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.

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.

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.

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.

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.

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.

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.

Download subprojects

Since 0.49.0

Meson will automatically download needed subprojects during configure, unless --wrap-mode=nodownload option is passed. It is sometimes preferable to download all subprojects in advance, so the meson configure can be performed offline. The command-line meson subprojects download can be used for that, it will download all missing subprojects, but will not update already fetched subprojects.

Update subprojects

Since 0.49.0

Once a subproject has been fetched, Meson will not update it automatically. For example if the wrap file tracks a git branch, it won't pull latest commits.

To pull latest version of all your subprojects at once, just run the command: meson subprojects update.

  • If the wrap file comes from wrapdb, the latest version of the wrap file will be pulled and used next time meson reconfigure the project. This can be triggered using meson --reconfigure. Previous source tree is not deleted, to prevent from any loss of local changes.
  • If the wrap file points to a git commit or tag, a checkout of that commit is performed.
  • If the wrap file points to a git branch, and the current branch has the same name, a git pull is performed.
  • If the wrap file points to a git branch, and the current branch is different, it is skipped. Unless --rebase option is passed in which case git pull --rebase is performed.

Start a topic branch across all git subprojects

Since 0.49.0

The command-line meson subprojects checkout <branch_name> will checkout a branch, or create one with -b argument, in every git subprojects. This is useful when starting local changes across multiple subprojects. It is still your responsability to commit and push in each repository where you made local changes.

To come back to the revision set in wrap file (i.e. master), just run meson subprojects checkout with no branch name.

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.

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