Contributing to Meson

A large fraction of Meson is contributed by people outside the core team. This documentation explains some of the design rationales of Meson as well as how to create and submit your patches for inclusion to Meson.

Thank you for your interest in participating to the development.

Submitting patches

All changes must be submitted as pull requests to GitHub. This causes them to be run through the CI system. All submissions must pass a full CI test run before they are even considered for submission.

Keeping pull requests up to date

It is possible that while your pull request is being reviewed, other changes are committed to master that cause merge conflicts that must be resolved. The basic rule for this is very simple: keep your pull request up to date using rebase only.

Do not merge head back to your branch. Any merge commits in your pull request make it not acceptable for merging into master and you must remove them.

Special procedure for new features

Every new feature requires some extra steps, namely:

  • Must include a project test under test cases/, or if that's not possible or if the test requires a special environment, it must go into
  • Must be registered with the FeatureChecks framework that will warn the user if they try to use a new feature while targeting an older Meson version.
  • Needs a release note snippet inside docs/markdown/snippets/ with a heading and a brief paragraph explaining what the feature does with an example.

Acceptance and merging

The kind of review and acceptance any merge proposal gets depends on the changes it contains. All pull requests must be reviewed and accepted by someone with commit rights who is not the original submitter. Merge requests can be roughly split into three different categories.

The first one consists of MRs that only change the markdown documentation under docs/markdown. Anyone with access rights can push changes to these directly to master. For major changes it is still recommended to create a MR so other people can comment on it.

The second group consists of merges that don't change any functionality, fixes to the CI system and bug fixes that have added regression tests (see below) and don't change existing functionality. Once successfully reviewed anyone with merge rights can merge these to master.

The final kind of merges are those that add new functionality or change existing functionality in a backwards incompatible way. These require the approval of the project lead.

In a simplified list form the split would look like the following:

  • members with commit access can do:
    • documentation changes (directly to master if warranted)
    • bug fixes that don't change functionality
    • refactorings
    • new dependency types
    • new tool support (e.g. a new Doxygen-kind of tool)
    • support for new compilers to existing languages
  • project leader decision is needed for:
    • new modules
    • new functions in the Meson language
    • syntax changes for Meson files
    • changes breaking backwards compatibility
    • support for new languages

A green CI run is mandatory for merging

No merge request may be merged until it has a fully green CI run. It does not matter why CI fails, it is a hard blocker. Even if the MR could possibly not have anything to do with the failure and clearly should be permitted, it may not be merged. Only MRs that fix the CI issue are allowed to land in trunk.

There is one, and only one, exception to this. At the time of writing the Apple CI is unreliable and sometimes fails with clock skew errors.

If a merge causes CI failure any developer can revert it out of master. It is then the responsibility of the original submitter to resubmit a fixed version.

Strategy for merging pull requests to trunk

Meson's merge strategy should fulfill the following guidelines:

  • preserve as much history as possible

  • have as little junk in the repo as possible

  • everything in the "master lineage" should always pass all tests

These goals are slightly contradictory so the correct thing to do often requires some judgement on part of the person doing the merge. GitHub provides three different merge options, The rules of thumb for choosing between them goes like this:

  • single commit pull requests should always be rebased

  • a pull request with one commit and one "fixup" commit (such as testing something to see if it passes CI) should be squashed

  • large branches with many commits should be merged with a merge commit, especially if one of the commits does not pass all tests (which happens in e.g. large and difficult refactorings)

If in doubt, ask for guidance on IRC.


All new features must come with automatic tests that thoroughly prove that the feature is working as expected. Similarly bug fixes must come with a unit test that demonstrates the bug, proves that it has been fixed and prevents the feature from breaking in the future.

Sometimes it is difficult to create a unit test for a given bug. If this is the case, note this in your pull request. We may permit bug fix merge requests in these cases. This is done on a case by case basis. Sometimes it may be easier to write the test than convince the maintainers that one is not needed. Exercise judgment and ask for help in problematic cases.

The tests are split into two different parts: unit tests and full project tests. To run all tests, execute ./ Unit tests can be run with ./ and project tests with ./

Project tests

Subsets of project tests can be selected with ./ --only option. This can save a great deal of time when only a certain part of Meson is being tested. For example, a useful and easy contribution to Meson is making sure the full set of compilers is supported. One could for example test various Fortran compilers by setting FC=ifort or FC=flang or similar with ./ --only fortran. Some families of tests require a particular backend to run. For example, all the CUDA project tests run and pass on Windows via ./ --only cuda --backend ninja

Each project test is a standalone project that can be compiled on its own. They are all in the test cases subdirectory. The simplest way to run a single project test is to do something like ./ test\ cases/common/1\ trivial builddir. The one exception to this is test cases/unit directory discussed below.

The test cases in the common subdirectory are meant to be run always for all backends. They should only depend on C and C++, without any external dependencies such as libraries. Tests that require those are in the test cases/frameworks directory. If there is a need for an external program in the common directory, such as a code generator, it should be implemented as a Python script. The goal of test projects is also to provide sample projects that end users can use as a base for their own projects.

All project tests follow the same pattern: they are configured, compiled, tests are run and finally install is run. Passing means that configuring, building and tests succeed and that installed files match those expected.

Any tests that require more thorough analysis, such as checking that certain compiler arguments can be found in the command line or that the generated pkg-config files actually work should be done with a unit test.


  • crossfile.ini and nativefile.ini are passed to the configure step with --cross-file and --native-file options, respectively.

  • mlog.cmd_ci_include() can be called from anywhere inside Meson to capture the contents of an additional file into the CI log on failure.

Projects needed by unit tests are in the test cases/unit subdirectory. They are not run as part of ./

Configuring project tests

The (optional) test.json file, in the root of a test case, is used for configuring the test. All of the following root entries in the test.json are independent of each other and can be combined as needed.

Example test.json:

  "env": {
    "VAR": "VAL"
  "installed": [
    { "type": "exe", "file": "usr/bin/testexe" },
    { "type": "pdb", "file": "usr/bin/testexe" },
    { "type": "shared_lib", "file": "usr/lib/z", "version": "1.2.3" },
  "matrix": {
    "options": {
      "opt1": [
        { "val": "abc"   },
        { "val": "qwert" },
        { "val": "bad"   }
      "opt2": [
        { "val": null    },
        { "val": "true"  },
        { "val": "false" },
    "exclude": [
      { "opt1": "qwert", "opt2": "false" },
      { "opt1": "bad"                    }
  "tools": {
    "cmake": ">=3.11"


The env key contains a dictionary which specifies additional environment variables to be set during the configure step of the test.

There is some basic support for configuring the string with the @<VAR>@ syntax:

  • @ROOT@: absolute path of the source directory
  • @PATH@: current value of the PATH env variable


The installed dict contains a list of dicts, describing which files are expected to be installed. Each dict contains the following keys:

  • file
  • type
  • platform (optional)
  • version (optional)
  • language (optional)

The file entry contains the relative path (from the install root) to the actually installed file.

The type entry specifies how the file path should be interpreted based on the current platform. The following values are currently supported:

type Description
file No postprocessing, just use the provided path
python_file Use the provided path while replacing the python directory.
dir To include all files inside the directory (for generated docs, etc). The path must be a valid directory
exe For executables. On Windows the .exe suffix is added to the path in file
shared_lib For shared libraries, always written as name. The appropriate suffix and prefix are added by platform
python_lib For python libraries, while replacing the python directory. The appropriate suffix is added by platform
pdb For Windows PDB files. PDB entries are ignored on non Windows platforms
implib For Windows import libraries. These entries are ignored on non Windows platforms
py_implib For Windows import libraries. These entries are ignored on non Windows platforms
implibempty Like implib, but no symbols are exported in the library
expr file is an expression. This type should be avoided and removed if possible

Except for the file, python_file and expr types, all paths should be provided without a suffix.

Argument Applies to Description
version shared_lib, pdb Sets the version to look for appropriately per-platform
language pdb Determines which compiler/linker determines the existence of this file

The shared_lib and pdb types takes an optional additional parameter, version, this is us a string in X.Y.Z format that will be applied to the library. Each version to be tested must have a single version. The harness will apply this correctly per platform:

The python_file, python_lib, and py_implib types have basic support for configuring the string with the @<VAR>@ syntax:

  • @PYTHON_PLATLIB@: python get_install_dir directory relative to prefix
  • @PYTHON_PURELIB@: python get_install_dir(pure: true) directory relative to prefix

pdb takes an optional language argument. This determines which compiler/linker should generate the pdb file. Because it's possible to mix compilers that do and don't generate pdb files (dmd's optlink doesn't). Currently this is only needed when mixing D and C code.

  "type": "shared_lib", "file": "usr/lib/lib",
  "type": "shared_lib", "file": "usr/lib/lib", "version": "1",
  "type": "shared_lib", "file": "usr/lib/lib", "version": "1.2.3.",

This will be applied appropriately per platform. On windows this expects lib.dll and lib-1.dll. on MacOS it expects liblib.dylib and liblib.1.dylib. On other Unices it expects,, and

If the platform key is present, the installed file entry is only considered if the platform matches. The following values for platform are currently supported:

platform Description
msvc Matches when a msvc like compiler is used (msvc, clang-cl, etc.)
gcc Not msvc
cygwin Matches when the platform is cygwin
!cygwin Not cygwin


The matrix section can be used to define a test matrix to run project tests with different Meson options.

In the options dict, all possible options and their values are specified. Each key in the options dict is a Meson option. It stores a list of all potential values in a dict format.

Each value must contain the val key for the value of the option. null can be used for adding matrix entries without the current option.

The skip_on_env key (as described below) may be used in the value to skip that matrix entry, based on the current environment.

The expect_skip_on_jobname and expect_skip_on_os keys (as described below) may be used to expect that the test will be skipped, based on the current environment.

Similarly, the compilers key can be used to define a mapping of compilers to languages that are required for this value.

  "compilers": {
    "c": "gcc",
    "cpp": "gcc",
    "d": "gdc"

Specific option combinations can be excluded with the exclude section. It should be noted that exclude does not require exact matches. Instead, any matrix entry containing all option value combinations in exclude will be excluded. Thus an empty dict ({}) to will match all elements in the test matrix.

The above example will produce the following matrix entries:

  • opt1=abc
  • opt1=abc opt2=true
  • opt1=abc opt2=false
  • opt1=qwert
  • opt1=qwert opt2=true


Currently supported values are:

  • prefix
  • libdir


This section specifies a dict of tool requirements in a simple key-value format. If a tool is specified, it has to be present in the environment, and the version requirement must be fulfilled. Otherwise, the entire test is skipped (including every element in the test matrix).


The stdout key contains a list of dicts, describing the expected stdout.

Each dict contains the following keys:

  • line
  • match (optional)
  • count (optional)

Each item in the list is matched, in order, against the remaining actual stdout lines, after any previous matches. If the actual stdout is exhausted before every item in the list is matched, the expected output has not been seen, and the test has failed.

The match element of the dict determines how the line element is matched:

Type Description
literal Literal match (default)
re regex match

The count element determines how many times the line is expected, and allowed, to be in the output. If unspecified, it must appear "any number of times, but at least once".


The skip_on_env key can be used to specify a list of environment variables. If at least one environment variable in the skip_on_env list is present, the test is skipped.


The expect_skip_on_jobname key contains a list of strings. If the MESON_CI_JOBNAME environment variable is set, and any of them are a sub-string of it, the test is expected to be skipped (that is, it is expected that the test will output MESON_SKIP_TEST, because the CI environment is not one in which it can run, for whatever reason).

The test is failed if it either skips unexpectedly or runs unexpectedly.


The expect_skip_on_os key can be used to specify a list of OS names (or their negations, prefixed with a !). If at least one item in the expect_skip_on_os list is matched, the test is expected to be skipped.

The test is failed if it either skips unexpectedly or runs unexpectedly.

Skipping integration tests

Meson uses several continuous integration testing systems that have slightly different interfaces for indicating a commit should be skipped.

Continuous integration systems currently used:

To promote consistent naming policy, use:

  • [skip ci] in the commit title if you want to disable all integration tests


The docs directory contains the full documentation that will be used to generate the Meson web site. Line length in most cases should not exceed 70 characters (lines containing links or examples are usually exempt). Every change in functionality must change the documentation pages. In most cases this means updating the reference documentation page but bigger changes might need changes in other documentation, too.

All new functionality needs to have a mention in the release notes. These features should be written in standalone files in the docs/markdown/snippets directory. The release manager will combine them into one page when doing the release.

Python Coding style

Meson follows the basic Python coding style. Additional rules are the following:

  • indent 4 spaces, no tabs ever
  • indent files with two spaces
  • try to keep the code as simple as possible
  • contact the mailing list before embarking on large scale projects to avoid wasted effort

Meson uses Flake8 for style guide enforcement. The Flake8 options for the project are contained in .flake8.

To run Flake8 on your local clone of Meson:

$ python3 -m pip install flake8
$ cd meson
$ flake8

To run it automatically before committing:

$ flake8 --install-hook=git
$ git config --bool flake8.strict true

C/C++ coding style

Meson has a bunch of test code in several languages. The rules for those are simple.

  • indent 4 spaces, no tabs ever
  • brace always on the same line as if/for/else/function definition

External dependencies

The goal of Meson is to be as easily usable as possible. The user experience should be "get Python3 and Ninja, run", even on Windows. Unfortunately this means that we can't have dependencies on projects outside of Python's standard library. This applies only to core functionality, though. For additional helper programs etc the use of external dependencies may be ok. If you feel that you are dealing with this kind of case, please contact the developers first with your use case.

Turing completeness

The main design principle of Meson is that the definition language is not Turing complete. Any change that would make Meson Turing complete is automatically rejected. In practice this means that defining your own functions inside files and generalised loops will not be added to the language.

Do I need to sign a CLA in order to contribute?

No you don't. All contributions are welcome.

No lingering state

Meson operates in much the same way as functional programming languages. It has inputs, which include files, values of options, compilers and so on. These are passed to a function, which generates output build definition. This function is pure, which means that:

  • for any given input the output is always the same
  • running Meson twice in a row always produce the same output in both runs

The latter one is important, because it enforces that there is no way for "secret state" to pass between consecutive invocations of Meson. This is the reason why, for example, there is no set_option function even though there is a get_option one.

If this were not the case, we could never know if the build output is "stable". For example suppose there were a set_option function and a boolean variable flipflop. Then you could do this:

set_option('flipflop', not get_option('flipflop'))

This piece of code would never converge. Every Meson run would change the value of the option and thus the output you get out of this build definition would be random.

Meson does not permit this by forbidding these sorts of covert channels.

There is one exception to this rule. Users can call into external commands with run_command. If the output of that command does not behave like a pure function, this problem arises. Meson does not try to guard against this case, it is the responsibility of the user to make sure the commands they run behave like pure functions.

Environment variables

Environment variables are like global variables, except that they are also hidden by default. Envvars should be avoided whenever possible, all functionality should be exposed in better ways such as command line switches.

Random design points that fit nowhere else

  • All features should follow the 90/9/1 rule. 90% of all use cases should be easy, 9% should be possible and it is totally fine to not support the final 1% if it would make things too complicated.

  • Any build directory will have at most two toolchains: one native and one cross.

  • Prefer specific solutions to generic frameworks. Solve the end user's problems rather than providing them tools to do it themselves.

  • Never use features of the Unix shell (or Windows shell for that matter). Doing things like forwarding output with > or invoking multiple commands with && are not permitted. Whenever these sorts of requirements show up, write an internal Python script with the desired functionality and use that instead.


YAML Reference manual – Editing and maintaining the Reference manual

Meson CI setup

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