cross compile for android on linux with __ANDROID_API__ predefinition fails my program

Issue

I setup conan for cross building android app on my linux, i have my_profile below for conan for this cross building, which that I run conan create . user/testing -pr=my_profile

include(default)
target_host=aarch64-linux-android
android_ndk=$HOME/android-ndk-r21
api_level=21
[settings]
arch=armv8
build_type=Release
compiler=clang
compiler.libcxx=libc++
compiler.version=9
os=Android
os.api_level=$api_level
[build_requires]
[options]
[env]
PATH=[$android_ndk/toolchains/llvm/prebuilt/linux-x86_64/bin]
CHOST=$target_host
AR=$target_host-ar
AS=$target_host-as
LD=$target_host-ld
STRIP=$target_host-strip
RANLIB=$target_host-ranlib
CC=$target_host$api_level-clang
CXX=$target_host$api_level-clang++
CONAN_MAKE_PROGRAM=$android_ndk/prebuilt/linux-x86_64/bin/make
CONAN_CMAKE_TOOLCHAIN_FILE=$android_ndk/build/cmake/android.toolchain.cmake

and I made a very simple file:

#include <fcntl.h>
int raw_fallocate(int fd, off_t length) {
  if (fallocate(fd, 0, 0, length) == 0)    return 0;
  return -1;
}

I found that in the fcntl.h, it only defines fallocate when __ANDROID_API >=21 with #ifdef so in my CMakeLists.txt, I need to put target_compile_definitions(hello PRIVATE __ANDROID_API__=21) to make it compile, otherwise, the compiler will complain it cannot find definition of fallocate.

That all make sense. However, when I put this preprocessor definition, I still got a warning message saying:

In file included from <built-in>:413:
<command line>:1:9: warning: '__ANDROID_API__' macro redefined [-Wmacro-redefined]
#define __ANDROID_API__ 21
        ^
<built-in>:405:9: note: previous definition is here
#define __ANDROID_API__ 16
        ^
1 warning generated.

What I don't understand is I could not find this built-in thing..., i searched my whole android_ndk folder, and could not find where is this #define __ANDROID_API__ 16

Also I only have android_ndk v21 installed, I have no idea where this version of 16 came from.

Any idea?

Solution

This #define __ANDROID_API__ 16 comes from the NDK itself (this is the lowest supported API for android-ndk-r21. To set it to 21, you must pass ANDROID_PLATFORM parameter to CMake. Update: this is actually wrong. For ABI arm64-v8a the minimal API is 21. So, the problem is that arch=armv8 didn't work.

According to the conan instructions, set os.api_level=21 should have worked. But with this approach, you should not supply the CONAN_CMAKE_TOOLCHAIN_FILE. Update: this does not work because conan is not compatible with NDK r21.

I assume that when you do supply CONAN_CMAKE_TOOLCHAIN_FILE, all the settings, like CC= and AR= become irrelevant. Same for os.api_level and arch. But if you replace cmake with a script that calls the original cmake binary and sets the necessarycommand-line parameters, including -DANDROID_PLATFORM=android-21, you should be set. Simply add to my_profile: CONAN_CMAKE_PROGRAM=cmake-wrapper

This approach is used in https://github.com/bincrafters/conan-android_ndk_installer package.

Answered By - Alex Cohn