cmake-toolchains(7)

Introduction

CMake uses a toolchain of utilities to compile, link libraries and create archives, and other tasks to drive the build. The toolchain utilities available are determined by the languages enabled. In normal builds, CMake automatically determines the toolchain for host builds based on system introspection and defaults. In cross-compiling scenarios, a toolchain file may be specified with information about compiler and utility paths.

Languages

Languages are enabled by the project() command. Language-specific built-in variables, such as CMAKE_CXX_COMPILER, CMAKE_CXX_COMPILER_ID etc are set by invoking the project() command. If no project command is in the top-level CMakeLists file, one will be implicitly generated. By default the enabled languages are C and CXX:

project(C_Only C)

A special value of NONE can also be used with the project() command to enable no languages:

project(MyProject NONE)

The enable_language() command can be used to enable languages after the project() command:

enable_language(CXX)

When a language is enabled, CMake finds a compiler for that language, and determines some information, such as the vendor and version of the compiler, the target architecture and bitwidth, the location of corresponding utilities etc.

The ENABLED_LANGUAGES global property contains the languages which are currently enabled.

Variables and Properties

Several variables relate to the language components of a toolchain which are enabled. CMAKE_<LANG>_COMPILER is the full path to the compiler used for <LANG>. CMAKE_<LANG>_COMPILER_ID is the identifier used by CMake for the compiler and CMAKE_<LANG>_COMPILER_VERSION is the version of the compiler.

The CMAKE_<LANG>_FLAGS variables and the configuration-specific equivalents contain flags that will be added to the compile command when compiling a file of a particular language.

As the linker is invoked by the compiler driver, CMake needs a way to determine which compiler to use to invoke the linker. This is calculated by the LANGUAGE of source files in the target, and in the case of static libraries, the language of the dependent libraries. The choice CMake makes may be overridden with the LINKER_LANGUAGE target property.

Toolchain Features

CMake provides the try_compile() command and wrapper macros such as CheckCXXSourceCompiles, CheckCXXSymbolExists and CheckIncludeFile to test capability and availability of various toolchain features. These APIs test the toolchain in some way and cache the result so that the test does not have to be performed again the next time CMake runs.

Some toolchain features have built-in handling in CMake, and do not require compile-tests. For example, POSITION_INDEPENDENT_CODE allows specifying that a target should be built as position-independent code, if the compiler supports that feature. The <LANG>_VISIBILITY_PRESET and VISIBILITY_INLINES_HIDDEN target properties add flags for hidden visibility, if supported by the compiler.

Cross Compiling

If cmake(1) is invoked with the command line parameter -DCMAKE_TOOLCHAIN_FILE=path/to/file, the file will be loaded early to set values for the compilers. The CMAKE_CROSSCOMPILING variable is set to true when CMake is cross-compiling.

Cross Compiling for Linux

A typical cross-compiling toolchain for Linux has content such as:

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)

set(CMAKE_SYSROOT /home/devel/rasp-pi-rootfs)
set(CMAKE_STAGING_PREFIX /home/devel/stage)

set(tools /home/devel/gcc-4.7-linaro-rpi-gnueabihf)
set(CMAKE_C_COMPILER ${tools}/bin/arm-linux-gnueabihf-gcc)
set(CMAKE_CXX_COMPILER ${tools}/bin/arm-linux-gnueabihf-g++)

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)

The CMAKE_SYSTEM_NAME is the CMake-identifier of the target platform to build for.

The CMAKE_SYSTEM_PROCESSOR is the CMake-identifier of the target architecture to build for.

The CMAKE_SYSROOT is optional, and may be specified if a sysroot is available.

The CMAKE_STAGING_PREFIX is also optional. It may be used to specify a path on the host to install to. The CMAKE_INSTALL_PREFIX is always the runtime installation location, even when cross-compiling.

The CMAKE_<LANG>_COMPILER variables may be set to full paths, or to names of compilers to search for in standard locations. In cases where CMake does not have enough information to extract information from the compiler, the CMakeForceCompiler module can be used to bypass some of the checks.

CMake find_* commands will look in the sysroot, and the CMAKE_FIND_ROOT_PATH entries by default in all cases, as well as looking in the host system root prefix. Although this can be controlled on a case-by-case basis, when cross-compiling, it can be useful to exclude looking in either the host or the target for particular artifacts. Generally, includes, libraries and packages should be found in the target system prefixes, whereas executables which must be run as part of the build should be found only on the host and not on the target. This is the purpose of the CMAKE_FIND_ROOT_PATH_MODE_* variables.

Cross Compiling for the Cray Linux Environment

Cross compiling for compute nodes in the Cray Linux Environment can be done without needing a separate toolchain file. Specifying -DCMAKE_SYSTEM_NAME=CrayLinuxEnvironment on the CMake command line will ensure that the appropriate build settings and search paths are configured. The platform will pull its configuration from the current environment variables and will configure a project to use the compiler wrappers from the Cray Programming Environment’s PrgEnv-* modules if present and loaded.

The default configuration of the Cray Programming Environment is to only support static libraries. This can be overridden and shared libraries enabled by setting the CRAYPE_LINK_TYPE environment variable to dynamic.

Running CMake without specifying CMAKE_SYSTEM_NAME will run the configure step in host mode assuming a standard Linux environment. If not overridden, the PrgEnv-* compiler wrappers will end up getting used, which if targeting the either the login node or compute node, is likely not the desired behavior. The exception to this would be if you are building directly on a NID instead of cross-compiling from a login node. If trying to build software for a login node, you will need to either first unload the currently loaded PrgEnv-* module or explicitly tell CMake to use the system compilers in /usr/bin instead of the Cray wrappers. If instead targeting a compute node is desired, just specify the CMAKE_SYSTEM_NAME as mentioned above.

Cross Compiling using Clang

Some compilers such as Clang are inherently cross compilers. The CMAKE_<LANG>_COMPILER_TARGET can be set to pass a value to those supported compilers when compiling:

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)

set(triple arm-linux-gnueabihf)

set(CMAKE_C_COMPILER clang)
set(CMAKE_C_COMPILER_TARGET ${triple})
set(CMAKE_CXX_COMPILER clang++)
set(CMAKE_CXX_COMPILER_TARGET ${triple})

Similarly, some compilers do not ship their own supplementary utilities such as linkers, but provide a way to specify the location of the external toolchain which will be used by the compiler driver. The CMAKE_<LANG>_COMPILER_EXTERNAL_TOOLCHAIN variable can be set in a toolchain file to pass the path to the compiler driver.

Cross Compiling for QNX

As the Clang compiler the QNX QCC compile is inherently a cross compiler. And the CMAKE_<LANG>_COMPILER_TARGET can be set to pass a value to those supported compilers when compiling:

set(CMAKE_SYSTEM_NAME QNX)

set(arch gcc_ntoarmv7le)

set(CMAKE_C_COMPILER qcc)
set(CMAKE_C_COMPILER_TARGET ${arch})
set(CMAKE_CXX_COMPILER QCC)
set(CMAKE_CXX_COMPILER_TARGET ${arch})

Cross Compiling for Windows CE

Cross compiling for Windows CE requires the corresponding SDK being installed on your system. These SDKs are usually installed under C:/Program Files (x86)/Windows CE Tools/SDKs.

A toolchain file to configure a Visual Studio generator for Windows CE may look like this:

set(CMAKE_SYSTEM_NAME WindowsCE)

set(CMAKE_SYSTEM_VERSION 8.0)
set(CMAKE_SYSTEM_PROCESSOR arm)

set(CMAKE_GENERATOR_TOOLSET CE800) # Can be omitted for 8.0
set(CMAKE_GENERATOR_PLATFORM SDK_AM335X_SK_WEC2013_V310)

The CMAKE_GENERATOR_PLATFORM tells the generator which SDK to use. Further CMAKE_SYSTEM_VERSION tells the generator what version of Windows CE to use. Currently version 8.0 (Windows Embedded Compact 2013) is supported out of the box. Other versions may require one to set CMAKE_GENERATOR_TOOLSET to the correct value.

Cross Compiling for Windows 10 Universal Applications

A toolchain file to configure a Visual Studio generator for a Windows 10 Universal Application may look like this:

set(CMAKE_SYSTEM_NAME WindowsStore)
set(CMAKE_SYSTEM_VERSION 10.0)

A Windows 10 Universal Application targets both Windows Store and Windows Phone. Specify the CMAKE_SYSTEM_VERSION variable to be 10.0 to build with the latest available Windows 10 SDK. Specify a more specific version (e.g. 10.0.10240.0 for RTM) to build with the corresponding SDK.

Cross Compiling for Windows Phone

A toolchain file to configure a Visual Studio generator for Windows Phone may look like this:

set(CMAKE_SYSTEM_NAME WindowsPhone)
set(CMAKE_SYSTEM_VERSION 8.1)

Cross Compiling for Windows Store

A toolchain file to configure a Visual Studio generator for Windows Store may look like this:

set(CMAKE_SYSTEM_NAME WindowsStore)
set(CMAKE_SYSTEM_VERSION 8.1)