Check CMake Cache Variable in Toolchain File - cmake

Question

I'm having trouble setting a configuration variable via the command line. I can't determine it from the system, so I expect the user to specify:
cmake -DCMAKE_TOOLCHAIN_FILE=../android.toolchain -DANDROID_ABI:STRING="arm64" ..
Inside my android.toolchain, I have the following:
message(STATUS "Android ABI: ${ANDROID_ABI}")
if( "${ANDROID_ABI}" STREQUAL "" )
message(FATAL_ERROR "Please specifiy ABI at cmake call -DANDROID_ABI:STRING=armeabi or -DANDROID_ABI:STRING=arm64")
endif()
No matter what, it fails at this line EVEN THOUGH it prints out the correct arm64:
-- Android ABI: arm64
CMake Error at yaml-cpp/android.toolchain:45 (message):
Please specifiy ABI at cmake call -DANDROID_ABI:STRING=armeabi or -DANDROID_ABI:STRING=arm64
Could anyone direct me to what I'm doing wrong?
I think this has to do with:
-D adds a cache variable instead of a normal variable
This is in a toolchain file... it seems to ignore cache variables
Any thoughts or suggestions?

Answer 1

I don't pretend to fully understand what's going on behind the scenes, but here's a workaround that works for me:
# Problem: CMake runs toolchain files multiple times, but can't read cache variables on some runs.
# Workaround: On first run (in which cache variables are always accessible), set an intermediary environment variable.
if (FOO)
# Environment variables are always preserved.
set(ENV{_FOO} "${FOO}")
else ()
set(FOO "$ENV{_FOO}")
endif ()

Answer 2

CMake 3.6 introduces variable CMAKE_TRY_COMPILE_PLATFORM_VARIABLES which contains a list of variables, automatically passed from the main project to the project, created with try_compile.
A toolchain may add its variables to that list, so they could be extracted in a subproject:
message(STATUS "Android ABI: ${ANDROID_ABI}")
if( "${ANDROID_ABI}" STREQUAL "" )
message(FATAL_ERROR "Please specifiy ABI at cmake call -DANDROID_ABI:STRING=armeabi or -DANDROID_ABI:STRING=arm64")
endif()
# propagate the variable into "inner" subprojects.
list(APPEND CMAKE_TRY_COMPILE_PLATFORM_VARIABLES "ANDROID_ABI")
Caveats:
This approach affects only to source flow of try_compile command. It won't work when try_compile is used for create fully-fledged CMake project with signature
try_compile(<resultVar> <bindir> <srcdir> <projectName> ...)
(Approach with setting environment variable, as described in the #sorbet answer, works perfectly in this case.)
This approach won't work for a subproject, created with ExternalProject_Add.
(Approach with setting environment variable fails in that case too.)