[FR] warn user if their cmake_minimum_required setting is incompatible with android

[andreya108]

Description

Trying to build project with new NDK we got the following for each component:

-- The C compiler identification is Clang 18.0.1
-- The CXX compiler identification is Clang 18.0.1
CMake Error at /home/andrey/android-sdk-linux/ndk/27.0.11902837/build/cmake/flags.cmake:50 (if):
  if given arguments:

    "hwaddress" "IN_LIST" "ANDROID_SANITIZE"

  Unknown arguments specified
Call Stack (most recent call first):
  /snap/cmake/1399/share/cmake-3.29/Modules/Platform/Android-Clang.cmake:23 (include)
  /snap/cmake/1399/share/cmake-3.29/Modules/Platform/Android-Clang-C.cmake:1 (include)
  /snap/cmake/1399/share/cmake-3.29/Modules/CMakeCInformation.cmake:48 (include)
  CMakeLists.txt:3 (project)


-- Configuring incomplete, errors occurred!

Adding

if(POLICY CMP0057)                                                                                                                                                          
  cmake_policy(SET CMP0057 NEW)
endif()

to the beginning of file ndk/27.0.11902837/build/cmake/flags.cmake solves the problem.

Why it is not set there (https://android.googlesource.com/platform/ndk/+/refs/heads/main/build/cmake/flags.cmake) by default?

P.S.: using $ANDROID_SDK/cmake/3.22.1 gives the same result

Affected versions

r27

Canary version

No response

Host OS

Linux

Host OS version

Ubuntu 20.04

Affected ABIs

arm64-v8a

Build system

CMake

Other build system

No response

minSdkVersion

26

Device API level

No response

[DanAlbert]

Why it is not set there (https://android.googlesource.com/platform/ndk/+/refs/heads/main/build/cmake/flags.cmake) by default?

Because we should not be making policy decisions on your behalf. Any policies we set would change the behavior of your CMakeLists.txt.

Presumably your project either doesn't have a cmake_minimum_required, or has a very old one that's not actually correct? Anything 3.3 or newer would enable that policy by default. That's not something toolchain files are supposed to do (that's my understanding from reading the docs and having spoken with the CMake owner a few times), so we don't set it in the toolchain file, but if you're building android you minimum cannot be lower than 3.6. The minimum may even be higher than that, but I know for sure that you must use at least 3.6.

https://cmake.org/cmake/help/latest/command/cmake_minimum_required.html says that we could probably add something like:

if (CMAKE_MINIMUM_REQUIRED LESS 3.6)
  message(FATAL_ERROR "Android requires cmake_minimum_required(3.6) or greater")
endif()

That's probably the best we can do. Probably a worthwhile improvement, but doesn't actually fix any bugs (and might break some builds if I'm wrong and there's actually a safe minimum between 3.3 and 3.6), so I'm not going to do that so late in the cycle in r27. We can do that for r28 though.

[DanAlbert]

(if any of that analysis is wrong, that's the best I can do without a repro case, and that's why we always ask for one, so upload one if you think I'm wrong)

[FeodorFitsner]

I had the same error while building libjpeg. Indeed, changing minimum CMake version in CMakeLists.txt worked (it's 2.8.12 in the source archive):

cmake_minimum_required(VERSION 3.3)

[DanAlbert]

Okay, so it's a project that wasn't written for android, and for non-android targets it doesn't require anything newer than 2.8.12, but CMake (for good reasons) doesn't allow per-target minimum versions. That makes sense. I don't think we can do any better than a warning for this case, but definitely the warning would be useful.

[dg0yt]

if (CMAKE_MINIMUM_REQUIRED LESS 3.6)
  message(FATAL_ERROR "Android requires cmake_minimum_required(3.6) or greater")
endif()

That's probably the best we can do. Probably a worthwhile improvement, but doesn't actually fix any bugs (and might break some builds if I'm wrong and there's actually a safe minimum between 3.3 and 3.6), so I'm not going to do that so late in the cycle in r27. We can do that for r28 though.

This topic comes up in vcpkg now.
The current state is: An error occurs in the toolchain cmake code, without good diagnostics for the uninitiated, for some ports.
What could have been done for better UX is: Check if cmake policy CMP0057 is set to NEW, and suggest setting CMAKE_POLICY_DEFAULT_CMP0057 to NEW as a config option, in an early FATAL_ERROR.
(Admittedly, not yet tested with CMAKE_POLICY_DEFAULT_CMP0057.)