[cmake-developers] Android variables

[Florent Castelli]

On 12/11/2016 06:53, Ruslan Baratov wrote:
> On 12-Nov-16 08:21, Florent Castelli wrote:
>> On 10/11/2016 16:05, Ruslan Baratov via cmake-developers wrote:
>>> Hi,
>>>
>>> I wonder if it's possible to introduce next variables describing Android
>>> tools:
>>> * C preprocessor. Similar to CMAKE_CXX_COMPILER the variable that will
>>> contain the path to preprocessor. Example:
>>>    * CMAKE_CXX_COMPILER =
>>> <ndk-root>/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-g++
>>>
>>>    * CMAKE_C_PREPROCESSOR =
>>> <ndk-root>/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-cpp
>>>
>>> * ANDROID_TOOLCHAIN_MACHINE_NAME. In fact I'm not sure what this
>>> variable mean but it's an important part of move from taka-no-me
>>> toolchain to CMake 3.7. I guess it's like
>>>    * CMAKE_CXX_COMPILER =
>>> <ndk-root>/toolchains/${ANDROID_TOOLCHAIN_MACHINE_NAME}-4.9/prebuilt/linux-x86_64/bin/${ANDROID_TOOLCHAIN_MACHINE_NAME}-g++
>>>
>>>
>>> Of course if I'm not missing something and they are not already
>>> somewhere :)
>>>
>>> Ruslo
>> How about ${CMAKE_C/CXX_COMPILER} -E to call the preprocessor?
> I have no idea and not planning to test it because the old code was
> using `cpp` preprocessor and I see no point of changing it.
Well, you can define your preprocessor to be "${CMAKE_C_COMPILER} -E" 
locally and it will work with GCC-like compilers.
That's a more robust way to call it as there is no CMAKE_C_PREPROCESSOR 
standard value in any CMake file.
Nobody should call "cpp" manually anymore, but use the compiler driver 
instead to do it for you.

That's why there's no standard CPP variable in CMake, it's not needed at 
all.
For the same reasons, you rarely have to call the linker directly, it's 
usually done through the compiler driver.

>> Calling it directly is kind of dangerous though since you will
>> probably be missing the proper language information and other options
>> that might affect the predefined preprocessor defines.
> I'm not calling it directly, it was used by `configure` script of 3rd party.
>
>> gcc has different binaries for different target archs and has some
>> defines baked in, but this won't apply to Clang that uses only one
>> binary and a target triple.
> For clang based android toolchain the CMAKE_CXX_ANDROID_TOOLCHAIN_PREFIX
> variable points to the directory with gcc, so it will find `cpp`
> preprocessor too. I don't know if it's good or bad :)

I think that's an implementation detail you shouldn't rely on. The 
compilers in the NDK are changing, GCC is deprecated and you shouldn't 
expect that.
Clang is mostly self contained and using very little from the GCC 
distribution (binutils), it will be gone at some point, so you should be 
ready for the future.

If you're updating CMake and changing the toolchain file, then I would 
expect that some work has to be done, including making changes to the 
invocation of a 3rd party script that is using custom variables from the 
old toolchain.


Overall, I consider that taka-no-me Android toolchain to be tech debt. 
It hasn't been updated for a long time, doesn't work with recent NDK 
(unless you resort to use a standalone toolchain), locks people with 
some custom variables making them believe they are standard and has tons 
of other small bugs and issues.
I understand its appeal for most people, but to me, it has caused more 
trouble than help. It could have been done so much better.


/Florent