CMakeToolchain

Warning

These tools are experimental and subject to breaking changes.

The CMakeToolchain is the toolchain generator for CMake. It will generate toolchain files that can be used in the command line invocation of CMake with the -DCMAKE_TOOLCHAIN_FILE=conantoolchain.cmake. This generator translates the current package configuration, settings, and options, into CMake toolchain syntax.

It can be declared as:

from conans import ConanFile

class Pkg(ConanFile):
    generators = "CMakeToolchain"

Or fully instantiated in the generate() method:

from conans import ConanFile
from conan.tools.cmake import CMakeToolchain

class App(ConanFile):
    settings = "os", "arch", "compiler", "build_type"
    requires = "hello/0.1"
    generators = "cmake_find_package_multi"
    options = {"shared": [True, False], "fPIC": [True, False]}
    default_options = {"shared": False, "fPIC": True}

    def generate(self):
        tc = CMakeToolchain(self)
        tc.variables["MYVAR"] = "MYVAR_VALUE"
        tc.preprocessor_definitions["MYDEFINE"] = "MYDEF_VALUE"
        tc.generate()

This will generate the following files after a conan install (or when building the package in the cache) with the information provided in the generate() method as well as information translated from the current settings:

conan_toolchain.cmake file, containing the translation of Conan settings to CMake variables. Some things that will be defined in this file:
- Definition of the CMake generator platform and generator toolset
- Definition of the CMake build_type
- Definition of the CMAKE_POSITION_INDEPENDENT_CODE, based on fPIC option.
- Definition of the C++ standard as necessary
- Definition of the standard library used for C++
- Deactivation of rpaths in OSX
conanbuild.json: The toolchain can also generate a conanbuild.json file that contains arguments to the command line CMake() helper used in the recipe build() method. At the moment it contains only the CMake generator and the CMake toolchain file. The CMake generator will be deduced from the current Conan compiler settings:
- For settings.compiler="Visual Studio", the CMake generator is a direct mapping of compiler.version, as this version represents the IDE version, not the compiler version.
- For settings.compiler=msvc, the CMake generator will be by default the one of the Visual Studio that introduced this compiler version (msvc 19.0 => Visual Studio 14, msvc 19.1 => Visual Studio 15, etc). This can be changed, using the tools.microsoft.msbuild:vs_version [conf] configuration. If it is defined, that Visual Studio version will be used as the CMake generator, and the specific compiler version and toolset will be defined in the conan_toolchain.cmake file.
conanvcvars.bat: In some cases, the Visual Studio environment needs to be defined correctly for building, like when using the Ninja or NMake generators. If necessary, the CMakeToolchain will generate this script, so defining the correct Visual Studio prompt is easier.

constructor

def __init__(self, conanfile, generator=None):

Most of the arguments are optional and will be deduced from the current settings, and not necessary to define them.

preprocessor_definitions

This attribute allows defining compiler preprocessor definitions, for multiple configurations (Debug, Release, etc).

def generate(self):
    tc = CMakeToolchain(self)
    tc.preprocessor_definitions["MYDEF"] = "MyValue"
    tc.preprocessor_definitions.debug["MYCONFIGDEF"] = "MyDebugValue"
    tc.preprocessor_definitions.release["MYCONFIGDEF"] = "MyReleaseValue"
    tc.generate()

This will be translated to:

One add_definitions() definition for MYDEF in conan_toolchain.cmake file.
One add_definitions() definition, using a cmake generator expression in conan_toolchain.cmake file, using the different values for different configurations.

variables

This attribute allows defining CMake variables, for multiple configurations (Debug, Release, etc).

def generate(self):
    tc = CMakeToolchain(self)
    tc.variables["MYVAR"] = "MyValue"
    tc.variables.debug["MYCONFIGVAR"] = "MyDebugValue"
    tc.variables.release["MYCONFIGVAR"] = "MyReleaseValue"
    tc.generate()

This will be translated to:

One set() definition for MYVAR in conan_toolchain.cmake file.
One set() definition, using a cmake generator expression in conan_toolchain.cmake file, using the different values for different configurations.

Generators

The CMakeToolchain is intended to run with the CMakeDeps dependencies generator. It might temporarily work with others like cmake_find_package and cmake_find_package_multi, but this will be removed soon.

Using a custom toolchain file

There are two ways of providing a custom CMake toolchain file:

The conan_toolchain.cmake file can be completely skipped and replaced by a user one, defining the tools.cmake.cmaketoolchain:toolchain_file=<filepath> configuration value
A custom user toolchain file can be added (included from) the conan_toolchain.cmake one, by using the user_toolchain block described below, and defining the tools.cmake.cmaketoolchain:user_toolchain=<filepath> configuration value.

Using the toolchain in developer flow

One of the advantages of using Conan toolchains is that they can help to achieve the exact same build with local development flows, than when the package is created in the cache.

With the CMakeToolchain it is possible to do, for multi-configuration systems like Visual Studio (assuming we are using the cmake_find_package_multi generator):

# Lets start in the folder containing the conanfile.py
$ mkdir build && cd build
# Install both debug and release deps and create the toolchain
$ conan install ..
$ conan install .. -s build_type=Debug
# the conan_toolchain.cmake is common for both configurations
# Need to pass the generator WITHOUT the platform, that matches your default settings
$ cmake .. -G "Visual Studio 15" -DCMAKE_TOOLCHAIN_FILE=conan_toolchain.cmake
# Now you can open the IDE, select Debug or Release config and build
# or, in the command line
$ cmake --build . --config Release
$ cmake --build . --config Debug

NOTE: The platform (Win64), is already encoded in the toolchain. The command line shouldn’t pass it, so using -G "Visual Studio 15" instead of the -G "Visual Studio 15 Win64"

For single-configuration build systems:

# Lets start in the folder containing the conanfile.py
$ mkdir build_release && cd build_release
$ conan install ..
# the build type Release is encoded in the toolchain already.
# This conan_toolchain.cmake is specific for release
$ cmake .. -G "Unix Makefiles" -DCMAKE_TOOLCHAIN_FILE=conan_toolchain.cmake
$ cmake --build .  # or just "make"

# debug build requires its own folder
$ cd .. && mkdir build_debug && cd build_debug
$ conan install .. -s build_type=Debug
# the build type Debug is encoded in the toolchain already.
# This conan_toolchain.cmake is specific for debug
$ cmake .. -G "Unix Makefiles" -DCMAKE_TOOLCHAIN_FILE=conan_toolchain.cmake
$ cmake --build .  # or just "make"

Extending and customizing CMakeToolchain

Since Conan 1.36, CMakeToolchain implements a powerful capability for extending and customizing the resulting toolchain file.

The following predefined blocks are available, and added in this order:

user_toolchain: Allows to include a user toolchain from the conan_toolchain.cmake file. If the configuration tools.cmake.cmaketoolchain:user_toolchain=xxxx is defined, its value will be include(xxx) as the first line in conan_toolchain.cmake.
generic_system: Defines CMAKE_GENERATOR_PLATFORM, CMAKE_GENERATOR_TOOLSET, CMAKE_C_COMPILER,``CMAKE_CXX_COMPILER`` and CMAKE_BUILD_TYPE
android_system: Defines ANDROID_PLATFORM, ANDROID_STL, ANDROID_ABI and includes CMAKE_ANDROID_NDK/build/cmake/android.toolchain.cmake where CMAKE_ANDROID_NDK comes defined in tools.android:ndk_path configuration value.
apple_system: Defines CMAKE_SYSTEM_NAME, CMAKE_SYSTEM_VERSION, CMAKE_OSX_ARCHITECTURES, CMAKE_OSX_SYSROOT for Apple systems.
fpic: Defines the CMAKE_POSITION_INDEPENDENT_CODE when there is a options.fPIC
arch_flags: Defines C/C++ flags like -m32, -m64 when necessary.
libcxx: Defines -stdlib=libc++ flag when necessary as well as _GLIBCXX_USE_CXX11_ABI.
vs_runtime: Defines the CMAKE_MSVC_RUNTIME_LIBRARY variable, as a generator expression for multiple configurations.
cppstd: defines CMAKE_CXX_STANDARD, CMAKE_CXX_EXTENSIONS
parallel: defines /MP parallel build flag for Visual.
cmake_flags_init: defines CMAKE_XXX_FLAGS variables based on previously defined Conan variables. The blocks above only define CONAN_XXX variables, and this block will define CMake ones like set(CMAKE_CXX_FLAGS_INIT "${CONAN_CXX_FLAGS}" CACHE STRING "" FORCE)`.
try_compile: Stop processing the toolchain, skipping the blocks below this one, if IN_TRY_COMPILE CMake property is defined.
find_paths: Defines CMAKE_FIND_PACKAGE_PREFER_CONFIG, CMAKE_MODULE_PATH, CMAKE_PREFIX_PATH so the generated files from CMakeDeps are found.
rpath: Defines CMAKE_SKIP_RPATH. By default it is disabled, and it is needed to define self.blocks["rpath"].skip_rpath=True if you want to activate CMAKE_SKIP_RPATH
shared: defines BUILD_SHARED_LIBS

Blocks can be customized in different ways:

# remove an existing block
def generate(self):
    tc = CMakeToolchain(self)
    tc.blocks.remove("generic_system")

# modify the template of an existing block
def generate(self):
    tc = CMakeToolchain(self)
    tmp = tc.blocks["generic_system"].template
    new_tmp = tmp.replace(...)  # replace, fully replace, append...
    tc.blocks["generic_system"].template = new_tmp

# modify one or more variables of the context
def generate(self):
    tc = CMakeToolchain(conanfile)
    # block.values is the context dictionary
    build_type = tc.blocks["generic_system"].values["build_type"]
    tc.blocks["generic_system"].values["build_type"] = "Super" + build_type

# modify the whole context values
def generate(self):
    tc = CMakeToolchain(conanfile)
    tc.blocks["generic_system"].values = {"build_type": "SuperRelease"}

# modify the context method of an existing block
import types

def generate(self):
    tc = CMakeToolchain(self)
    generic_block = toolchain.blocks["generic_system"]

    def context(self):
        assert self  # Your own custom logic here
        return {"build_type": "SuperRelease"}
    generic_block.context = types.MethodType(context, generic_block)

# completely replace existing block
def generate(self):
    tc = CMakeToolchain(self)
    # this could go to a python_requires
    class MyGenericBlock(Block):
        template = "HelloWorld"

        def context(self):
            return {}

    tc.blocks["generic_system"] = MyBlock

# add a completely new block
def generate(self):
    tc = CMakeToolchain(self)
    # this could go to a python_requires
    class MyBlock(Block):
        template = "Hello {{myvar}}!!!"

        def context(self):
            return {"myvar": "World"}

    tc.blocks["mynewblock"] = MyBlock


# extend from an existing block
def generate(self):
    tc = CMakeToolchain(self)
    # this could go to a python_requires
    class MyBlock(GenericSystemBlock):
        template = "Hello {{build_type}}!!"

        def context(self):
            c = super(MyBlock, self).context()
            c["build_type"] = c["build_type"] + "Super"
            return c

    tc.blocks["generic_system"] = MyBlock

Recall that this is a very experimental feature, and these interfaces might change in the following releases.

For more information about these blocks, please have a look at the source code.

Cross building

The generic_system block contains some basic cross-building capabilities. In the general case, the user would want to provide their own user toolchain defining all the specifics, which can be done with the configuration tools.cmake.cmaketoolchain:user_toolchain. If this conf value is defined, the generic_system block will include the provided file, but no further define any CMake variable for cross-building.

If user_toolchain is not defined and Conan detects it is cross-building, because the build and host profiles contain different OS or architecture, it will try to define the following variables:

CMAKE_SYSTEM_NAME: tools.cmake.cmaketoolchain:system_name configuration if defined, otherwise, it will try to autodetect it. This block will consider cross-building if not Apple or Android systems (that is managed by other blocks), and not 64bits to 32bits builds in x86_64, sparc and ppc systems.
CMAKE_SYSTEM_VERSION: tools.cmake.cmaketoolchain:system_version conf if defined, otherwise os.version subsetting (host) when defined
CMAKE_SYSTEM_PROCESSOR: tools.cmake.cmaketoolchain:system_processor conf if defined, otherwise arch setting (host) if defined