meson/docs/markdown/Dependencies.md

---
short-description: Dependencies for external libraries and frameworks
...

# Dependencies

Very few applications are fully self-contained, but rather they use
external libraries and frameworks to do their work. Meson makes it
very easy to find and use external dependencies. Here is how one would
use the zlib compression library.

```meson
zdep = dependency('zlib', version : '>=1.2.8')
exe = executable('zlibprog', 'prog.c', dependencies : zdep)
```

First Meson is told to find the external library `zlib` and error out
if it is not found. The `version` keyword is optional and specifies a
version requirement for the dependency. Then an executable is built
using the specified dependency. Note how the user does not need to
manually handle compiler or linker flags or deal with any other
minutiae.

If you have multiple dependencies, pass them as an array:

```meson
executable('manydeps', 'file.c', dependencies : [dep1, dep2, dep3, dep4])
```

If the dependency is optional, you can tell Meson not to error out if
the dependency is not found and then do further configuration.

```meson
opt_dep = dependency('somedep', required : false)
if opt_dep.found()
  # Do something.
else
  # Do something else.
endif
```

You can pass the `opt_dep` variable to target construction functions
whether the actual dependency was found or not. Meson will ignore
non-found dependencies.

Meson also allows to get variables that are defined in the
`pkg-config` file. This can be done by using the
`get_pkgconfig_variable` function.

```meson
zdep_prefix = zdep.get_pkgconfig_variable('prefix')
```

These variables can also be redefined by passing the `define_variable`
parameter, which might be useful in certain situations:

```meson
zdep_prefix = zdep.get_pkgconfig_variable('libdir', define_variable: ['prefix', '/tmp'])
```

The dependency detector works with all libraries that provide a
`pkg-config` file. Unfortunately several packages don't provide
pkg-config files. Meson has autodetection support for some of these,
and they are described [later in this
page](#dependencies-with-custom-lookup-functionality).

# Declaring your own

You can declare your own dependency objects that can be used
interchangeably with dependency objects obtained from the system. The
syntax is straightforward:

```meson
my_inc = include_directories(...)
my_lib = static_library(...)
my_dep = declare_dependency(link_with : my_lib,
  include_directories : my_inc)
```

This declares a dependency that adds the given include directories and
static library to any target you use it in.

# Building dependencies as subprojects

Many platforms do not provide a system package manager. On these
systems dependencies must be compiled from source. Meson's subprojects
make it simple to use system dependencies when they are available and
to build dependencies manually when they are not.

To make this work, the dependency must have Meson build definitions
and it must declare its own dependency like this:

```meson
    foo_dep = declare_dependency(...)
```

Then any project that wants to use it can write out the following
declaration in their main `meson.build` file.

```meson
    foo_dep = dependency('foo', fallback : ['foo', 'foo_dep'])
```

What this declaration means is that first Meson tries to look up the
dependency from the system (such as by using pkg-config). If it is not
available, then it builds subproject named `foo` and from that
extracts a variable `foo_dep`. That means that the return value of
this function is either an external or an internal dependency
object. Since they can be used interchangeably, the rest of the build
definitions do not need to care which one it is. Meson will take care
of all the work behind the scenes to make this work.

# Dependency method

You can use the keyword `method` to let meson know what method to use
when searching for the dependency. The default value is `auto`.
Aditional dependencies methods are `pkg-config`, `config-tool`, `cmake`,
`system`, `sysconfig`, `qmake`, `extraframework` and `dub`.

```meson
cups_dep = dependency('cups', method : 'pkg-config')
```

The dependency method order for `auto` is:

  1. `pkg-config`
  2. `cmake`
  3. `extraframework` (OSX only)

## CMake

Meson can use the CMake `find_package()` function to detect
dependencies with the builtin `Find<NAME>.cmake` modules and exported
project configurations (usually in `/usr/lib/cmake`). Meson is able
to use both the old-style `<NAME>_LIBRARIES` variables as well as
imported targets.

It is possible to manually specify a list of CMake targets that should
be used with the `modules` property. Howerver, this step is optional
since meson tries to automatically guess the correct target based on the
name of the dependency.

Depending on the dependency it may be neccessary to explicitly specify
a CMake target with the `modules` property if meson is unable to guess
it automatically.

```meson
    cmake_dep = dependency('ZLIB', method : 'cmake', modules : ['ZLIB::ZLIB'])
```

### Some notes on Dub

Please understand that meson is only able to find dependencies that
exist in the local Dub repository. You need to manually fetch and
build the target dependencies.

For `urld`.
```
dub fetch urld
dub build urld
```

Other thing you need to keep in mind is that both meson and Dub need
to be using the same compiler. This can be achieved using Dub's
`-compiler` argument and/or manually setting the `DC` environment
variable when running meson.
```
dub build urld --compiler=dmd
DC="dmd" meson builddir
```

# Dependencies with custom lookup functionality

Some dependencies have specific detection logic.

Generic dependency names are case-sensitive<sup>[1](#footnote1)</sup>,
but these dependency names are matched case-insensitively.  The
recommended style is to write them in all lower-case.

In some cases, more than one detection method exists, and the `method` keyword
may be used to select a detection method to use.  The `auto` method uses any
checking mechanisms in whatever order meson thinks is best.

e.g. libwmf and CUPS provide both pkg-config and config-tool support. You can
force one or another via the `method` keyword:

```meson
cups_dep = dependency('cups', method : 'pkg-config')
wmf_dep = dependency('libwmf', method : 'config-tool')
```

## Dependencies using config tools

[CUPS](#cups), [LLVM](#llvm), [pcap](#pcap), [WxWidgets](#wxwidgets),
[libwmf](#libwmf), [GCrypt](#libgcrypt), and GnuStep either do not provide pkg-config
modules or additionally can be detected via a config tool
(cups-config, llvm-config, libgcrypt-config, etc). Meson has native support for these
tools, and they can be found like other dependencies:

```meson
pcap_dep = dependency('pcap', version : '>=1.0')
cups_dep = dependency('cups', version : '>=1.4')
llvm_dep = dependency('llvm', version : '>=4.0')
libgcrypt_dep = dependency('libgcrypt', version: '>= 1.8')
```

## AppleFrameworks

Use the `modules` keyword to list frameworks required, e.g.

```meson
dep = dependency('appleframeworks', modules : 'foundation')
```

These dependencies can never be found for non-OSX hosts.

## Boost

Boost is not a single dependency but rather a group of different
libraries. To use Boost headers-only libraries, simply add Boost as a
dependency.

```meson
boost_dep = dependency('boost')
exe = executable('myprog', 'file.cc', dependencies : boost_dep)
```

To link against boost with Meson, simply list which libraries you
would like to use.

```meson
boost_dep = dependency('boost', modules : ['thread', 'utility'])
exe = executable('myprog', 'file.cc', dependencies : boost_dep)
```

You can call `dependency` multiple times with different modules and
use those to link against your targets.

If your boost headers or libraries are in non-standard locations you
can set the BOOST_ROOT, BOOST_INCLUDEDIR, and/or BOOST_LIBRARYDIR
environment variables.

You can set the argument `threading` to `single` to use boost
libraries that have been compiled for single-threaded use instead.

## CUPS

`method` may be `auto`, `config-tool`, `pkg-config`, `cmake` or `extraframework`.

## GL

This finds the OpenGL library in a way appropriate to the platform.

`method` may be `auto`, `pkg-config` or `system`.

## GTest and GMock

GTest and GMock come as sources that must be compiled as part of your
project. With Meson you don't have to care about the details, just
pass `gtest` or `gmock` to `dependency` and it will do everything for
you. If you want to use GMock, it is recommended to use GTest as well,
as getting it to work standalone is tricky.

You can set the `main` keyword argument to `true` to use the `main()`
function provided by GTest:
```
gtest_dep = dependency('gtest', main : true, required : false)
e = executable('testprog', 'test.cc', dependencies : gtest_dep)
test('gtest test', e)
```

## libwmf

*(added 0.44.0)*

`method` may be `auto`, `config-tool` or `pkg-config`.

## LLVM

Meson has native support for LLVM going back to version LLVM version 3.5.
It supports a few additional features compared to other config-tool based
dependencies.

As of 0.44.0 Meson supports the `static` keyword argument for
LLVM. Before this LLVM >= 3.9 would always dynamically link, while
older versions would statically link, due to a quirk in `llvm-config`.

### Modules, a.k.a. Components

Meson wraps LLVM's concept of components in it's own modules concept.
When you need specific components you add them as modules as meson
will do the right thing:

```meson
llvm_dep = dependency('llvm', version : '>= 4.0', modules : ['amdgpu'])
```

As of 0.44.0 it can also take optional modules (these will affect the arguments
generated for a static link):

```meson
llvm_dep = dependency(
  'llvm', version : '>= 4.0', modules : ['amdgpu'], optional_modules : ['inteljitevents'],
)
```

## MPI

*(added 0.42.0)*

MPI is supported for C, C++ and Fortran. Because dependencies are
language-specific, you must specify the requested language using the
`language` keyword argument, i.e.,
 * `dependency('mpi', language: 'c')` for the C MPI headers and libraries
 * `dependency('mpi', language: 'cpp')` for the C++ MPI headers and libraries
 * `dependency('mpi', language: 'fortran')` for the Fortran MPI headers and libraries

Meson prefers pkg-config for MPI, but if your MPI implementation does
not provide them, it will search for the standard wrapper executables,
`mpic`, `mpicxx`, `mpic++`, `mpifort`, `mpif90`, `mpif77`. If these
are not in your path, they can be specified by setting the standard
environment variables `MPICC`, `MPICXX`, `MPIFC`, `MPIF90`, or
`MPIF77`, during configuration.

## OpenMP

*(added 0.46.0)*

This dependency selects the appropriate compiler flags and/or libraries to use
for OpenMP support.

The `language` keyword may used.

## pcap

*(added 0.42.0)*

`method` may be `auto`, `config-tool` or `pkg-config`.

## libgcrypt

*(added 0.49.0)*

`method` may be `auto`, `config-tool` or `pkg-config`.

## Python3

Python3 is handled specially by meson:
1. Meson tries to use `pkg-config`.
2. If `pkg-config` fails meson uses a fallback:
    - On Windows the fallback is the current `python3` interpreter.
    - On OSX the fallback is a framework dependency from `/Library/Frameworks`.

Note that `python3` found by this dependency might differ from the one used in
`python3` module because modules uses the current interpreter, but dependency tries
`pkg-config` first.

`method` may be `auto`, `extraframework`, `pkg-config` or `sysconfig`

## Qt4 & Qt5

Meson has native Qt support. Its usage is best demonstrated with an
example.

```meson
qt5_mod = import('qt5')
qt5widgets = dependency('qt5', modules : 'Widgets')

processed = qt5_mod.preprocess(
  moc_headers : 'mainWindow.h',   # Only headers that need moc should be put here
  moc_sources : 'helperFile.cpp', # must have #include"moc_helperFile.cpp"
  ui_files    : 'mainWindow.ui',
  qresources  : 'resources.qrc',
)

q5exe = executable('qt5test',
  sources     : ['main.cpp',
                 'mainWindow.cpp',
                 processed],
  dependencies: qt5widgets)
```

Here we have an UI file created with Qt Designer and one source and
header file each that require preprocessing with the `moc` tool. We
also define a resource file to be compiled with `rcc`. We just have to
tell Meson which files are which and it will take care of invoking all
the necessary tools in the correct order, which is done with the
`preprocess` method of the `qt5` module. Its output is simply put in
the list of sources for the target. The `modules` keyword of
`dependency` works just like it does with Boost. It tells which
subparts of Qt the program uses.

You can set the `main` keyword argument to `true` to use the `WinMain()`
function provided by qtmain static library (this argument does nothing on platforms
other than Windows).

Setting the optional `private_headers` keyword to true adds the private header
include path of the given module(s) to the compiler flags.  (since v0.47.0)

**Note** using private headers in your project is a bad idea, do so at your own
risk.

`method` may be `auto`, `pkgconfig` or `qmake`.

## SDL2

SDL2 can be located using `pkg-confg`, the `sdl2-config` config tool, or as an
OSX framework.

`method` may be `auto`, `config-tool`, `extraframework` or `pkg-config`.

## Threads

This dependency selects the appropriate compiler flags and/or libraries to use
for thread support.

See [threads](Threads.md).

## Valgrind

Meson will find valgrind using `pkg-config`, but only uses the compilation flags
and avoids trying to link with it's non-PIC static libs.

## Vulkan

*(added 0.42.0)*

Vulkan can be located using `pkg-config`, or the `VULKAN_SDK` environment variable.

`method` may be `auto`, `pkg-config` or `system`.

## WxWidgets

Similar to [Boost](#boost), WxWidgets is not a single library but rather
a collection of modules. WxWidgets is supported via `wx-config`.
Meson substitutes `modules` to `wx-config` invocation, it generates
- `compile_args` using `wx-config --cxxflags $modules...`
- `link_args` using `wx-config --libs $modules...`

### Example

```meson
wx_dep = dependency(
  'wxwidgets', version : '>=3.0.0', modules : ['std', 'stc'],
)
```

```shell
# compile_args:
$ wx-config --cxxflags std stc

# link_args:
$ wx-config --libs std stc
```

<hr>
<a name="footnote1">1</a>: They may appear to be case-insensitive, if the
    underlying file system happens to be case-insensitive.