How do I detect that I am cross-compiling in CMakeLists.txt? - cmake

Question

The CMake documentation suggests that CMAKE_CROSSCOMPILING is set when cross-compiling. In my CMakeLists.txt I have the lines:
IF(CMAKE_CROSSCOMPILING)
message(STATUS "Cross-compiling so skipping unit tests.")
option(GAME_PORTAL_UNIT_TEST "Enable unit testing of Game Portal code" OFF)
ELSE()
message(STATUS "Enabling unit testing of Game Portal code")
option(GAME_PORTAL_UNIT_TEST "Enable unit testing of Game Portal code" ON)
ENDIF()
The output from running:
cmake -DCMAKE_TOOLCHAIN_FILE=../crosscompile/raspberry_pi/CMakeCross.txt .
Includes the text "Enabling unit testing of Game Portal code", so clearly this variable is not being set, or not so it evaluates to true anyway.
I tried modifying CMakeCross.txt to include:
set(CMAKE_CROSSCOMPILING ON CACHE BOOL "Cross-compiling" FORCE)
and after cleaning the old CMakeCache.txt and rerunning my cmake command I can see that the new CMakeCache.txt now includes this variable, but I still get the same result as previously with regards to the unit tests being enabled.
How can I reliably detect that I am cross-compiling so I can properly disable the unit tests?
As requested, the full cross-compile file is:
# Set minimum cmake version required for cross-compiling to work.
cmake_minimum_required(VERSION 2.6)
# Build with rm CMakeCache.txt; cmake -DCMAKE_TOOLCHAIN_FILE=/home/crosscompile/dev/raspberry_pi/CMakeCross.txt ..
# Set target system name.
SET (CMAKE_SYSTEM_NAME Linux)
# Set compiler name.
SET (CMAKE_C_COMPILER arm-linux-gnueabihf-gcc)
SET (CMAKE_CXX_COMPILER arm-linux-gnueabihf-g++)
# Set path(s) to search for libraries/binaries/headers.
SET (CMAKE_FIND_ROOT_PATH /home/crosscompile/dev/raspberry_pi/rootfs/)
# Ensure only cross-compiler directories are searched.
SET (ONLY_CMAKE_FIND_ROOT_PATH TRUE)
# search for programs in the build host directories
SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
# for libraries and headers in the target directories
SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
# Set output/install directory to safe place.
SET (CMAKE_INSTALL_PREFIX /home/crosscompile/dev/raspberry_pi/install/)
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -rpath-link=/lib/arm-linux-gnueabihf")
set(THREADS_PTHREAD_ARG 0)
set(CMAKE_CROSSCOMPILING ON CACHE BOOL "Cross-compiling" FORCE)

Answer 1

The test for CMAKE_CROSSCOMPILING must come after the "project" instruction in CMakeLists.txt.

Answer 2

With in-source builds, one need to manually cleanup build files when change configuration parameters a lot.
E.g., if you did native build before, and then decide to cross-compile, you need to perform manual cleanup: CMake cannot automatically adjust build directory from one build type to another.
This is one of the reasons why in-source builds are not recommended and should be replaced with out-of-source builds.

Answer 3

This is working in my example:
CMakeLists.txt
cmake_minimum_required(VERSION 3.0)
project(STM32F4Examples C)
set(CMAKE_CROSSCOMPILE OFF CACHE BOOL "is crosscompiled")
message(STATUS "CMAKE_CROSSCOMPILE ${CMAKE_CROSSCOMPILE}")
CMakeToolChain_STM32F4.txt
# cmake toolchain
# Use this file with cmake -DCMAKE_TOOLCHAIN_FILE=[PATH/TO/This/FILE] PATH/TO/SOURCES
set(CMAKE_CROSSCOMPILE ON CACHE BOOL "is crosscompiled" FORCE)

Answer 4

This cmake -DCMAKE_TOOLCHAIN_FILE=.. command will set CMAKE_TOOLCHAIN_FILE.
Check cross compiling with following:
if (CMAKE_TOOLCHAIN_FILE)
# This is in cross compiling condition.
set(PROJECT_OUT_NAME ${PROJECT_NAME}.elf)
else ()
set(PROJECT_OUT_NAME ${PROJECT_NAME})
endif()
add_executable(${PROJECT_OUT_NAME} "main.cpp")