# libyang

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libyang is a YANG data modelling language parser and toolkit written (and

providing API) in C. The library is used e.g. in [libnetconf2](https://github.com/CESNET/libnetconf2),

[Netopeer2](https://github.com/CESNET/Netopeer2) or [sysrepo](https://github.com/sysrepo/sysrepo) projects.

## Provided Features

* Parsing (and validating) schemas in YANG format.

* Parsing (and validating) schemas in YIN format.

* Parsing, validating and printing instance data in XML format.

* Parsing, validating and printing instance data in JSON format

  ([RFC 7951](https://tools.ietf.org/html/rfc7951)).

* Manipulation with the instance data.

* Support for default values in the instance data ([RFC 6243](https://tools.ietf.org/html/rfc6243)).

* Support for YANG extensions.

* Support for YANG Metadata ([RFC 7952](https://tools.ietf.org/html/rfc6243)).

* [yanglint](#yanglint) - feature-rich YANG tool.

Current implementation covers YANG 1.0 ([RFC 6020](https://tools.ietf.org/html/rfc6020))

as well as YANG 1.1 ([RFC 7950](https://tools.ietf.org/html/rfc7950)).

## Packages

We are using openSUSE Build Service to automaticaly prepare binary packages for number of GNU/Linux distros. Check

[this](https://software.opensuse.org//download.html?project=home%3Aliberouter&package=libyang) page and follow the

instructions for your distro to install `libyang` package. The `libyang` package is built once a day from the

master branch. If you want the latest code from the devel branch, install `libyang-experimental` package.

## Requirements

### Build Requirements

* C compiler (gcc >= 4.9, clang >= 3.0, ...)

* cmake >= 2.8.12

* libpcre (devel package)

 * note, that PCRE is supposed to be compiled with unicode support (configure's options

   `--enable-utf` and `--enable-unicode-properties`)

* cmocka >= 1.0.0 (for tests only, see [Tests](#Tests))

#### Optional

* doxygen (for generating documentation)

* valgrind (for enhanced testing)

### Runtime Requirements

* libpcre

## Building

```

$ mkdir build; cd build

$ cmake ..

$ make

# make install

```

### Documentation

The library documentation can be generated directly from the source codes using

Doxygen tool:

```

$ make doc

$ google-chrome ../doc/html/index.html

```

The documentation is also built hourly and available at

[netopeer.liberouter.org](https://netopeer.liberouter.org/doc/libyang/master/).

### Useful CMake Options

#### Changing Compiler

Set `CC` variable:

```

$ CC=/usr/bin/clang cmake ..

```

#### Changing Install Path

To change the prefix where the library, headers and any other files are installed,

set `CMAKE_INSTALL_PREFIX` variable:

```

$ cmake -DCMAKE_INSTALL_PREFIX:PATH=/usr ..

```

Default prefix is `/usr/local`.

#### Build Modes

There are two build modes:

* Release.

  This generates library for the production use without any debug information.

* Debug.

  This generates library with the debug information and disables optimization

  of the code.

The `Debug` mode is currently used as the default one. to switch to the

`Release` mode, enter at the command line:

```

$ cmake -D CMAKE_BUILD_TYPE:String="Release" ..

```

#### Changing Extensions Plugins Directory

As for YANG extensions, libyang allows loading extension plugins. By default, the

directory to store the plugins is LIBDIR/libyang. To change it, use the following

cmake option with the value specifying the desired directory:

```

$ cmake -DPLUGINS_DIR:PATH=`pwd`"/src/extensions/" ..

```

The directory path can be also changed runtime via environment variable, e.g.:

```

$ LIBYANG_EXTENSIONS_PLUGINS_DIR=`pwd`/my/relative/path yanglint

```

#### Optimizations

Whenever the latest revision of a schema is supposed to be loaded (import without specific revision),

it is performed in the standard way, the first time. By default, every other time when the latest

revision of the same schema is needed, the one initially loaded is reused. If you know this can cause

problems meaning the latest available revision of a schema can change during operation, you can force

libyang to always search for the schema anew by:

```

$ cmake -DENABLE_LATEST_REVISIONS=OFF ..

```

Also, it can be efficient to store certain information about schemas that is generated during parsing

so that it does not need to be generated every time the schema is used, but it will consume some

additional space. You can enable this cache with:

```

$ cmake -DENABLE_CACHE=ON ..

```

### CMake Notes

Note that, with CMake, if you want to change the compiler or its options after

you already ran CMake, you need to clear its cache first - the most simple way

to do it is to remove all content from the 'build' directory.

## Usage

All libyang functions are available via the main header:

```

#include <libyang/libyang.h>

```

To compile your program with libyang, it is necessary to link it with libyang using the

following linker parameters:

```

-lyang

```

Note, that it may be necessary to call `ldconfig(8)` after library installation and if the

library was installed into a non-standard path, the path to libyang must be specified to the

linker. To help with setting all the compiler's options, there is `libyang.pc` file for

`pkg-config(1)` available in the source tree. The file is installed with the library.

If you are using `cmake` in you project, it is also possible to use the provided

`FindLibYANG.cmake` file to detect presence of the libyang library in the system.

## yanglint

libyang project includes a feature-rich tool called `yanglint(1)` for validation

and conversion of the schemas and YANG modeled data. The source codes are

located at [`/tools/lint`](./tools/lint) and can be used to explore how an

application is supposed to use the libyang library. `yanglint(1)` binary as

well as its man page are installed together with the library itself.

There is also [README](./tools/lint/examples/README.md) describing some examples of

using `yanglint`.

libyang supports YANG extensions via a plugin mechanism. Some of the plugins (for

NACM or Metadata) are available out of the box and installed together with libyang.

However, when libyang is not installed and `yanglint(1)` is used from the build

directory, the plugins are not available. There are two options:

1. Install libyang.

```

# make install

```

2. Set environment variable `LIBYANG_EXTENSIONS_PLUGINS_DIR` to contain path to the

   built extensions plugin (`./src/extensions` from the build directory).

```

$ LIBYANG_EXTENSIONS_PLUGINS_DIR="`pwd`/src/extensions" ./yanglint

```

## Tests

libyang includes several tests built with [cmocka](https://cmocka.org/). The tests

can be found in `tests` subdirectory and they are designed for checking library

functionality after code changes.

The tests are by default built in the `Debug` build mode by running

```

$ make

```

In case of the `Release` mode, the tests are not built by default (it requires

additional dependency), but they can be enabled via cmake option:

```

$ cmake -DENABLE_BUILD_TESTS=ON ..

```

Note that if the necessary [cmocka](https://cmocka.org/) headers are not present

in the system include paths, tests are not available despite the build mode or

cmake's options.

Tests can be run by the make's `test` target:

```

$ make test

```

## Bindings

We provide bindings for high-level languages using [SWIG](http://www.swig.org/)

generator. The bindings are optional and to enable building of the specific

binding, the appropriate cmake option must be enabled, for example:

```

$ cmake -DJAVASCRIPT_BINDING=ON ..

```

More information about the specific binding can be found in their README files.

Currently supported bindings are:

* JavaScript

 * cmake option: `JAVASCRIPT_BINDING`

 * [README](./swig/javascript/README.md)