meson/docs/markdown/Subprojects.md

---
short-description: Using meson projects as subprojects within other meson projects
...

# Subprojects

Some platforms do not provide a native packaging system. In these
cases it is common to bundle all third party libraries in your source
tree. This is usually frowned upon because it makes it hard to add
these kinds of projects into e.g. those Linux distributions that
forbid bundled libraries.

Meson tries to solve this problem by making it extremely easy to
provide both at the same time. The way this is done is that Meson
allows you to take any other Meson project and make it a part of your
build without (in the best case) any changes to its Meson setup. It
becomes a transparent part of the project. 

It should be noted that this only works for subprojects that are built
with Meson. It can not be used with any other build system. The reason
is the simple fact that there is no possible way to do this reliably
with mixed build systems.

## A subproject example

Usually dependencies consist of some header files plus a library to link against. 
To declare this internal dependency use `declare_dependency` function.  

As an example, suppose we have a simple project that provides a shared
library. It's `meson.build` would look like this.

```meson
project('libsimple', 'c')

inc = include_directories('include')
libsimple = shared_library('simple', 
  'simple.c', 
  include_directories : inc, 
  install : true)

libsimple_dep = declare_dependency(include_directories : inc, 
  link_with : libsimple)
```

### Naming convention for dependency variables

Ideally the dependency variable name should be of `<project_name>_dep` form. 
This way one can just use it without even looking inside build definitions of that subproject.

In cases where there are multiple dependencies need to be declared, the default one 
should be named as `<project_name>_dep` (e.g. `gtest_dep`), and others can have 
`<project_name>_<other>_<name>_dep` form (e.g. `gtest_main_dep` - gtest with main function).

There may be exceptions to these rules where common sense should be applied.

### Build options in subproject

All Meson features of the subproject, such as project options keep
working and can be set in the master project. There are a few
limitations, the most important being that global compiler arguments
must be set in the main project before calling subproject. Subprojects
must not set global arguments because there is no way to do that
reliably over multiple subprojects. To check whether you are running
as a subproject, use the `is_subproject` function.

## Using a subproject

All subprojects must be inside `subprojects` directory.  
The `subprojects` directory must be at the top level of your project.  
Subproject declaration must be in your top level `meson.build`.  

### A simple example

Let's use `libsimple` as a subproject.

At the top level of your project create `subprojects` directory. 
Then copy `libsimple` into `subprojects` directory. 

Your project's `meson.build` should look like this.

```meson
project('my_project', 'cpp')

libsimple_proj = subproject('libsimple')
libsimple_dep = libsimple_proj.get_variable('libsimple_dep')

executable('my_project', 
  'my_project.cpp', 
  dependencies : libsimple_dep, 
  install : true)
```

Note that the subproject object is *not* used as the dependency, but
rather you need to get the declared dependency from it with
`get_variable` because a subproject may have multiple declared
dependencies.

### Toggling between system libraries and embedded sources

When building distro packages it is very important that you do not
embed any sources. Some distros have a rule forbidding embedded
dependencies so your project must be buildable without them or
otherwise the packager will hate you.

Here's how you would use system libraries and fall back to embedding sources 
if the dependency is not available.

```meson
project('my_project', 'cpp')

libsimple_dep = dependency('libsimple', required : false)

if not libsimple_dep.found()
  libsimple_proj = subproject('libsimple')
  libsimple_dep = libsimple_proj.get_variable('libsimple_dep')
endif

executable('my_project', 
  'my_project.cpp', 
  dependencies : libsimple_dep, 
  install : true)
```

Because this is such a common operation, Meson provides a shortcut for
this use case.

```meson
dep = dependency('foo', fallback : [subproject_name, variable_name])
```

The `fallback` keyword argument takes two items, the name of the
subproject and the name of the variable that holds the dependency. If
you need to do something more complicated, such as extract several
different variables, then you need to do it yourself with the manual
method described above.

Using this shortcut the build definition would look like this.

```meson
project('my_project', 'cpp')

libsimple_dep = dependency('libsimple', fallback : ['libsimple', 'libsimple_dep'])

executable('my_project', 
  'my_project.cpp', 
  dependencies : libsimple_dep, 
  install : true)
```

With this setup when libsimple is provided by the system, we use it. When
that is not the case we use the embedded version (the one from subprojects). 

Note that `libsimple_dep` can point to an external or an internal dependency but
you don't have to worry about their differences. Meson will take care
of the details for you.

### Subprojects depending on other subprojects

Subprojects can use other subprojects, but all subprojects must reside
in the top level `subprojects` directory. Recursive use of subprojects
is not allowed, though, so you can't have subproject `a` that uses
subproject `b` and have `b` also use `a`.

## Obtaining subprojects

Meson ships with a dependency system to automatically obtain
dependency subprojects. It is documented in the [Wrap dependency
system manual](Wrap-dependency-system-manual.md).

## Command-line options

The usage of subprojects can be controlled by users and distros with
the following command-line options:

* **--wrap-mode=nodownload**

    Meson will not use the network to download any subprojects or
    fetch any wrap information. Only pre-existing sources will be used.
    This is useful (mostly for distros) when you want to only use the
    sources provided by a software release, and want to manually handle
    or provide missing dependencies.
    
* **--wrap-mode=nofallback**

    Meson will not use subproject fallbacks for any dependency
    declarations in the build files, and will only look for them in the
    system. Note that this does not apply to unconditional subproject()
    calls, and those are meant to be used for sources that cannot be
    provided by the system, such as copylibs.

* **--wrap-mode=forcefallback**

    Meson will not look at the system for any dependencies which have
    subproject fallbacks available, and will *only* use subprojects for
    them. This is useful when you want to test your fallback setup, or
    want to specifically build against the library sources provided by
    your subprojects.

## Why must all subprojects be inside a single directory?

There are several reasons.

First of all, to maintain any sort of sanity, the system must prevent going
inside other subprojects with `subdir()` or variations thereof. Having the
subprojects in well defined places makes this easy. If subprojects could be
anywhere at all, it would be a lot harder.

Second of all it is extremely important that end users can easily see what
subprojects any project has. Because they are in one, and only one, place,
reviewing them becomes easy.

This is also a question of convention. Since all Meson projects have the same
layout w.r.t subprojects, switching between projects becomes easier. You don't
have to spend time on a new project traipsing through the source tree looking
for subprojects. They are always in the same place.

Finally if you can have subprojects anywhere, this increases the possibility of
having many different (possibly incompatible) versions of a dependency in your
source tree. Then changing some code (such as changing the order you traverse
directories) may cause a completely different version of the subproject to be
used by accident.