Help: Error Trying to Compile the c++ library in Android using CMake.

[perojas3]

Hello.
I'm trying to compile or-tools in Android, but there is always an error in the building process, and don't know what to do to continue.
This is the error:

ninja: warning: phony target '_deps/or-tools-build/host_tools' names itself as an input; ignoring [-w phonycycle=warn]
ninja: error: build.ninja:748: multiple rules generate _deps/or-tools-build/host_tools [-w dupbuild=err]
	at com.android.build.gradle.internal.cxx.process.ExecuteProcessKt.execute(ExecuteProcess.kt:274)
	at com.android.build.gradle.internal.cxx.process.ExecuteProcessKt$executeProcess$1.invoke(ExecuteProcess.kt:106)
	at com.android.build.gradle.internal.cxx.process.ExecuteProcessKt$executeProcess$1.invoke(ExecuteProcess.kt:104)
	at com.android.build.gradle.internal.cxx.timing.TimingEnvironmentKt.time(TimingEnvironment.kt:32)
	at com.android.build.gradle.internal.cxx.process.ExecuteProcessKt.executeProcess(ExecuteProcess.kt:104)
	at com.android.build.gradle.internal.cxx.process.ExecuteProcessKt.executeProcess$default(ExecuteProcess.kt:84)
	at com.android.build.gradle.internal.cxx.build.CxxRegularBuilder.executeProcessBatch(CxxRegularBuilder.kt:331)
	at com.android.build.gradle.internal.cxx.build.CxxRegularBuilder.build(CxxRegularBuilder.kt:128)
	at com.android.build.gradle.tasks.ExternalNativeBuildTask$doTaskAction$$inlined$recordTaskAction$1.invoke(BaseTask.kt:70)
	at com.android.build.gradle.internal.tasks.Blocks.recordSpan(Blocks.java:51)
	at com.android.build.gradle.tasks.ExternalNativeBuildTask.doTaskAction(ExternalNativeBuildTask.kt:145)
	at com.android.build.gradle.internal.tasks.UnsafeOutputsTask$taskAction$$inlined$recordTaskAction$1.invoke(BaseTask.kt:65)
	at com.android.build.gradle.internal.tasks.Blocks.recordSpan(Blocks.java:51)
	at com.android.build.gradle.internal.tasks.UnsafeOutputsTask.taskAction(UnsafeOutputsTask.kt:63)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at org.gradle.internal.reflect.JavaMethod.invoke(JavaMethod.java:125)
	... 113 more

I'm building from source using CMake, building all the dependencies set(BUILD_DEPS ON) to make sure to have the necessary ones to function. Followed the instructions here: https://github.com/google/or-tools/blob/stable/cmake/README.md
The error does not seem to come from the dependencies, though. But from host_tools, I assume either of these files:

1. https://github.com/google/or-tools/blob/stable/cmake/host.cmake
2. https://github.com/google/or-tools/blob/stable/cmake/host.CMakeLists.txt

But I don't know how to continue debugging from here.

The project is multi-platform, I have tried compiling it in Linux (Manjaro) and worked. The issue only occurs in Android. Right now, I am compiling using the C++17 Android Studio Pipeline. I'm developing using a Linux machine. And testing the code using the Android emulator, tried various images offered by Android Studio (api 27 x86, api 30 x86 and api 34 x86_64), and the problem is the same in all the cases. I'm unable to test in a real device right now.

Also, added compile setting set(__PORTABLE_PLATFORM__) because I found that it was recommended for compiling OR Tools in Android. Tested with and without that flag, and the result is the same.

I'm using the version 9.7 of or-tools, calling it from CMake like this:

FetchContent_Declare(
        or-tools
        GIT_REPOSITORY https://github.com/google/or-tools.git
        GIT_TAG        v9.7
)

Also tried to manually download the repo git clone and add the subdirectory in CMake add_subdirectory(or-tools). Both cases gave the same error.

I prepared a repository with the minimum code necessary to replicate the error. It is basically the default template for an Android Project with of C++, only that I added the lines of code to link the or-tools library. Here: https://github.com/perojas3/Example-on-Trying-to-compile-OR-Tools-in-Android

I know that you do not officially support Android, I'm just asking for pointers on how to solve this problem.
I have read in some issues that the development team build for Android internally. (These: #537 and #1130)
So assume there is a chance you can help me with this.
I hope you can.
Thanks in advance

[cengizkandemir]

One binary (protoc) under host_tools, which is required for cross-compilation, is missing. I am not sure if the binary is intentionally left out (proprietary?). Can maintainers comment whether Android cross-compilation is still not supported?

[Mizux]

protoc is part of the protobuf project when using a CMake crosstool we built it

Start digging here:
https://github.com/google/or-tools/blob/main/cmake/host.cmake
most setup
https://github.com/google/or-tools/blob/main/cmake/host.CMakeLists.txt

Do you know if android provide this variable CMAKE_CROSSCOMPILING (when using the android NDK and CMake cross-toolchain)

or-tools/cmake/host.cmake

Lines 14 to 17 in 546f612

	if(NOT CMAKE_CROSSCOMPILING)
	set(PROTOC_PRG protobuf::protoc)
	return()
	endif()

[perojas3]

Alright. Apologies for taking so long, had problems with getting a machine to try this.
I tried adding that tag in cmake set(CMAKE_CROSSCOMPILING).
And now throws a different error, this time in build time rather than in cmake.
So I would rather say that no, android does not provide the CMAKE_CROSSCOMPILING variable.

I'm unable to understand the error, I will leave the build logs attached as a txt file:
error.txt

Don't know what to do next, this happens while performing the task of [489/1689] Building CXX object _deps/eigen3-build/lapack/CMakeFiles/eigen_lapack.dir/complex_double.cpp.o. Any ideas?

Also, I made a branch here with the code to replicate this mistake (is just one more line of code, but just in case): https://github.com/perojas3/Example-on-Trying-to-compile-OR-Tools-in-Android/tree/trying_adding_the_cmake_crosscompiling_variable