There are several different ways to check the current system/ platform in CMake, and each interacts with cross-compiling in different ways.
== Platform Variables ==
CMake sets certain variables to true depending on the current platform and toolchain in use. These always describe the ''target'' platform.
; UNIX : is TRUE on all UNIX-like OS's, including Apple OS X and ''CygWin''
; WIN32 : is TRUE on Windows. Prior to 2. 8.4 this included ''CygWin''
; APPLE : is TRUE on Apple systems. Note this does ''not'' imply the system is Mac OS X, only that __APPLE__ is #defined in C/ C++ header files.
; MINGW : is TRUE when using the MinGW compiler in Windows
; MSYS : is TRUE when using the MSYS developer environment in Windows
; CYGWIN : is TRUE on Windows when using the ''CygWin'' version of cmake
== CMake System ==
A cleaner and more cross- compiling-compatible way to check the current platform is through the CMAKE_SYSTEM variables.
; CMAKE_SYSTEM : the complete system name, e.g. "Linux-2.4.22", "FreeBSD-5.4-RELEASE" or "Windows 5.1"
; CMAKE_SYSTEM_NAME : The name of the system targeted by the build. The three common values are '''Windows''', '''Darwin''', and '''Linux''', though several others exist, such as '''Android''', '''FreeBSD''', and '''CrayLinuxEnvironment'''. Platforms without an operating system, such as embedded devices, are given '''Generic''' as a system name.
; CMAKE_SYSTEM_VERSION : Version of the operating system. Generally the ''kernel'' version.
; CMAKE_SYSTEM_PROCESSOR : the processor name (e.g. "Intel(R) Pentium(R) M processor 2.00GHz")
; CMAKE_HOST_SYSTEM_NAME : The name of the system hosting the build. Has the same possible values as '''CMAKE_SYSTEM_NAME'''.