Compiling Vala applications and libraries

Meson supports compiling applications and libraries written in Vala and Genie . A skeleton meson.build file:

project('vala app', 'vala', 'c')

dependencies = [
    dependency('glib-2.0'),
    dependency('gobject-2.0'),
]

sources = files('app.vala')

executable('app_name', sources, dependencies: dependencies)

You must always specify the glib-2.0 and gobject-2.0 libraries as dependencies, because all current Vala applications use them. GLib is used for basic data types and GObject is used for the runtime type system.

Using libraries

Meson uses the dependency() function to find the relevant VAPI, C headers and linker flags when it encounters a Vala source file in a build target. Vala needs a VAPI file and a C header or headers to use a library. The VAPI file helps map Vala code to the library's C programming interface. It is the pkg-config tool that makes finding these installed files all work seamlessly behind the scenes. When a pkg-config file doesn't exist for the library then the find_library() method of the compiler object needs to be used. Examples are given later.

Note Vala uses libraries that follow the C Application Binary Interface (C ABI). The library, however, could be written in C, Vala, Rust, Go, C++ or any other language that can generate a binary compatible with the C ABI and so provides C headers.

The simplest case

This first example is a simple addition to the meson.build file because:

  • the library has a pkg-config file, gtk+-3.0.pc
  • the VAPI is distributed with Vala and so installed with the Vala compiler
  • the VAPI is installed in Vala's standard search path
  • the VAPI, gtk+-3.0.vapi, has the same name as the pkg-config file

Everything works seamlessly in the background and only a single extra line is needed:

project('vala app', 'vala', 'c')

dependencies = [
    dependency('glib-2.0'),
    dependency('gobject-2.0'),
    dependency('gtk+-3.0'),
]

sources = files('app.vala')

executable('app_name', sources, dependencies: dependencies)

GTK+ is the graphical toolkit used by GNOME, elementary OS and other desktop environments. The binding to the library, the VAPI file, is distributed with Vala.

Other libraries may have a VAPI that is distributed with the library itself. Such libraries will have their VAPI file installed along with their other development files. The VAPI is installed in Vala's standard search path and so works just as seamlessly using the dependency() function.

Targeting a version of GLib

Meson's dependency() function allows a version check of a library. This is often used to check a minimum version is installed. When setting a minimum version of GLib, Meson will also pass this to the Vala compiler using the --target-glib option.

This is needed when using GTK+'s user interface definition files with Vala's [GtkTemplate], [GtkChild] and [GtkCallback] attributes. This requires --target-glib 2.38, or a newer version, to be passed to Vala. With Meson this is simply done with:

project('vala app', 'vala', 'c')

dependencies = [
    dependency('glib-2.0', version: '>=2.38'),
    dependency('gobject-2.0'),
    dependency('gtk+-3.0'),
]

sources = files('app.vala')

executable('app_name', sources, dependencies: dependencies)

Using [GtkTemplate] also requires the GTK+ user interface definition files to be built in to the binary as GResources. For completeness, the next example shows this:

project('vala app', 'vala', 'c')

dependencies = [
    dependency('glib-2.0', version: '>=2.38'),
    dependency('gobject-2.0'),
    dependency('gtk+-3.0'),
]

sources = files('app.vala')

sources += import( 'gnome' ).compile_resources(
    'project-resources',
    'src/resources/resources.gresource.xml',
    source_dir: 'src/resources',
)

executable('app_name', sources, dependencies: dependencies)

Adding to Vala's search path

So far we have covered the cases where the VAPI file is either distributed with Vala or the library. A VAPI can also be included in the source files of your project. The convention is to put it in the vapi directory of your project.

This is needed when a library does not have a VAPI or your project needs to link to another component in the project that uses the C ABI. For example if part of the project is written in C.

The Vala compiler's --vapidir option is used to add the project directory to the VAPI search path. In Meson this is done with the add_project_arguments() function:

project('vala app', 'vala', 'c')

vapi_dir = meson.current_source_dir() / 'vapi'

add_project_arguments(['--vapidir', vapi_dir], language: 'vala')

dependencies = [
    dependency('glib-2.0'),
    dependency('gobject-2.0'),
    dependency('foo'), # 'foo.vapi' will be resolved as './vapi/foo.vapi'
]

sources = files('app.vala')

executable('app_name', sources, dependencies: dependencies)

If the VAPI is for an external library then make sure that the VAPI name corresponds to the pkg-config file name.

The vala-extra-vapis repository is a community maintained repository of VAPIs that are not distributed. Developers use the repository to share early work on new bindings and improvements to existing bindings. So the VAPIs can frequently change. It is recommended VAPIs from this repository are copied in to your project's source files.

This also works well for starting to write new bindings before they are shared with the vala-extra-vapis repository.

Libraries without pkg-config files

A library that does not have a corresponding pkg-config file may mean dependency() is unsuitable for finding the C and Vala interface files. In this case it is necessary to use the find_library() method of the compiler object.

The first example uses Vala's POSIX binding. There is no pkg-config file because POSIX includes the standard C library on Unix systems. All that is needed is the VAPI file, posix.vapi. This is included with Vala and installed in Vala's standard search path. Meson just needs to be told to only find the library for the Vala compiler:

project('vala app', 'vala', 'c')

dependencies = [
    dependency('glib-2.0'),
    dependency('gobject-2.0'),
    meson.get_compiler('vala').find_library('posix'),
]

sources = files('app.vala')

executable('app_name', sources, dependencies: dependencies)

The next example shows how to link with a C library where no additional VAPI is needed. The standard maths functions are already bound in glib-2.0.vapi, but the GNU C library requires linking to the maths library separately. In this example Meson is told to find the library only for the C compiler:

project('vala app', 'vala', 'c')

dependencies = [
    dependency('glib-2.0'),
    dependency('gobject-2.0'),
    meson.get_compiler('c').find_library('m', required: false),
]

sources = files('app.vala')

executable('app_name', sources, dependencies: dependencies)

The required: false means the build will continue when using another C library that does not separate the maths library. See Add math library (-lm) portably.

The final example shows how to use a library that does not have a pkg-config file and the VAPI is in the vapi directory of your project source files:

project('vala app', 'vala', 'c')

vapi_dir = meson.current_source_dir() / 'vapi'

add_project_arguments(['--vapidir', vapi_dir], language: 'vala')

dependencies = [
    dependency('glib-2.0'),
    dependency('gobject-2.0'),
    meson.get_compiler('c').find_library('foo'),
    meson.get_compiler('vala').find_library('foo', dir: vapi_dir),
]

sources = files('app.vala')

executable('app_name', sources, dependencies: dependencies)

The find_library() method of the C compiler object will try to find the C header files and the library to link with.

The find_library() method of the Vala compiler object needs to have the dir keyword added to include the project VAPI directory. This is not added automatically by add_project_arguments().

Building libraries

Changing C header and VAPI names

Meson's library target automatically outputs the C header and the VAPI. They can be renamed by setting the vala_header and vala_vapi arguments respectively:

foo_lib = shared_library('foo', 'foo.vala',
                  vala_header: 'foo.h',
                  vala_vapi: 'foo-1.0.vapi',
                  dependencies: [glib_dep, gobject_dep],
                  install: true,
                  install_dir: [true, true, true])

In this example, the second and third elements of the install_dir array indicate the destination with true to use default directories (i.e. include and share/vala/vapi).

GObject Introspection and language bindings

A 'binding' allows another programming language to use a library written in Vala. Because Vala uses the GObject type system as its runtime type system it is very easy to use introspection to generate a binding. A Meson build of a Vala library can generate the GObject introspection metadata. The metadata is then used in separate projects with language specific tools to generate a binding.

The main form of metadata is a GObject Introspection Repository (GIR) XML file. GIRs are mostly used by languages that generate bindings at compile time. Languages that generate bindings at runtime mostly use a typelib file, which is generated from the GIR.

Meson can generate a GIR as part of the build. For a Vala library the vala_gir option has to be set for the library:

foo_lib = shared_library('foo', 'foo.vala',
                  vala_gir: 'Foo-1.0.gir',
                  dependencies: [glib_dep, gobject_dep],
                  install: true,
                  install_dir: [true, true, true, true])

The true value in install_dir tells Meson to use the default directory (i.e. share/gir-1.0 for GIRs). The fourth element in the install_dir array indicates where the GIR file will be installed.

To then generate a typelib file use a custom target with the g-ir-compiler program and a dependency on the library:

g_ir_compiler = find_program('g-ir-compiler')
custom_target('foo typelib', command: [g_ir_compiler, '--output', '@OUTPUT@', '@INPUT@'],
              input: meson.current_build_dir() / 'Foo-1.0.gir',
              output: 'Foo-1.0.typelib',
              depends: foo_lib,
              install: true,
              install_dir: get_option('libdir') / 'girepository-1.0')

The results of the search are