CMake variable persisting between runs even though it was reset

I'm trying to cross compile from Ubuntu to Windows, using CMake. I have everything configured (Installed both mingw32 and mingw64), and I have the relevant toolchain files for both. The problem is that a variable that I set in one toolchain file, after it is used, persists in the second run with the other toolchain file one as well.

These are my toolchain files:

Windows 64 bit: (Toolchain-Ubuntu-mingw64.cmake)

set(CMAKE_SYSTEM_NAME Windows)

unset(PROCESSOR_ARCHITECTURE CACHE)
set(PROCESSOR_ARCHITECTURE x64)
message("===Processor architecture: " ${PROCESSOR_ARCHITECTURE})
set(TOOLCHAIN_PREFIX x86_64-w64-mingw32)

set(CMAKE_C_COMPILER ${TOOLCHAIN_PREFIX}-gcc)
set(CMAKE_CXX_COMPILER ${TOOLCHAIN_PREFIX}-g++)
set(CMAKE_RC_COMPILER ${TOOLCHAIN_PREFIX}-windres)

set(CMAKE_FIND_ROOT_PATH /usr/${TOOLCHAIN_PREFIX})

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

Windows 32 bit: (Toolchain-Ubuntu-mingw32.cmake)

set(CMAKE_SYSTEM_NAME Windows)

unset(PROCESSOR_ARCHITECTURE CACHE)
set(PROCESSOR_ARCHITECTURE x86)
message("===Processor architecture: " ${PROCESSOR_ARCHITECTURE})
set(TOOLCHAIN_PREFIX i686-w64-mingw32)

set(CMAKE_C_COMPILER ${TOOLCHAIN_PREFIX}-gcc)
set(CMAKE_CXX_COMPILER ${TOOLCHAIN_PREFIX}-g++)
set(CMAKE_RC_COMPILER ${TOOLCHAIN_PREFIX}-windres)

set(CMAKE_FIND_ROOT_PATH /usr/${TOOLCHAIN_PREFIX})

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

And this is the top of my CMakeLists.txt file:

cmake_minimum_required(VERSION 2.8)

project("project")

set(SOURCE_FILE_LIST main.c)

set(ARCH_PROJECT_NAME ${PROJECT_NAME}${PROCESSOR_ARCHITECTURE})
message("Project - " ${ARCH_PROJECT_NAME})

...

When I run

cmake -DCMAKE_TOOLCHAIN_FILE=../cmake-toolchains/Toolchain-Ubuntu-mingw64.cmake ../src/

The output starts with:

===Processor architecture: x64
Project - projectx64

The unexpected part comes when I run:

cmake -DCMAKE_TOOLCHAIN_FILE=../cmake-toolchains/Toolchain-Ubuntu-mingw32.cmake ../src/

and the output still starts with

===Processor architecture: x64
Project - projectx64

What can I do to fix this behaviour?

Solution

CMake will cache many of the values it finds during the setup process. Frustratingly it does this to the point that attempting to recreate the projects with a new option specified on the command line simply doesn't work.

To force the new variable to be picked up you can either

delete the entire build directory
delete the cache file containing the variables in <buildDir>/CMakeCache.txt
Edit the <buildDir>/CMakeCache.txt to find the value you want to update and just delete it, then re-configure the project specifying the new value.

Depending on how the variable actually gets used in your build, the third option has the possibility of triggering the least amount of re-building, BUT it's also very possible that if it impacts the preprocessor defines in any way you're going to end up doing a full rebuild anyway, so going with option one is often the simplest.