CMake/Help/manual/cmake-toolchains.7.rst

.. cmake-manual-description: CMake Toolchains Reference

cmake-toolchains(7)
*******************

.. only:: html or latex

   .. contents::

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 :command:`project` command.  Language-specific
built-in variables, such as
:variable:`CMAKE_CXX_COMPILER <CMAKE_<LANG>_COMPILER>`,
:variable:`CMAKE_CXX_COMPILER_ID <CMAKE_<LANG>_COMPILER_ID>` etc are set by
invoking the :command:`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:

.. code-block:: cmake

  project(C_Only C)

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

.. code-block:: cmake

  project(MyProject NONE)

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

.. code-block:: cmake

  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 :prop_gbl:`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. :variable:`CMAKE_<LANG>_COMPILER` is the full path to the compiler used
for ``<LANG>``. :variable:`CMAKE_<LANG>_COMPILER_ID` is the identifier used
by CMake for the compiler and :variable:`CMAKE_<LANG>_COMPILER_VERSION` is the
version of the compiler.

The :variable:`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
:prop_sf:`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 :prop_tgt:`LINKER_LANGUAGE` target property.

Toolchain Features
==================

CMake provides the :command:`try_compile` command and wrapper macros such as
:module:`CheckCXXSourceCompiles`, :module:`CheckCXXSymbolExists` and
:module:`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, :prop_tgt:`POSITION_INDEPENDENT_CODE` allows
specifying that a target should be built as position-independent code, if
the compiler supports that feature. The :prop_tgt:`<LANG>_VISIBILITY_PRESET`
and :prop_tgt:`VISIBILITY_INLINES_HIDDEN` target properties add flags for
hidden visibility, if supported by the compiler.

.. _`Cross Compiling Toolchain`:

Cross Compiling
===============

If :manual:`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. A typical cross-compiling toolchain has content such
as:

.. code-block:: cmake

  set(CMAKE_SYSTEM_NAME Linux)

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

  set(CMAKE_C_COMPILER /home/devel/gcc-4.7-linaro-rpi-gnueabihf/bin/arm-linux-gnueabihf-gcc)
  set(CMAKE_CXX_COMPILER /home/devel/gcc-4.7-linaro-rpi-gnueabihf/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 :variable:`CMAKE_SYSTEM_NAME` is the CMake-identifier of the target platform
to build for.

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

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

The :variable:`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
:module:`CMakeForceCompiler` module can be used to bypass some of the checks.

CMake ``find_*`` commands will look in the sysroot, and the :variable:`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.

Some compilers are inherently cross compilers, such as Clang and the QNX QCC
compiler. The :variable:`CMAKE_<LANG>_COMPILER_TARGET` can be set to pass a
value to those supported compilers when compiling:

.. code-block:: cmake

  set(CMAKE_SYSTEM_NAME Linux)

  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})

Or, for QCC:

.. code-block:: cmake

  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})


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
:variable:`CMAKE_<LANG>_COMPILER_EXTERNAL_TOOLCHAIN` variable can be set in a
toolchain file to pass the path to the compiler driver.

The :variable:`CMAKE_CROSSCOMPILING` variable is set to true when CMake is
cross-compiling.