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

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.