[SOLVED] How to use CMake cached variables inside subprocess called by custom target?

Issue

My project contains a custom target which generates some output via .cmake script. It looks like this:

add_custom_target(TargetName
    COMMAND ${CMAKE_COMMAND} -P ${CMAKE_SOURCE_DIR}/cmake/script.cmake
    BYPRODUCTS ${CMAKE_CURRENT_BINARY_DIR}/generated/output
    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
    VERBATIM
)

But now I want to set come cache variables inside the script. I tried doing like that:

message("MY_CACHE_VARIABLE = ${MY_CACHE_VARIABLE}")
set(MY_CACHE_VARIABLE "VALUE" CACHE INTERNAL "")

And I faced with the problem that cache variables are not saved. It always prints me empty output:

MY_CACHE_VARIABLE =

I already tried setting working directory as CMAKE_BINARY_DIR, or passing CMAKE_BINARY_DIR of the last argument of cmake command, or passing -B ${CMAKE_BINARY_DIR} or -C ${CMAKE_BINARY_DIR}/CMakeCache.txt as arguments and etc. None of these worked.
So is there any way to reuse existing cache inside CMake subprocess or I just should write my own cache inside the script?

Solution

You have to distinguish between running CMake to generate build files (for Make, Ninja, etc.) and running CMake in script mode:

Script mode simply runs the commands in the given CMake Language source file and does not generate a build system. It does not allow CMake commands that define build targets or actions.
-- cmake-language(7)

No configure or generate step is performed and the cache is not modified.
-- cmake(1)

So in script mode (-P), CMake is not aware of the cache or any variable/target/etc. defined in your regular CMakeLists.txt files. It is more similar to executing a bash/shell script than to processing a "usual" CMakeLists.txt.

But don't worry, there is still a solution to your problem. You can simply pass your arguments as -D options to your script:

add_custom_target(TargetName
  COMMAND ${CMAKE_COMMAND}
    -DMY_VAR="..."
    -DANOTHER_VAR="..."
    -P ${CMAKE_SOURCE_DIR}/cmake/script.cmake
  ...
)

Note however:

If variables are defined using -D, this must be done before the -P argument.
-- cmake(1)

Answered By - peter_w