CMakeToolchain

Warning

These tools are still experimental (so subject to breaking changes) but with very stable syntax. We encourage the usage of it to be prepared for Conan 2.0.

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=conan_toolchain.cmake. This generator translates the current package configuration, settings, and options, into CMake toolchain syntax.

Important

This class will require very soon to define both the “host” and “build” profiles. It is very recommended to start defining both profiles immediately to avoid future breaking. Furthermore, some features, like trying to cross-compile might not work at all if the “build” profile is not provided.

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()

This will generate the following files after a call to 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_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
CMakePresets.json: The toolchain also generates a CMakePresets.json standard file, check the documentation here. It is currently using version “3” of the JSON schema. Conan creates a default configure preset with the following information:
- The CMake generator to be used.
- The path to the generated conan_toolchain.cmake file.
- Some cache variables corresponding to the specified settings cannot work if specified in the toolchain.
- The CMAKE_BUILD_TYPE variable when using single-configuration generators.
CMakeUserPresets.json: If you declare a layout() in the recipe and your CMakeLists.txt file is found in the conanfile.source_folder folder, Conan will place a CMakeUserPresets.json to include the CMakePresets.json generated with the above specification. This will allow your IDE to allow your IDE (Visual Studio, Visual Studio Code, CLion…) or cmake tool to locate the Conan-generated``CMakePresets.json``. The version schema of the generated CMakeUserPresets.json is “4” and requires CMake >= 3.23. Note: Conan will skip the generation if it already exists and was not previously generated by Conan.
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):

conanfile: the current recipe object. Always use self.

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.

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 ...).

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.

Generators

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.

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()

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.

# 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

If you are using a multi-configuration generator:

# 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"

If you are using a single-configuration generator:

$ cmake ..  -DCMAKE_TOOLCHAIN_FILE=conan_toolchain.cmake -DCMAKE_BUILD_TYPE=Release
$ cmake --build

It is recommended to use the cmake_layout(self) in the layout() method of your conanfile.py. If a layout is declared, the CMakeUserPresets.json file will be generated in the same folder of your CMakeLists.txt file, so you can use the --preset argument from cmake >= 3.23 or use an IDE:

# The conan_toolchain.cmake is common for both configurations and will be located at "build/generators"
$ conan install .
$ conan install . -s build_type=Debug

# For single-configuration generator
$ cmake --preset Debug
$ cmake --build --preset Debug
$ cmake --preset Release
$ cmake --build --preset Release

# For multi-configuration generator
$ cmake --preset default
$ cmake --build --preset Debug
$ cmake --build --preset Release

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.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.

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 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.

Note

In Conan 1.45 the CMakeToolchain doesn’t append the root package folder of the dependencies (declared in the cpp_info.builddirs) to the CMAKE_PREFIX_PATH variable. That interfered with the find_file, find_path and find_program, making, for example, impossible to locate only the executables from the build context. In Conan 2.0, the cppinfo.builddirs won’t contain by default the '' entry (root package).

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
    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

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 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