libidn2  2.3.4
About: GNU Libidn2 is an implementation of the IDNA2008, Punycode and TR46 specifications used for (Unicode) internationalized domain names.
  Fossies Dox: libidn2-2.3.4.tar.gz  ("unofficial" and yet experimental doxygen-generated source code documentation)  

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libidn2 Documentation

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Libidn2 README -- Introduction information

Libidn2 is a free software implementation of IDNA2008, Punycode and Unicode TR46. Its purpose is to encode and decode internationalized domain names.

For technical reference, see:

The library contains functionality to convert internationalized domain names to and from ASCII Compatible Encoding (ACE).

The API consists of two main functions, idn2_to_ascii_8z for converting data from UTF-8 to ASCII Compatible Encoding (ACE), and idn2_to_unicode_8z8z to convert ACE names into UTF-8 format. There are several variations of these main functions, which accept UTF-32, or input in the local system encoding. All functions assume zero-terminated strings.

This library is backwards (API) compatible with the libidn library. Replacing the idna.h header with idn2.h into a program is sufficient to switch the application from IDNA2003 to IDNA2008 as supported by this library.

Libidn2 is believed to be a complete IDNA2008 and TR46 implementation, it contains an extensive test-suite, and is included in the continuous fuzzing project OSS-Fuzz.

You can check the current test code coverage here and the current fuzzing code coverage here.


The installed C library libidn2 is dual-licensed under LGPLv3+|GPLv2+, while the rest of the package is GPLv3+. See the file COPYING for detailed information.

Online docs

API reference


Obtaining the source

Software releases of libidn2 can be downloaded from and

Development of libidn2 is organized through GitLab website, and there is an issue tracker for reporting bugs.

Building & Dependencies

To build Libidn2 you will need a POSIX shell, the Unix "make" tool, and a C compiler.

When building from a released .tar. archive, after unpacking you build the package like this:

make check

The Libidn2 library may use GNU libunistring for Unicode processing and GNU libiconv for character set conversion. It is recommended to install them before building and installing libidn2. See the following links for more information on these packages:

The iconv dependency is optional -- it is required for the functions involving locale to UTF8 conversions -- but is recommended.

When the recommended libunistring is not available, libidn2 uses internal replacement functionality which increases the size of the library. To use the internal libunistring-replacement rather than the system libunistring (even when deemed to be sufficient) you may use:

./configure --with-included-libunistring


See the contributing document.

Estimating code coverage


  • lcov (for code coverage)

To test the code coverage of the test suite use the following:

./configure --enable-code-coverage
make && make check && make code-coverage-capture

The current coverage report can be found here.


Libidn2 is being continuously fuzzed by OSS-Fuzz.

Of course you can do local fuzzing on your own, see fuzz/ for instructions.

The code coverage of our fuzzers can be found here.