CMAKE_SYSTEM_PROCESSOR value in toolchain file not visible in top level CMakeLists.txt file

Question

I have cmake project which I've successfully set up and used for cross compiling with a toolchain file.

I have a separate variable I want to set in my top level cmake file based on the value of the CMAKE_SYSTEM_PROCESSOR variable that is set in the tool chain file. But when referenced in the top level cmake file, the value of CMAKE_SYSTEM_PROCESSOR does not have the value from the toolchain file.

I can reproduce the conditions with a simple cmake file and one line toolchain file.

If I have the following cmake file:

project(ToolchainTest C)
message("CMAKE_SYSTEM_PROCESSOR ${CMAKE_SYSTEM_PROCESSOR}")

and run

cmake .

I get

CMAKE_SYSTEM_PROCESSOR x86_64

as I expect.

With the following one line toolchain file "toolchain.cmake":

set(CMAKE_SYSTEM_PROCESSOR aarch64)

when I run

cmake . -DCMAKE_TOOLCHAIN_FILE=toolchain.cmake

I get exactly the same output, x86_64, and not aarch64 as I expected.

I looked over the following related posts, but as I'm just printing the value and not doing a string match (in the example) I don't think they apply.

Can I use CMAKE_SYSTEM_PROCESSOR, defined in a toolchain file, in CMakeLists?

CMake compare to empty string with STREQUAL failed

I'm using cmake version 3.13.4 on Ubuntu 19.04.

Answer 1

I had a similar issue. The answer is that when CMAKE_SYSTEM_NAME is not set, CMake overwrites CMAKE_SYSTEM_PROCESSOR (source code). This is arguably a bug but regardless is how CMake behaves.

The solution is to set both, even if you're compiling for the same system:

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR aarch64)