meson/docs/markdown/CMake-module.md

# CMake module

**Note**: the functionality of this module is governed by [Meson's
  rules on mixing build systems](Mixing-build-systems.md).

This module provides helper tools for generating cmake package files.
It also supports the usage of CMake based subprojects, similar to
the normal [Meson subprojects](Subprojects.md).


## Usage

To use this module, just do: **`cmake = import('cmake')`**. The
following functions will then be available as methods on the object
with the name `cmake`. You can, of course, replace the name `cmake`
with anything else.

It is generally recommended to use the latest Meson version and
CMake >=3.17 for best compatibility. CMake subprojects will
usually also work with older CMake versions. However, this can
lead to unexpected issues in rare cases.

## CMake subprojects

Using CMake subprojects is similar to using the "normal" Meson
subprojects. They also have to be located in the `subprojects`
directory.

Example:

```cmake
add_library(cm_lib SHARED ${SOURCES})
```

```meson
cmake = import('cmake')

# Configure the CMake project
sub_proj = cmake.subproject('libsimple_cmake')

# Fetch the dependency object
cm_lib = sub_proj.dependency('cm_lib')

executable(exe1, ['sources'], dependencies: [cm_lib])
```

The `subproject` method is almost identical to the normal Meson
`subproject` function. The only difference is that a CMake project
instead of a Meson project is configured.

The returned `sub_proj` supports the same options as a "normal"
subproject. Meson automatically detects CMake build targets, which can
be accessed with the methods listed [below](#subproject-object).

It is usually enough to just use the dependency object returned by the
`dependency()` method in the build targets. This is almost identical
to using `declare_dependency()` object from a normal Meson subproject.

It is also possible to use executables defined in the CMake project as code
generators with the `target()` method:

```cmake
add_executable(cm_exe ${EXE_SRC})
```

```meson
cmake = import('cmake')

# Subproject with the "code generator"
sub_pro = cmake.subproject('cmCodeGen')

# Fetch the code generator exe
sub_exe = sub_pro.target('cm_exe')

# Use the code generator
generated = custom_target(
  'cmake-generated',
  input: [],
  output: ['test.cpp'],
  command: [sub_exe, '@OUTPUT@']
)
```

It should be noted that not all projects are guaranteed to work. The
safest approach would still be to create a `meson.build` for the
subprojects in question.

### Configuration options

*New in meson 0.55.0*

Meson also supports passing configuration options to CMake and overriding
certain build details extracted from the CMake subproject.

```meson
cmake   = import('cmake')
opt_var = cmake.subproject_options()

# Call CMake with `-DSOME_OTHER_VAR=ON`
opt_var.add_cmake_defines({'SOME_OTHER_VAR': true})

# Globally override the C++ standard to c++11
opt_var.set_override_option('cpp_std', 'c++11')

# Override the previous global C++ standard
# with c++14 only for the CMake target someLib
opt_var.set_override_option('cpp_std', 'c++14', target: 'someLib')

sub_pro = cmake.subproject('someLibProject', options: opt_var)

# Further changes to opt_var have no effect
```

See [the CMake options object](#cmake-options-object) for a complete
reference of all supported functions.

The CMake configuration options object is very similar to the
[[@cfg_data]] object] object returned
by [[configuration_data]]. It
is generated by the `subproject_options` function

All configuration options have to be set *before* the subproject is
configured and must be passed to the `subproject` method via the
`options` key. Altering the configuration object won't have any effect
on previous `cmake.subproject` calls.

In earlier Meson versions CMake command-line parameters could be set
with the `cmake_options` kwarg. However, this feature is deprecated
since 0.55.0 and only kept for compatibility. It will not work
together with the `options` kwarg.

### `subproject` object

This object is returned by the `subproject` function described above
and supports the following methods:

 - `dependency(target)` returns a dependency object for any CMake target. The
   `include_type` kwarg *(new in 0.56.0)* controls the include type of the
   returned dependency object similar to the same kwarg in the
   [[dependency]] function.
 - `include_directories(target)` returns a Meson [[@inc]]
   object for the specified target. Using this function is not necessary
   if the dependency object is used.
 - `target(target)` returns the raw build target.
 - `target_type(target)` returns the type of the target as a string
 - `target_list()` returns a list of all target *names*.
 - `get_variable(name)` fetches the specified variable from inside
   the subproject. Usually `dependency()` or `target()` should be
   preferred to extract build targets.
 - `found` returns true if the subproject is available, otherwise false
   *new in Meson 0.53.2*

### `cmake options` object

This object is returned by the `subproject_options()` function and
consumed by the `options` kwarg of the `subproject` function. The
following methods are supported:

 - `add_cmake_defines({'opt1': val1, ...})` add additional CMake commandline defines
 - `set_override_option(opt, val)` set specific [build options](Build-options.md)
   for targets. This will effectively add `opt=val` to the `override_options`
   array of the [[build_target]]
 - `set_install(bool)` override whether targets should be installed or not
 - `append_compile_args(lang, arg1, ...)` append compile flags for a specific
   language to the targets
 - `append_link_args(arg1, ...)` append linger args to the targets
 - `clear()` reset all data in the `cmake options` object

The methods `set_override_option`, `set_install`,
`append_compile_args` and `append_link_args` support the optional
`target` kwarg. If specified, the set options affect the specific
target. The effect of the option is global for the subproject
otherwise.

If, for instance, `opt_var.set_install(false)` is called, no target
will be installed regardless of what is set by CMake. However, it is
still possible to install specific targets (here `foo`) by setting the
`target` kwarg: `opt_var.set_install(true, target: 'foo')`

Options that are not set won't affect the generated subproject. So, if
for instance, `set_install` was not called then the values extracted
from CMake will be used.

### Cross compilation

*New in 0.56.0*

Meson will try to automatically guess most of the required CMake
toolchain variables from existing entries in the cross and native
files. These variables will be stored in an automatically generate
CMake toolchain file in the build directory. The remaining variables
that can't be guessed can be added by the user in the `[cmake]`
cross/native file section (*new in 0.56.0*).

Adding a manual CMake toolchain file is also supported with the
`cmake_toolchain_file` setting in the `[properties]` section. Directly
setting a CMake toolchain file with
`-DCMAKE_TOOLCHAIN_FILE=/path/to/some/Toolchain.cmake` in the
`meson.build` is **not** supported since the automatically generated
toolchain file is also used by Meson to inject arbitrary code into
CMake to enable the CMake subproject support.

The closest configuration to only using a manual CMake toolchain file
would be to set these options in the machine file:

```ini
[properties]

cmake_toolchain_file = '/path/to/some/Toolchain.cmake'
cmake_defaults       = false

[cmake]

# No entries in this section
```

This will result in a toolchain file with just the bare minimum to
enable the CMake subproject support and `include()` the
`cmake_toolchain_file` as the last instruction.

For more information see the [cross and native file
specification](Machine-files.md).

## CMake configuration files

### cmake.write_basic_package_version_file()

This function is the equivalent of the corresponding [CMake
function](https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html#command:write_basic_package_version_file),
it generates a `name` package version file.

* `name`: the name of the package.
* `version`: the version of the generated package file.
* `compatibility`: a string indicating the kind of compatibility, the accepted values are
`AnyNewerVersion`, `SameMajorVersion`, `SameMinorVersion` or `ExactVersion`.
It defaults to `AnyNewerVersion`. Depending on your cmake installation some kind of
compatibility may not be available.
* `arch_independent`: *new in 0.62.0*, if true the generated package file
  will skip architecture checks. Useful for header-only libraries.
* `install_dir`: optional installation directory, it defaults to `$(libdir)/cmake/$(name)`


Example:

```meson
cmake = import('cmake')

cmake.write_basic_package_version_file(name: 'myProject', version: '1.0.0')
```

### cmake.configure_package_config_file()

This function is the equivalent of the corresponding [CMake
function](https://cmake.org/cmake/help/v3.11/module/CMakePackageConfigHelpers.html#generating-a-package-configuration-file),
it generates a `name` package configuration file from the `input`
template file. Just like the cmake function in this file the
`@PACKAGE_INIT@` statement will be replaced by the appropriate piece
of cmake code. The equivalent `PATH_VARS` argument is given through
the `configuration` parameter.

* `name`: the name of the package.
* `input`: the template file where that will be treated for variable substitutions contained in `configuration`.
* `install_dir`: optional installation directory, it defaults to `$(libdir)/cmake/$(name)`.
* `configuration`: a `configuration_data` object that will be used for variable substitution in the template file.


Example:

meson.build:

```meson
cmake = import('cmake')

conf = configuration_data()
conf.set_quoted('VAR', 'variable value')

cmake.configure_package_config_file(
    name: 'myProject',
    input: 'myProject.cmake.in',
    configuration: conf
)
```

myProject.cmake.in:

```text
@PACKAGE_INIT@

set(MYVAR VAR)
```