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: This toolchain generates a standard CMakePresets.json file. For more information, refer to the documentation here. It currently uses version “3” of the JSON schema. Conan adds configure, build, and test preset entries to the JSON file:
- configurePresets storing the following information:
  
  The generator to be used.
  
  The path to the conan_toolchain.cmake.
  
  Cache variables corresponding to the specified settings that cannot work if specified in the toolchain.
  
  The CMAKE_BUILD_TYPE variable for single-configuration generators.
  
  The BUILD_TESTING variable set to OFF when the configuration tools.build:skip_test is true.
  
  An environment section, setting all the environment information related to the VirtualBuildEnv, if applicable. This environment can be modified in the generate() method of the recipe by passing an environment through the CMakeToolchain.presets_build_environment attribute. Generation of this section can be skipped by using the tools.cmake.cmaketoolchain:presets_environment configuration.
  
  By default, preset names will be conan-xxxx, but the “conan-” prefix can be customized with the CMakeToolchain.presets_prefix = “conan” attribute.
  
  Preset names are controlled by the layout() self.folders.build_folder_vars definition, which can contain a list of settings and options like [“settings.compiler”, “settings.arch”, “options.shared”].
  
  If CMake is found as a direct tool_requires dependency, or if tools.cmake:cmake_program is set, the configure preset will include a cmakeExecutable field. This field represents the path to the CMake executable to be used for this preset. As stated in the CMake documentation, this field is reserved for use by IDEs and is not utilized by CMake itself.
- buildPresets storing the following information:
  
  The configurePreset associated with this build preset.
- testPresets storing the following information:
  
  The configurePreset associated with this build preset.
  
  An environment section, setting all the environment information related to the VirtualRunEnv, if applicable. This environment can be modified in the generate() method of the recipe by passing an environment through the CMakeToolchain.presets_run_environment attribute. Please note that since this preset inherits from a configurePreset, it will also inherit its environment. Generation of this section can be skipped by using the`tools.cmake.cmaketoolchain:presets_environment` configuration.
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. The file name of this file can be configured with the CMakeToolchain.user_presets_path = "CMakeUserPresets.json"` attribute, so if you want to generate a “ConanPresets.json” instead to be included from your own file, you can define tc.user_presets_path = "ConanPresets.jon" in the generate() method. See extending your own CMake presets for a full example.

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

Note: To list all available presets, use the cmake --list-presets command:

Note

The version schema of the generated CMakeUserPresets.json is 4 (compatible with CMake>=3.23) and the schema for the CMakePresets.json is 3 (compatible with CMake>=3.21).

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"
    # Setting to None will add the definition with no value
    tc.preprocessor_definitions["NOVALUE_DEF"] = None
    tc.generate()

This will be translated to:

One add_compile_definitions() definition for MYDEF in conan_toolchain.cmake file.
One add_compile_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()

presets_build_environment, presets_run_environment¶

These attributes enable the modification of the build and run environments associated with the presets, respectively, by assigning an Environment. This can be accomplished in the generate() method.

For example, you can override the value of an environment variable already set in the build environment:

def generate(self):
    buildenv = VirtualBuildEnv(self)
    buildenv.environment().define("MY_BUILD_VAR", "MY_BUILDVAR_VALUE_OVERRIDDEN")
    buildenv.generate()

    tc = CMakeToolchain(self)
    tc.presets_build_environment = buildenv.environment()
    tc.generate()

Or generate a new environment and compose it with an already existing one:

def generate(self):
    runenv = VirtualRunEnv(self)
    runenv.environment().define("MY_RUN_VAR", "MY_RUNVAR_SET_IN_GENERATE")
    runenv.generate()

    env = Environment()
    env.define("MY_ENV_VAR", "MY_ENV_VAR_VALUE")
    env = env.vars(self, scope="run")
    env.save_script("other_env")

    tc = CMakeToolchain(self)
    tc.presets_run_environment = runenv.environment().compose_env(env)
    tc.generate()

Extra compilation flags¶

You can use the following attributes to append extra compilation flags to the toolchain:

extra_cxxflags (defaulted to []) for additional cxxflags
extra_cflags (defaulted to []) for additional cflags
extra_sharedlinkflags (defaulted to []) for additional shared link flags
extra_exelinkflags (defaulted to []) for additional exe link flags

Note

flags order of preference: Flags specified in the tools.build configuration, such as cxxflags, cflags, sharedlinkflags and exelinkflags, will always take precedence over those set by the CMakeToolchain attributes.

presets_prefix¶

By default it is "conan", and it will generate CMake presets named “conan-xxxx”. This is done to avoid potential name clashes with users own presets.

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.
linker_scripts: Defines the flags for any provided linker scripts.
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 (recall to call tc.generate() after the customization):

# tc.generate() should be called at the end of every one

# remove an existing block, the generated conan_toolchain.cmake
# will not contain code for that block at all
def generate(self):
    tc = CMakeToolchain(self)
    tc.blocks.remove("generic_system")

# remove several blocks
def generate(self):
    tc = CMakeToolchain(self)
    tc.blocks.remove("generic_system", "cmake_flags_init")

# keep one block, remove all the others
# If you want to generate conan_toolchain.cmake with only that
# block
def generate(self):
    tc = CMakeToolchain(self)
    tc.blocks.select("generic_system")

# keep several blocks, remove the other blocks
def generate(self):
    tc = CMakeToolchain(self)
    tc.blocks.select("generic_system", "cmake_flags_init")

# iterate blocks
def generate(self):
    tc = CMakeToolchain(self)
    for block_name in tc.blocks.keys():
        # do something with block_name
    for block_name, block in tc.blocks.items():
        # do something with block_name and block

# 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.android:cmake_legacy_toolchain: boolean value for ANDROID_USE_LEGACY_TOOLCHAIN_FILE. It will only be defined in conan_toolchain.cmake if given a value. This is taken into account by the CMake toolchain inside the Android NDK specified in the tools.android:ndk_path config, for versions r23c and above. It may be useful to set this to False if compiler flags are defined via tools.build:cflags or tools.build:cxxflags to prevent Android’s legacy CMake toolchain from overriding the values. If setting this to False, please ensure you are using CMake 3.21 or above.
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.cmaketoolchain:toolset_cuda: (Experimental) Will add the ,cuda=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_environment: Set to 'disabled' to prevent the addition of the environment section to the generated CMake presets.
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.apple:sdk_path value for CMAKE_OSX_SYSROOT. In the general case it’s not needed and will be passed to CMake by the settings values.
tools.apple:enable_bitcode boolean value to enable/disable Bitcode Apple Clang flags, e.g., CMAKE_XCODE_ATTRIBUTE_ENABLE_BITCODE.
tools.apple:enable_arc boolean value to enable/disable ARC Apple Clang flags, e.g., CMAKE_XCODE_ATTRIBUTE_CLANG_ENABLE_OBJC_ARC.
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.microsoft:winsdk_version Defines the CMAKE_SYSTEM_VERSION or the CMAKE_GENERATOR_PLATFORM according to CMake policy CMP0149.
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.