to the user

There are basically four functions that output stuff: mxinfo(), mxverb(), mxwarn() and mxerror(). Each takes a string. Formatting is done via the fmt::format() if required (see the fmt documentation if you’re not familiar with it), and mxverb() also takes a verbosity level argument.

mxinfo() is supposed to be used for messages that are always output. Its messages must be translatable by using the Y() macro, Example:

mxinfo(fmt::format(Y("Hello, {0}\n")) % user_name);

The same applies to mxwarn() and mxerror(). However, both prefix their messages with “Warning:” and “Error:” respectively. Also they modify mkvmerge’s exit code – “1” if mxwarn() has been used at least once and “2” if mxerror() is used. mxerror() also instantly exits the program. The GUI shows such warnings and errors in separate areas as to draw the user’s attention to them. Therefore mxwarn() should be used for stuff the user really should know, and mxerror() for stuff that prevents further processing altogether.

mxverb() is a debugging facility. mkvmerge starts out at verbosity level 1. Therefore mxverb(1, ...) is equivalent to mxinfo(...). You can raise the verbosity level by adding the “-v” or “–verbose” command line parameters multiple times. Messages output with mxverb() should not be translatable, meaning you shouldn’t use the _() macro.



Most classes are nameed *_c, and the corresponding counted pointer implementation is called *_cptr. A counted pointer is a reference counter implementation that automatically deletes its instance once the counter reaches zero.

memory_c (from common/memory.h)

Stores a pointer to a buffer and its size. Can free the memory if the buffer has been malloced.

Allocating memory:

memory_cptr mem = memory_c::alloc(12345);

Copying/reading something into it:

memcpy(mem->get_buffer(), source_pointer, mem->get_size());
file->read(mem, 20);

mm_io_c (from common/mm_io.h)

Base class for input/output. Several derived classes implement access to binary files (mm_file_io_c), text files (mm_text_io_c, including BOM handling), memory buffers (mm_mem_io_c) etc.

mm_file_io_c (from common/mm_io.h)

File input/output class. Opening a file for reading and retrieving its size:

mm_file_io_cptr file(new mm_file_io_c(file_name));
int64_t size = file->get_size();


// Seek to absolute position 123:
file->setFilePointer(123, seek_beginning);
// Seek forward 61 bytes:
file->setFilePointer(61, seek_current);


size_t num_read = file->read(some_buffer, 12345);

track_info_c (from src/merge/pr_generic.h)

A file and track information storage class. It is created by the command line parser with the source file name and all the command line options the user has passed in.

The instance is then passed to the reader which stores a copy in the m_ti variable. The reader also passes that copy to each packetizer it creates which in turn stores its own copy (again in the m_ti variable).


A reader is a class that demuxes a certain file type. It has to parse the file content, create one packetizer for each track, report the progress etc. Each reader class is derived from generic_reader_c (from src/merge/pr_generic.h).

An example for a rather minimal implementation is the MP3 reader in src/input/r_mp3.h.

Constructor & virtual destructor

The constructor usually only takes a track_info_c argument. A virtual destructor must be present, even if it is empty. Otherwise you’ll get linker errors.

The constructor must open the file and parse all its headers so that all track information has been processed. This will most likely be split up into a separate function in the future.

read(generic_packetizer_c *ptzr, bool force = false)

Requests that the reader reads data for a specific track. mkvmerge uses a pulling model: the core asks each packetizer to provide data. The packetizers in turn ask the reader they’ve been created from to read data by calling this function.

If the reader supports arbitrary access to track data (e.g. for AVI and MP4/MOV files) then the reader should only read data for the requested track in order not to consume too much memory. If the file format doesn’t allow for direct access to that data then the reader can simply read the next packet regardless which track that packet belongs to. The packetizer will call the reader’s read() function as often as necessary until it has enough data.

The reader must return FILE_STATUS_MOREDATA if more data is available and FILE_STATUS_DONE when the end has been reached.

Each data packet is stored in an instance of packet_c. If the source container provides a timestamp then that timestamp should be set in the packet as well.

create_packetizer(int64_t track_id)

Has to create a packetizer for the track with the specific ID. This ID is the same number that the user uses on the command line as well as the number used in the call to id_result_track() (see below).

This function has to verify that the user actually wants this track processed. This is checked with the demuxing_requested() function. Example from the MP3 reader (as the MP3 file format can only contain one track the ID is always 0; see the Matroska reader for more complex examples):

// Check if the audio demuxer with ID 0 is requested. Also make
// sure that the number of packetizers this reader has created is
// still 0.
if (!demuxing_requested('a', 0) || (NPTZR() != 0))

// Create the actual packetizer.
add_packetizer(new mp3_packetizer_c(this, m_ti, mp3header.sampling_frequency, mp3header.channels, false));

// Inform the user.
show_packetizer_info(0, PTZR0);

A lot of packetizers expect their codec private data to be constructed completely by the reader. This often requires that the reader processes at least a few packets. For a rather complex case see the MPEG PS reader’s handling of h264 tracks.


Returns the demuxing progress, a value between 0 and 100. Usually based on the number of bytes processed.


File identification. Has to call id_result_container() once for the container type.

For each supported track the function must call id_result_track(...) once.

Both the container and the track identification can contain extended information. For an extensive example see the Matroska reader’s identification function in src/input/r_matroska.cpp.


These are general hints for anyone wishing to translate MKVToolNix.

There are basically three things that can be translated in the MKVToolNix package:

  1. The programs and their messages themselves. This is the most important part.
  2. The manual pages.

All three are translated by the gettext system. This system uses .po-files which map the original English version to the translated version. There are several (OpenSource) applications available that can be used to translate such files, e.g. poedit if you’re using Windows.

Alternatively you can use the Transifex web service for translating MKVToolNix. Please apply to join the MKVToolNix project and drop me a line via email. I’ll add you to the project as soon as possible.

Please ask the author for an up-to-date template of the untranslated strings before you start doing anything else. He will attach the untranslated translation file (xyz.po) for the programs themselves (the first point of the three mentioned above). Take a look at it and at the poedit software. If you feel up to the task then simply start translating and send the translated .po file back to the author. The author can notify you whenever phrases change or are added.

If you want to test your translations you can do that as well:

  1. Install mkvtoolnix.
  2. Translate the xyz.po file.
  3. Whenever you want to test something compile the xyz.po to a .mo file with your po editor application.
  4. Replace one of the installed translation files (e.g. mkvtoolnix\locale\de\mkvtoolnix.mo) with your compiled file.
  5. Start the GUI, go to the preferences dialog and select ‘German’ as the language (the ‘de’ file you’ve replaced in step 4 is for German).

Please note that a lot of strings must not be translated, especially command line options and their values. If a value to an option is a fixed string then it must not be translated, but if it is a placeholder e.g. for a file name then you can of course translate the filename part. Example:

--stereo-mode <TID:[none|both]>

Here neither --stereo-mode nor none or both may be translated because mkvmerge expects all those in English. But in

--attach-file filename

you can of course translate filename.

Some other hints:

If you’re unsure what the author is trying to say (the author is not a native English speaker) or what you can and what you must not translate just drop him a note.

You can send the author your work in progress whenever you feel like it – you don’t have to finish everything at once.

Once you have shown that you are comfortable with translating you can also get write access to the project’s Git repository and push updates there yourself.

Adding new translations

This is a TODO list for adding a new translation (a .po file) to MKVToolNix: