Help: Add cmake-toolchains.7 manual
Create a manual for toolchain related docs.
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Stephen Kelly
Brad King
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@ -27,6 +27,7 @@ Reference Manuals
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/manual/cmake-commands.7
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/manual/cmake-generators.7
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/manual/cmake-toolchains.7
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/manual/cmake-modules.7
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/manual/cmake-policies.7
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/manual/cmake-properties.7
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@ -0,0 +1,160 @@
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.. cmake-manual-description: CMake Toolchains Reference
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cmake-toolchains(7)
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*******************
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.. only:: html or latex
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.. contents::
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Introduction
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============
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CMake uses a toolchain of utilities to compile, link libraries and create
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archives, and other tasks to drive the build. The toolchain utilities available
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are determined by the languages enabled. In normal builds, CMake automatically
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determines the toolchain for host builds based on system introspection and
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defaults. In cross-compiling scenarios, a toolchain file may be specified
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with information about compiler and utility paths.
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Languages
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=========
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Languages are enabled by the :command:`project` command. If no project command
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is in the top-level CMakeLists file, one will be implicitly generated. By default
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the enabled languages are C and CXX::
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project(C_Only C)
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A special value of NONE can also be used with the :command:`project` command
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to enable no languages::
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project(MyProject NONE)
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The :command:`enable_language` command can be used to enable languages after the
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:command:`project` command::
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enable_language(CXX)
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When a language is enabled, CMake finds a compiler for that language, and
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determines some information, such as the vendor and version of the compiler,
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the target architecture and bitwidth, the location of corresponding utilities
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etc.
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The :prop_gbl:`ENABLED_LANGUAGES` global property contains the languages which
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are currently enabled.
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Variables and Properties
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========================
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Several variables relate to the language components of a toolchain which are
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enabled. :variable:`CMAKE_<LANG>_COMPILER` is the full path to the compiler used
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for ``<LANG>``. :variable:`CMAKE_<LANG>_COMPILER_ID` is the identifier used
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by CMake for the compiler and :variable:`CMAKE_<LANG>_COMPILER_VERSION` is the
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version of the compiler.
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The :variable:`CMAKE_<LANG>_FLAGS` variables and the configuration-specific
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equivalents contain flags that will be added to the compile command when
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compiling a file of a particular language.
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As the linker is invoked by the compiler driver, CMake needs a way to determine
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which compiler to use to invoke the linker. This is calculated by the
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:prop_sf:`LANGUAGE` of source files in the target, and in the case of static
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libraries, the language of the dependent libraries. The choice CMake makes may
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be overridden with the :prop_tgt:`LINKER_LANGUAGE` target property.
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Toolchain Features
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==================
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CMake provides the :command:`try_compile` command and wrapper macros such as
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:module:`CheckCXXSourceCompiles`, :module:`CheckCXXSymbolExists` and
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:module:`CheckIncludeFile` to test capability and availability of various
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toolchain features. These APIs test the toolchain in some way and cache the
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result so that the test does not have to be performed again the next time
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CMake runs.
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Some toolchain features have built-in handling in CMake, and do not require
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compile-tests. For example, :prop_tgt:`POSITION_INDEPENDENT_CODE` allows
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specifying that a target should be built as position-independent code, if
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the compiler supports that feature. The :prop_tgt:`<LANG>_VISIBILITY_PRESET`
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and :prop_tgt:`VISIBILITY_INLINES_HIDDEN` target properties add flags for
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hidden visibility, if supported by the compiler.
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Cross Compiling
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===============
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If :manual:`cmake(1)` is invoked with the command line parameter
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``-DCMAKE_TOOLCHAIN_FILE=path/to/file``, the file will be loaded early to set
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values for the compilers. A typical cross-compiling toolchain has content such
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as::
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set(CMAKE_SYSTEM_NAME Linux)
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set(CMAKE_SYSROOT /home/devel/rasp-pi-rootfs)
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set(CMAKE_STAGING_PREFIX /home/devel/stage)
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set(CMAKE_C_COMPILER /home/devel/gcc-4.7-linaro-rpi-gnueabihf/bin/arm-linux-gnueabihf-gcc)
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set(CMAKE_CXX_COMPILER /home/devel/gcc-4.7-linaro-rpi-gnueabihf/bin/arm-linux-gnueabihf-g++)
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set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
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set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
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set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
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set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
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The :variable:`CMAKE_SYSTEM_NAME` is the CMake-identifier of the target platform
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to build for.
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The :variable:`CMAKE_SYSROOT` is optional, and may be specified if a sysroot
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is available.
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The :variable:`CMAKE_STAGING_PREFIX` is also optional. It may be used to specify
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a path on the host to install to. The :variable:`CMAKE_INSTALL_PREFIX` is always
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the runtime installation location, even when cross-compiling.
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The :variable:`CMAKE_<LANG>_COMPILER` variables may be set to full paths, or to
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names of compilers to search for in standard locations. In cases where CMake does
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not have enough information to extract information from the compiler, the
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:module:`CMakeForceCompiler` module can be used to bypass some of the checks.
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CMake ``find_*`` commands will look in the sysroot, and the :variable:`CMAKE_FIND_ROOT_PATH`
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entries by default in all cases, as well as looking in the host system root prefix.
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Although this can be controlled on a case-by-case basis, when cross-compiling, it
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can be useful to exclude looking in either the host or the target for particular
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artifacts. Generally, includes, libraries and packages should be found in the
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target system prefixes, whereas executables which must be run as part of the build
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should be found only on the host and not on the target. This is the purpose of
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the ``CMAKE_FIND_ROOT_PATH_MODE_*`` variables.
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Some compilers are inherently cross compilers, such as Clang and the QNX QCC
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compiler. The :variable:`CMAKE_<LANG>_COMPILER_TARGET` can be set to pass a
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value to those supported compilers when compiling::
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set(CMAKE_SYSTEM_NAME Linux)
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set(triple arm-linux-gnueabihf)
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set(CMAKE_C_COMPILER clang)
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set(CMAKE_C_COMPILER_TARGET ${triple})
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set(CMAKE_CXX_COMPILER clang++)
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set(CMAKE_CXX_COMPILER_TARGET ${triple})
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Or, for QCC::
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set(CMAKE_SYSTEM_NAME QNX)
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set(arch gcc_ntoarmv7le)
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set(CMAKE_C_COMPILER qcc)
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set(CMAKE_C_COMPILER_TARGET ${arch})
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set(CMAKE_CXX_COMPILER QCC)
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set(CMAKE_CXX_COMPILER_TARGET ${arch})
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Similarly, some compilers do not ship their own supplementary utilities
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such as linkers, but provide a way to specify the location of the external
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toolchain which will be used by the compiler driver. The
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:variable:`CMAKE_<LANG>_COMPILER_EXTERNAL_TOOLCHAIN` variable can be set in a
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toolchain file to pass the path to the compiler driver.
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The :variable:`CMAKE_CROSSCOMPILING` variable is set to true when CMake is
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cross-compiling.
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