C++ library for reading and writing MP4 (iTunes), ID3, Vorbis, Opus, FLAC and Matroska tags.

Supported formats

The tag library can read and write the following tag formats:

iTunes-style MP4/M4A tags (MP4-DASH is supported)
ID3v1 and ID3v2 tags
- conversion between ID3v1 and different versions of ID3v2 is possible
- mainly for use in MP3 files but can be added to any kind of file
Vorbis, Opus and FLAC comments in Ogg streams
- cover art via "METADATA_BLOCK_PICTURE" is supported
Vorbis comments and "METADATA_BLOCK_PICTURE" in raw FLAC streams
Matroska/WebM tags and attachments

Further remarks:

Unsupported file contents (such as unsupported tag formats) are generally preserved as-is.
Note that APE tags are not supported. APE tags in the beginning of a file are strongly unrecommended and thus discarded when applying changes. APE tags at the end of the file are preserved as-is when applying changes.

File layout options

Tag position

The library allows you to choose whether tags should be placed at the beginning or at the end of an MP4/Matroska file.

Padding

Padding allows adding additional tag information without rewriting the entire file or appending the tag. Usage of padding can be configured:

minimum/maximum padding: The file is rewritten if the padding would fall below/exceed the specified limits.
preferred padding: If the file needs to be rewritten the preferred padding is used.

Default value for minimum and maximum padding is zero. Hence the library will almost always have to rewrite the entire file to apply changes. To prevent this, set at least the maximum padding to a higher value.

It is also possible to force rewriting the entire file always.

Taking advantage of padding is currently not supported when dealing with Ogg streams (it is supported when dealing with raw FLAC streams).

Additional features

The library can also display technical information such as the ID, format, language, bitrate, duration, size, timestamps, sampling frequency, FPS and other information of the tracks.

It also allows to inspect and validate the element structure of MP4 and Matroska files.

Text encoding, Unicode support

The library is aware of different text encodings and can convert between different encodings using iconv.

Usage

For building/installing the library, checkout the build instructions below.
To use the library with CMake, use its find module (e.g. find_package(tagparser 10.1.0 REQUIRED)) which provides the imported target tagparser you can link against. Otherwise, use the pkg-config module tagparser to query the required compiler/linker flags.
For a code example that shows how to read and write tag fields in a format-independent way, have a look at example.cpp.
The most important class is TagParser::MediaFileInfo providing access to everything else.
IO errors are propagated via standard std::ios_base::failure.
Fatal processing errors are propagated by throwing a class derived from TagParser::Failure.
All operations which might generate warnings, non-fatal errors, ... take a TagParser::Diagnostics object to store those messages.
All operations which might be aborted or might provide progress feedback take a TagParser::AbortableProgressFeedback object for callbacks and aborting.
Field values are stored using TagParser::TagValue objects. Those objects erase the actual type similar to QVariant from the Qt framework. The documentation of TagParser::TagValue covers how different types and encodings are handled.

Further documentation

For more examples check out the command line interface of Tag Editor. API documentation can be generated using Doxygen with cmake --build … --target tagparser_apidoc.

Bugs, stability

Bugs can be reported on GitHub.

It is recommend to create backups before editing because I can not test whether the library works with all kinds of files. (When forcing rewrite a backup is always created.)

Build instructions

The tagparser library depends on c++utilities and is built in the same way. It also depends on zlib, iso-codes and requires at least CMake 3.19. Tests depend on CppUnit. For checking integrity of testfiles, the OpenSSL crypto library is required.

The location of the JSON file from iso-codes can be specified via the CMake variable LANGUAGE_FILE_ISO_639_2.

For building multiple projects in one go (c++utilities, tagparser and the tag editor), checkout the "Building this straight" instructions.

TODOs

Support more formats (EXIF, PDF metadata, Theora, ...)
Support adding cue-sheet to FLAC files

More TODOs are tracked in the issue section at GitHub.

Copyright notice and license

All code is licensed under [GPL-2-or-later](LICENSE).