Is t possible/feasible to use android's NDK toolchain to cross-compile for Pi?

Question

That's it, really. I have an x86_64 host that already cross-compiles a C application for android's 4 supported architectures, could I use the existing toolchain to build armeabi-v7a for a Pi (running Linux like Raspbian or whatever)?

Edit 1

I extracted the ext4 filesystem from Raspberry OS and monted it onto /opt/rpi/sysroot.img.

I then tried

~/Android/Sdk/ndk/22.0.7026061/toolchains/llvm/prebuilt/linux-x86_64/bin/clang \
-target arm-v7a-none-linux-eabi \
-mfloat-abi=hard \
-nostdinc \
-std=c99 \
-isysroot /opt/rpi/sysroot.img \
-c helloRaspi.c

but hit a sort of lib hell. The compiler complains it can't find stdio.h, then stddef.h, etc.

I was under the assumption -isysroot would change the root for the compiler search paths, but i noticed that's not quite what it does, as providing e.g. -I/opt/rpi/sysroot.img/usr/include/c++/8/tr1 will find stdarg.h but -I/usr/include/c++/8/tr1 won't. And what's the difference to --sysroot?

My current compilation line is at

~/Android/Sdk/ndk/22.0.7026061/toolchains/llvm/prebuilt/linux-x86_64/bin/clang 
-target arm-v7a-none-linux-eabi 
-mfloat-abi=hard 
-nostdinc 
-std=c99 
-isysroot /opt/rpi/sysroot.img 
-I/opt/rpi/sysroot.img/usr/include                          stdio.h
-I/opt/rpi/sysroot.img/usr/include/linux                    stddef.h
-I/opt/rpi/sysroot.img/usr/include/arm-linux-gnueabihf      bits/libc-header-start.h
-I/opt/rpi/sysroot.img/usr/include/c++/8                    tr1/cstdarg
-I/opt/rpi/sysroot.img/usr/include/c++/8/tr1                stdarg.h
-I/opt/rpi/sysroot.img/usr/include/arm-linux-gnueabihf/c++/8  bits/c++config.h
-c helloRaspi.c

(the file names were added of course), but now am getting errors in the included files themselves, such as

In file included from /opt/rpi/sysroot.img/usr/include/stdio.h:43:
/opt/rpi/sysroot.img/usr/include/arm-linux-gnueabihf/bits/types/struct_FILE.h:95:3: error: unknown type name 'size_t'
  size_t __pad5;

which makes me think i got the include path all wrong from the start.

For completeness this is my helloWorld:

#include <stdio.h>

int main(int argc, char* argv[]) {
    printf("hello world\n");
    return 0;
}

Answer 1

I couldn't grok -isysroot and --sysroot, maybe clang handles them differently than gcc (it's not exactly verbose regarding those two options), but I got it to compile as such:

export CLANG=/home/user/Android/Sdk/ndk/22.0.7026061/toolchains/llvm/prebuilt/linux-x86_64/bin/clang
export SROOT=/opt/rpi/sysroot.img
${CLANG} -v -target arm-hardfloat-linux-gnueabi -mfloat-abi=hard -std=c99 \
-nostdinc \
-I${SROOT}/usr/include \
-I${SROOT}/usr/include/arm-linux-gnueabihf \
-I${SROOT}/usr/lib/gcc/arm-linux-gnueabihf/8/include \
helloRaspi.c -o helloRaspi

Ran successfully on a Raspberry Pi 4 B.~

...whether this is feasible for a larger program remains to be seen.