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Error when attempting to configure cmake for cross compilation to android

I have been trying to build a c++ library for android using cmake cross compilation.
The library itself is a trivial test I made purely for testing the androind build process.
I have been using the cmake gui (v 3.25.0) on a windows machine.
I use a visual studio 2019 generator in cmake and specify a toolchain file.
Unfortunately, I seem to get the same error whatever I try. I tried searching for similar problems and trouble shooting, but have so far been unable to make any progress. If anyone with more experience could lend a hand, I would be very grateful!
I have attempted setting many different variables in the toolchain file in an attempt to resolve the problem, but the basic version of what I am working with is:
set(CMAKE_SYSTEM_NAME Android)
set(CMAKE_ANDROID_ARCH_ABI armeabi-v7a)
set(CMAKE_ANDROID_NDK C:/Microsoft/AndroidNDK64/android-ndk-r16b/)
The error I run into happens whenever I configure cmake. The outut I get from cmake looks something like this:
Android: Targeting API '27' with architecture 'arm', ABI 'armeabi-v7a', and processor 'armv7-a'
Android: Selected Clang toolchain 'arm-linux-androideabi-clang' with GCC toolchain 'arm-linux-androideabi-4.9'
The C compiler identification is Clang 5.0.300080
The CXX compiler identification is Clang 5.0.300080
Detecting C compiler ABI info
Detecting C compiler ABI info - failed
Check for working C compiler: C:/Microsoft/AndroidNDK64/android-ndk-r16b//toolchains/llvm/prebuilt/windows-x86_64/bin/clang.exe
Check for working C compiler: C:/Microsoft/AndroidNDK64/android-ndk-r16b//toolchains/llvm/prebuilt/windows-x86_64/bin/clang.exe - broken
CMake Error at C:/Program Files/CMake/share/cmake-3.25/Modules/CMakeTestCCompiler.cmake:70 (message):
The C compiler
"C:/Microsoft/AndroidNDK64/android-ndk-r16b//toolchains/llvm/prebuilt/windows-x86_64/bin/clang.exe"
is not able to compile a simple test program.
It fails with the following output:
Change Dir: C:/Users/username/Documents/Code Projects/Android Test/build/CMakeFiles/CMakeScratch/TryCompile-zqgcqm
Run Build Command(s):C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/MSBuild/Current/Bin/MSBuild.exe cmTC_b0f1d.vcxproj /p:Configuration=Debug /p:Platform=x64 /p:VisualStudioVersion=16.0 /v:m && Microsoft (R) Build Engine version 16.11.2+f32259642 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.
ANDROID_HOME=C:\\Microsoft\AndroidSDK\25
ANT_HOME=C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Apps\apache-ant-1.9.3
JAVA_HOME=C:\Program Files\Eclipse Foundation\jdk-8.0.302.8-hotspot
NDK_ROOT=C:\\Microsoft\AndroidNDK64\android-ndk-r16b
testCCompiler.c
clang.exe : warning : argument unused during compilation: '-mthumb' [-Wunused-command-line-argument] [C:\Users\username\Documents\Code Projects\Android Test\build\CMakeFiles\CMakeScratch\TryCompile-zqgcqm\cmTC_b0f1d.vcxproj]
clang.exe : warning : argument unused during compilation: '-mfpu=vfpv3-d16' [-Wunused-command-line-argument] [C:\Users\username\Documents\Code Projects\Android Test\build\CMakeFiles\CMakeScratch\TryCompile-zqgcqm\cmTC_b0f1d.vcxproj]
clang.exe : warning : argument unused during compilation: '-mfloat-abi=softfp' [-Wunused-command-line-argument] [C:\Users\username\Documents\Code Projects\Android Test\build\CMakeFiles\CMakeScratch\TryCompile-zqgcqm\cmTC_b0f1d.vcxproj]
CLANGCOMPILE : error : unknown target CPU 'armv7-a' [C:\Users\username\Documents\Code Projects\Android Test\build\CMakeFiles\CMakeScratch\TryCompile-zqgcqm\cmTC_b0f1d.vcxproj]
CMake will not be able to correctly generate this project.
Call Stack (most recent call first):
CMakeLists.txt:4 (project)
Configuring incomplete, errors occurred!
I initially used used the NDK version found in the android tools for visual studio found at: "C:/Microsoft/AndroidNDK64/android-ndk-r16b/"
as an NDK version.
I have also tried directly downloading the latest version of the NDK and pointing to that with CMAKE_ANDROID_NDK in the toolchain file, though this did not seem to make any change.
I have tried directly using the toolchain.cmake files found at "NDK/build/cmake/android.toolchain.cmake"
for both the NDK versions described above. This too gave the same error.
Any ideas what could be causing this or how to fix?

Ok, so I was misunderstanding a few things about cross-compiling to android.
First, if I want to be able to use the static library (e.g in android studio), I will need to generate and build a solution for more than one android ABI.
The ABI's to build for are: armeabi-v7a, arm64-v8a, x86_64, x86. I then need to check for each android ABI in the CMakeLists.txt file of Android studio (e.g. elseif(ANDROID_ABI STREQUAL arm64-v8a)) and swap out the location of the library to import to the build folder of that particular ABI.
As far as I am aware, it is unfortunately not possible to generate for all these ABI's as seperate platforms within the same visual studio solution. Given that, cmake must be run 4 seperate times to different build folders, each time targeting a different ABI and generator platform. To save the hassle of doing this manually, creating a batch (.bat) script to automate the process seems the best way to go. This will involve using cmake from the command line instead of the GUI version. The script can also be made to build the generated projects to save opening each one in visual studio and building there.
The batch script I have made to generate and build cross-compilation projects for android is as follows:
#echo OFF
set BUILD_DIR=build
set ANDROID_NDK=C:\Microsoft\AndroidNDK\android-ndk-r23c
set GENERATOR="Visual Studio 17 2022"
set CMAKE_GENERATOR=-G %GENERATOR%
set CMAKE_TOOLCHAIN_FILE=-DCMAKE_TOOLCHAIN_FILE=%ANDROID_NDK%\build\cmake\android.toolchain.cmake
set CMAKE_SYSTEM_NAME=-DCMAKE_SYSTEM_NAME=Android
set EXTRA_CMAKE_ARGS=-DBUILD_SHARED_LIBS=true -DANDROID_TOOLCHAIN=clang -DANDROID_STL=c++_static
CALL :build_android armeabi-v7a ARM 16
CALL :build_android arm64-v8a ARM64 21
CALL :build_android x86_64 x64 21
CALL :build_android x86 x86 16
EXIT /B %ERRORLEVEL%
:build_android
set ABI_VERSION=%~1
set GENERATOR_PLATFORM=%~2
set MINIMUM_API_LEVEL=%~3
set CMAKE_ANDROID_ARCH_ABI=-DANDROID_ABI=%ABI_VERSION%
set ABI_BUILD_DIR=%BUILD_DIR%\%ABI_VERSION%
set CMAKE_GENERATOR_PLATFORM=-A %GENERATOR_PLATFORM%
set CMAKE_BUILD_DIR=-B %ABI_BUILD_DIR%
set CMAKE_MIN_API=-DANDROID_PLATFORM=android-%MINIMUM_API_LEVEL%
set CMAKE_ARGS=%CMAKE_BUILD_DIR% %CMAKE_ANDROID_ARCH_ABI% %CMAKE_GENERATOR% %CMAKE_GENERATOR_PLATFORM% %CMAKE_SYSTEM_NAME% %CMAKE_TOOLCHAIN_FILE% %CMAKE_MIN_API%
echo building for android ABI: %ABI_VERSION%
echo cmake arguments = %CMAKE_ARGS%
echo:
cmake %CMAKE_ARGS% %EXTRA_CMAKE_ARGS%
echo:
cmake --build %ABI_BUILD_DIR% --target ALL_BUILD
echo:
echo:
EXIT /B 0
This worked for my simple test library, but I guess depending on the project the arguments to the cmake commands may need to be tweaked.

Unable to get qmake to cross compile correctly

I am new to this build system, but I've spent dozens of hours trying to get qmake + clang to produce a working android binary. I would appreciate any help or advice. Here is as simple an example as I can come up with:
First, here is a working build command. It gives me a shared object that I can call from android (with Unity, using C# native interops):
C:\Users\deltav\AppData\Local\Android\Sdk2\ndk\21.3.6528147\toolchains\llvm\prebuilt\windows-x86_64\bin\clang.exe ^
-target aarch64-none-linux-android21 -shared -v ^
-DANDROID_ABI=arm64-v8a -DANDROID_ARM_MODE=arm -DANDROID ^
M:\simpleExample\simpleExample.c ^
-o M:\simpleExample\libsimpleExample.so
Here is the verbose output of running that command: https://pastebin.com/18HcnDYh
On the other hand, here is the simpleExample.pro file that is NOT giving me a good binary: https://pastebin.com/wxrx6Myc
Here is the qmake output for building in release mode: https://pastebin.com/KWavwrb7
As you will know, qmake uses a kit to execute the project. My kit: https://imgur.com/a/Ehtmgin
The specific issue that has pushed me down this debugging path is from C# trying to load my shared object like this:
[DllImport("simpleExample")]
public static extern float getSpeedY();
The error that I get is this:
"DllNotFoundException: Unable to load DLL 'simpleExample': The
specified module could not be found."
Again, this error is not an issue if I just build from the CLI without qmake. I do not beleive this is a unity or C# error, as I worked the problem from that side first.
What I have tried so far:
I have used multiple ELF & DWARF tools to compare the ABIs of the generated binaries, which all told me the binaries were identical.
I tweaked and shaved down the clang calls that qmake generated until I got a vaild build command. Unfortunately I can't just replace qmake with the complile command because there are other projects that I need to apply this fix to.
QMake version 3.1
Ironically, CMake basically works out of the box.

Answer: Looking at the clang linking call, you can see QMake 3.1 was adding "libc++shared" as a dependency. I'm not sure why, as this was a C project.
The equivalent CMake project did not do this.
Packaging libc++shared with my binary fixed the issue.

go with networking on Android

I've been trying to get a Go application running on Android 10 and hit a snag. I'm pretty sure I now know why I'm hitting this snag, but not sure how to fix it. First, simple things work with cross compilation. For example,
package main
import "fmt"
func main() {
fmt.Println("Hello, Android!")
}
will run just fine compiled with GOARM=arm64 GOOS=linux go build
But now if I add some networking code:
package main
import (
"fmt"
"net/http"
"io/ioutil"
)
func main() {
fmt.Println("Hello, Android!")
client := &http.Client{}
resp, err := client.Get("https://www.google.com")
if err != nil {
fmt.Println(err)
return
}
bodyBytes, err := ioutil.ReadAll(resp.Body)
if err != nil {
fmt.Println(err)
return
}
fmt.Printf("%s", bodyBytes)
}
And compile the same way it will run, but I'll get an error like:
Get "https://www.google.com": dial tcp: lookup www.google.com on [::1]:53: read udp [::1]:47618->[::1]:53: read: connection refused
Initially I thought this was some networking config on my Android device, but after lots of messing around I'm pretty sure it has to do with the fact that Android networking is "different". Maybe this is a bionic vs libc thing (?) but at least one other person seems to have hit the same snag and solved by building using the toolchain that comes with the NDK.
Does anyone know how to actually get this to work though?
Initially I was hoping it was as easy as GOOS=android GOARCH=arm64 go build but that fails with:
# command-line-arguments
/usr/lib/golang/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
/usr/bin/ld: /tmp/go-link-384615804/go.o: Relocations in generic ELF (EM: 183)
<bunch of these>
/usr/bin/ld: /tmp/go-link-384615804/go.o: error adding symbols: file in wrong format
collect2: error: ld returned 1 exit status
Some Googling turned up https://github.com/golang/go/issues/30216 - which says you need to explicitly enable CGO. But GOARCH=arm64 GOOS=android CGO_ENABLED=1 go build still fails, this time with:
# runtime/cgo
gcc_android.c:6:10: fatal error: android/log.h: No such file or directory
6 | #include <android/log.h>
| ^~~~~~~~~~~~~~~
compilation terminated.
I messed around with a lot of other combinations and the closest I got was pointing Go to compilers in my NDK installation. Basically something like GOARCH=arm64 GOOS=android CC=$NDKROOT/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-gcc CGO_ENABLED=1 CGO_CFLAGS="--sysroot $NDKROOT/sysroot -I$NDKROOT/sysroot/usr/include/arm-linux-androideabi/" go build. But this failed with:
# runtime/cgo
In file included from _cgo_export.c:4:0:
cgo-gcc-export-header-prolog:25:14: error: size of array '_check_for_64_bit_pointer_matching_GoInt' is negative
There's something broken in the way I'm pointing to the compiler - for example I won't find any header files (stdlib.h) without specifying sysroot, and even then it won't find the architecture specific includes (asm/types.h) without specifying the additional include path. So the error is probably a mismatch between architecture and compiler I'm just not sure where or how to fix it.
If you've read all this, thanks! Some additional info:
go version: 1.14.13 linux/amd64
NDK version: r16b
I've also looked at things like gomobile but that seems more geared towards building Android applications (like apks) although maybe I'm missing something here.

Figured it out! Well, got my specific example working but I'm not really sure why this works. The answer came basically from https://github.com/golang/go/issues/20755 and you have to:
Build a standalone version of the NDK (the comment in the Python script that does this says it packages it in a more convenient way for other tools to use)
Rebuild Go's standard library using the cross compiler in your NDK
Build your application using the NDK cross compiler.
So for my example:
$ $(NDKROOT)/build/tools/make_standalone_toolchain.py --arch arm64 --api 24 --install-dir $(HOME)/ndk_standalone
$ CC=$(HOME)/ndk_standalone/bin/clang CXX=$(HOME)/ndk_standalone/bin/clang GOOS=android GOARCH=arm64 go install std
$ CC=$(HOME)/ndk_standalone/bin/clang CXX=$(HOME)/ndk_standalone/bin/clang GOOS=android GOARCH=arm64 CGO_ENABLED=1 go build
And networking works!
I'm not really sure why (or if) clang makes a difference as opposed to invoking the compiler directly. Maybe I'll play around with this more. Also, I had to still enabled CGO explicitly when compiling my application (but not for the go std library) otherwise linking failed.

Same android cmake git project compiles well on one Ubuntu machine but does not compile on another Ubuntu machine even the entire project is the same

I'm developing an android cmake project that compiles succesfully on my 32-bit Ubuntu 12.04 machine at work (I will call it WorkMachine). When I want to compile it at home, I installed a 32-bit Ubuntu 12.04 virtual machine (I will call it HomeMachine), I git cloned the project on it, and when I start the compilation, it will block with an error.
CMAKE_CFLAGS are:
CMAKE_CXXFLAGS are:
-- Configuring done
-- Generating done
-- Build files have been written to: /home/raa/Dropbox/Demo/android_build
[ 0%] Built target bitstream
[ 0%] Built target distrat
[ 1%] Built target fisher
[ 1%] Built target gtest
[ 7%] Built target jpeg-8c
[ 7%] Built target map
[ 8%] Built target resampler
[ 9%] Built target timer
[ 92%] Built target fftw3f-3
[ 96%] Built target vlfeat
[ 96%] Built target extract_shared
[ 97%] Built target shared
[ 97%] Built target cssc_train
Linking CXX executable ../../bin/extract
/home/raa/Dropbox/Demo/libraries/fftw-3.3.3/api/configure.c:28: error: undefined reference to 'fftwf_dft_conf_standard'
collect2: error: ld returned 1 exit status
make[2]: *** [../bin/extract] Error 1
make[1]: *** [src/CMakeFiles/extract.dir/all] Error 2
make: *** [all] Error 2
Here I am reporting gcc version (the same on both computers)
gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-linux-gnu/4.6/lto-wrapper
Target: i686-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.3-1ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --enable-objc-gc --enable-targets=all --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=i686-linux-gnu --host=i686-linux-gnu --target=i686-linux-gnu
Thread model: posix
gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5)
But how is that possible?
It seems the problem is not due to some cmake cache or similar chaches, because if I git clone the project on another directory on the WorkMachine, the code compiles well too.
What can I check in order to resolve the problem? If you need additional details, please ask me.
Thank you for your time.
Riccardo

the solution to Riccardos problem is to change the commandline parameter of ar from r to q (see man page for difference, basically q does not check for duplicats)
do this via
SET(CMAKE_C_ARCHIVE_APPEND "<CMAKE_AR> q <TARGET> <LINK_FLAGS> <OBJECTS>")
you probably want to check if CMAKE_AR is "ar" (might be different on other systems)

whoa! Solved! It was really a weird behaviour from cmake: I had three files conf.c that must be merged into a static library. When cmake run the AR program to create the library, it executed a command that was like:
ar cr dir1/source1.c dir1/source2.c ... dft/conf.c rdft/conf.c
ar r reodft/conf.c dir2/source3.c
so, maybe due to the huge number of object files that were to be linked, cmake splitted ar commands in two command, BUT second command overwrited one of the two file conf.c that were added with the first command, so symbols defined with the first ar command were lost! This is clearly a bug in ar because even if the source file is the same, maybe the content may vary, thus symbols are not to be deleted. Thanks to all! Your suggestions brought me to the right solution!

Error with building Android NDK Project

I wand build project in windows, I get same Errors:
bash C:\linux\android-ndk1\ndk-build V=1
cygwin warning:
MS-DOS style path detected: C:\dev\android\workspace4_android\FFWall
Preferred POSIX equivalent is: /cygdrive/c/dev/android/workspace4_android/FFWall
CYGWIN environment variable option "nodosfilewarning" turns off this warning.
Consult the user's guide for more details about POSIX paths:
http://cygwin.com/cygwin-ug-net/using.html#using-pathnames
Android NDK: ERROR:jni/Android.mk:bambuser-libavcore: LOCAL_SRC_FILES points to a missing file
/cygdrive/c/linux/android-ndk1/build/core/prebuilt-library.mk:43: *** Android NDK: Aborting . Stop.
Android NDK: Check that jni/ffmpeg-android/build/ffmpeg/armeabi/lib/libavcore.so exists or that its path is correct
Another way building:
C:\art\android-ndk\ndk-build V=1
Cannot run program "C:\art\android-ndk\ndk-build": Launching failed
Error: Program "C:/art/android-ndk/ndk-build" is not found in PATH
PATH=[C:\Art\android-ndk;C:/Program Files/Java/jre7/bin/client;C:/Program Files/Java/jre7/bin;C:/Program Files/Java/jre7/lib/i386;C:\Program Files\PC Connectivity Solution\;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;C:\Program Files\MATLAB\R2011a\runtime\win32;C:\Program Files\MATLAB\R2011a\bin;C:\Program Files\MATLAB\R2007b\bin;C:\Program Files\MATLAB\R2007b\bin\win3;C:\dev\android\android-sdk-windows\tools;C:\cygwin\bin;C:/art/android-ndk/;C:\linux\android-ndk-r7b;C:\dev\android\eclipse]
Why this happened?

From: http://developer.android.com/sdk/ndk/overview.html#reqs
Required development tools
For all development platforms, GNU Make 3.81 or later is required. Earlier versions of GNU Make might work but have not been tested.
A recent version of awk (either GNU Awk or Nawk) is also required.
For Windows, Cygwin 1.7 or higher is required. The NDK will not work with Cygwin 1.5 installations.
Meaning, that you will have to install Cygwin. After that you can call the ndk-build like:
$ProjectPath> /cygdrive/c/myNdkPath/ndk-build
Please also check the "Getting Started" part of:
http://developer.android.com/sdk/ndk/index.html