CMakeToolchain

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

It can be declared as:

from conan import ConanFile

class Pkg(ConanFile):
    generators = "CMakeToolchain"

Or fully instantiated in the generate() method:

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

class App(ConanFile):
    settings = "os", "arch", "compiler", "build_type"
    requires = "hello/0.1"
    generators = "CMakeDeps"
    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()

Note

The CMakeToolchain is intended to run with the CMakeDeps dependencies generator. Please do not use other CMake legacy generators (like cmake, or cmake_paths) with it.

Generated files

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: 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_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
  • 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.

  • CMakePresets.json: The toolchain also generates a CMakePresets.json standard file, check the documentation here. It is currently using the version “3” of the JSON schema. Conan creates a default configure preset with the information:

    • The generator to be used.
    • The path to the conan_toolchain.cmake.
    • Some cache variables corresponding to the specified settings cannot work if specified in the toolchain.
    • The CMAKE_BUILD_TYPE variable when using a single-configuration generators.
    • The BUILD_TESTING variable set to OFF, when configuration tools.build:skip_test is true.
    • If you run several conan install with different -s build_type values, it will generate the corresponding buildPresets and configurePresets.
  • CMakeUserPresets.json: If you declare a layout() in the recipe and your CMakeLists.txt file is found at the conanfile.source_folder folder, a CMakeUserPresets.json file will be generated (if doesn’t exist already) including automatically the CMakePresets.json (at the conanfile.generators_folder) to allow your IDE (Visual Studio, Visual Studio Code, CLion…) or cmake tool to locate the CMakePresets.json. The location of the generated CMakeUserPresets.json can be further tweaked by the user_presets_path attribute, as documented below. The version schema of the generated CMakeUserPresets.json is “4” and requires CMake >= 3.23.

    Note: Conan will skip the generation of the CMakeUserPresets.json if it already exists and was not generated by Conan.

By default, the version schema of the generated CMakeUserPresets.json is 4 and the schema for the CMakePresets.json is 3, so they require CMake >= 3.23. You can control the version of the generated CMakePresets.json and CMakeUserPresets.json with a configuration tools.cmake.cmaketoolchain.presets:max_schema_version.

It can be set in the global.conf, with -c in the conan install command, or in a profile. The minimum accepted value for this conf is 2:

conan install . -c tools.cmake.cmaketoolchain.presets:max_schema_version=2

Customization

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.

cache_variables

This attribute allows defining CMake cache-variables. These variables, unlike the variables, are single-config. They will be stored in the CMakePresets.json file (at the cacheVariables in the configurePreset) and will be applied with -D arguments when calling cmake.configure using the CMake() build helper.

def generate(self):
    tc = CMakeToolchain(self)
    tc.cache_variables["foo"] = True
    tc.cache_variables["foo2"] = False
    tc.cache_variables["var"] = "23"

The booleans assigned to a cache_variable will be translated to ON and OFF symbols in CMake.

variables

This attribute allows defining CMake variables, for multiple configurations (Debug, Release, etc). These variables should be used to define things related to the toolchain and for the majority of cases cache_variables is what you probably want to use. Also, take into account that as these variables are defined inside the conan_toolchain.cmake file, and the toolchain is loaded several times by CMake, the definition of these variables will be done at those points as well.

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.

The booleans assigned to a variable will be translated to ON and OFF symbols in CMake:

def generate(self):
    tc = CMakeToolchain(self)
    tc.variables["FOO"] = True
    tc.variables["VAR"] = False
    tc.generate()

Will generate the sentences: set(FOO ON ...) and set(VAR OFF ...).

user_presets_path

This attribute allows specifying the location of the generated CMakeUserPresets.json file. Accepted values:

  • An absolute path
  • A path relative to self.source_folder
  • The boolean value False, to suppress the generation of the file altogether.

For example, we can prevent the generator from creating CMakeUserPresets.json in the following way:

def generate(self):
    tc = CMakeToolchain(self)
    tc.user_presets_path = False
    tc.generate()

Using a custom toolchain file

There are two ways of providing custom CMake toolchain files:

  • 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) to the conan_toolchain.cmake one, by using the user_toolchain block described below, and defining the tools.cmake.cmaketoolchain:user_toolchain=["<filepath>"] configuration value.

    The configuration tools.cmake.cmaketoolchain:user_toolchain=["<filepath>"] can be defined in the global.conf. but also creating a Conan package for your toolchain and using self.conf_info to declare the toolchain file:

    import os
    from conan import ConanFile
    class MyToolchainPackage(ConanFile):
        ...
        def package_info(self):
            f = os.path.join(self.package_folder, "mytoolchain.cmake")
            self.conf_info.define("tools.cmake.cmaketoolchain:user_toolchain", [f])
    

    If you declare the previous package as a tool_require, the toolchain will be automatically applied.

  • If you have more than one tool_requires defined, you can easily append all the user toolchain values together using the append method in each of them, for instance:

    import os
    from conan import ConanFile
    class MyToolRequire(ConanFile):
        ...
        def package_info(self):
            f = os.path.join(self.package_folder, "mytoolchain.cmake")
            # Appending the value to any existing one
            self.conf_info.append("tools.cmake.cmaketoolchain:user_toolchain", f)
    

    So, they’ll be automatically applied by your CMakeToolchain generator without writing any extra code:

    from conan import ConanFile
    from conan.tools.cmake import CMake
    class Pkg(ConanFile):
        settings = "os", "compiler", "arch", "build_type"
        exports_sources = "CMakeLists.txt"
        tool_requires = "toolchain1/0.1", "toolchain2/0.1"
        generators = "CMakeToolchain"
    
        def build(self):
            cmake = CMake(self)
            cmake.configure()
    

Extending and advanced customization

CMakeToolchain implements a powerful capability for extending and customizing the resulting toolchain file.

The contents are organized by blocks that can be customized. The following predefined blocks are available, and added in this order:

  • user_toolchain: Allows to include user toolchains from the conan_toolchain.cmake file. If the configuration tools.cmake.cmaketoolchain:user_toolchain=["xxxx", "yyyy"] is defined, its values will be include(xxx)\ninclude(yyyy) as the first lines in conan_toolchain.cmake.

  • generic_system: Defines CMAKE_SYSTEM_NAME, CMAKE_SYSTEM_VERSION, CMAKE_SYSTEM_PROCESSOR, CMAKE_GENERATOR_PLATFORM, CMAKE_GENERATOR_TOOLSET, CMAKE_C_COMPILER, CMAKE_CXX_COMPILER

  • android_system: Defines ANDROID_PLATFORM, ANDROID_STL, ANDROID_ABI and includes ANDROID_NDK_PATH/build/cmake/android.toolchain.cmake where ANDROID_NDK_PATH comes defined in tools.android:ndk_path configuration value.

  • apple_system: Defines 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.

  • output_dirs: Define the CMAKE_INSTALL_XXX variables.

    • CMAKE_INSTALL_PREFIX: Is set with the package_folder, so if a “cmake install” operation is run, the artifacts go to that location.
    • CMAKE_INSTALL_BINDIR, CMAKE_INSTALL_SBINDIR and CMAKE_INSTALL_LIBEXECDIR: Set by default to bin.
    • CMAKE_INSTALL_LIBDIR: Set by default to lib.
    • CMAKE_INSTALL_INCLUDEDIR and CMAKE_INSTALL_OLDINCLUDEDIR: Set by default to include.
    • CMAKE_INSTALL_DATAROOTDIR: Set by default to res.

    If you want to change the default values, adjust the cpp.package object at the layout() method:

    def layout(self):
        ...
        # For CMAKE_INSTALL_BINDIR, CMAKE_INSTALL_SBINDIR and CMAKE_INSTALL_LIBEXECDIR, takes the first value:
        self.cpp.package.bindirs = ["mybin"]
        # For CMAKE_INSTALL_LIBDIR, takes the first value:
        self.cpp.package.libdirs = ["mylib"]
        # For CMAKE_INSTALL_INCLUDEDIR, CMAKE_INSTALL_OLDINCLUDEDIR, takes the first value:
        self.cpp.package.includedirs = ["myinclude"]
        # For CMAKE_INSTALL_DATAROOTDIR, takes the first value:
        self.cpp.package.resdirs = ["myres"]
    

    Note

    It is not valid to change the self.cpp_info at the package_info() method.

Customizing the content blocks

Every block 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
    toolset = tc.blocks["generic_system"].values["toolset"]
    tc.blocks["generic_system"].values["toolset"] = "other_toolset"

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

# 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 {"toolset": "other_toolset"}
    generic_block.context = types.MethodType(context, generic_block)

# completely replace existing block
from conan.tools.cmake import CMakeToolchain

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

        def context(self):
            return {}

    tc.blocks["generic_system"] = MyGenericBlock

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

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

    tc.blocks["mynewblock"] = MyBlock

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 or files, 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 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

Reference

class CMakeToolchain(conanfile, generator=None)
generate()

This method will save the generated files to the conanfile.generators_folder

conf

CMakeToolchain is affected by these [conf] variables:

  • tools.cmake.cmaketoolchain:toolchain_file user toolchain file to replace the conan_toolchain.cmake one.
  • tools.cmake.cmaketoolchain:user_toolchain list of user toolchains to be included from the conan_toolchain.cmake file.
  • tools.android:ndk_path value for ANDROID_NDK_PATH.
  • tools.cmake.cmaketoolchain:system_name is not necessary in most cases and is only used to force-define CMAKE_SYSTEM_NAME.
  • tools.cmake.cmaketoolchain:system_version is not necessary in most cases and is only used to force-define CMAKE_SYSTEM_VERSION.
  • tools.cmake.cmaketoolchain:system_processor is not necessary in most cases and is only used to force-define CMAKE_SYSTEM_PROCESSOR.
  • tools.cmake.cmaketoolchain:toolset_arch: Will add the ,host=xxx specifier in the CMAKE_GENERATOR_TOOLSET variable of conan_toolchain.cmake file.
  • tools.cmake.cmake_layout:build_folder_vars: Settings and Options that will produce a different build folder and different CMake presets names.
  • tools.cmake.cmaketoolchain.presets:max_schema_version: Generate CMakeUserPreset.json compatible with the supplied schema version.
  • tools.build:cxxflags list of extra C++ flags that will be appended to CMAKE_CXX_FLAGS_INIT.
  • tools.build:cflags list of extra of pure C flags that will be appended to CMAKE_C_FLAGS_INIT.
  • tools.build:sharedlinkflags list of extra linker flags that will be appended to CMAKE_SHARED_LINKER_FLAGS_INIT.
  • tools.build:exelinkflags list of extra linker flags that will be appended to CMAKE_EXE_LINKER_FLAGS_INIT.
  • tools.build:defines list of preprocessor definitions that will be used by add_definitions().
  • tools.build:tools.apple:enable_bitcode boolean value to enable/disable Bitcode Apple Clang flags, e.g., CMAKE_XCODE_ATTRIBUTE_ENABLE_BITCODE.
  • tools.build:tools.apple:enable_arc boolean value to enable/disable ARC Apple Clang flags, e.g., CMAKE_XCODE_ATTRIBUTE_CLANG_ENABLE_OBJC_ARC.
  • tools.build:tools.apple:enable_visibility boolean value to enable/disable Visibility Apple Clang flags, e.g., CMAKE_XCODE_ATTRIBUTE_GCC_SYMBOLS_PRIVATE_EXTERN.
  • tools.build:sysroot defines the value of CMAKE_SYSROOT.
  • tools.build:compiler_executables dict-like Python object which specifies the compiler as key and the compiler executable path as value. Those keys will be mapped as follows:
    • c: will set CMAKE_C_COMPILER in conan_toolchain.cmake.
    • cpp: will set CMAKE_CXX_COMPILER in conan_toolchain.cmake.
    • RC: will set CMAKE_RC_COMPILER in conan_toolchain.cmake.
    • objc: will set CMAKE_OBJC_COMPILER in conan_toolchain.cmake.
    • objcpp: will set CMAKE_OBJCXX_COMPILER in conan_toolchain.cmake.
    • cuda: will set CMAKE_CUDA_COMPILER in conan_toolchain.cmake.
    • fortran: will set CMAKE_Fortran_COMPILER in conan_toolchain.cmake.
    • asm: will set CMAKE_ASM_COMPILER in conan_toolchain.cmake.
    • hip: will set CMAKE_HIP_COMPILER in conan_toolchain.cmake.
    • ispc: will set CMAKE_ISPC_COMPILER in conan_toolchain.cmake.