#defineUsing #ifdef or equivalents is common when writing portable code. Which to use when can be quite tricky. This document describes the most common choices related to Android.
__BIONIC__If your code is specific to Android's C library, bionic, use __BIONIC__. This is typically a good choice when you use libc API that's only in bionic, such as the system property functions. Common alternatives on this dimension are __GLIBC__, __APPLE__, or _WIN32. Note that although bionic is most often seen on Android devices, it is possible to use bionic on the host too.
__ANDROID__If your code is specific to Android devices, use __ANDROID__. This isn't useful as you might think, and one of the other choices on this page is usually more appropriate. This is typically a good choice if you have code that's part of the OS and needs to behave differently on the host than on the device. Genuine cases are quite rare, and __BIONIC__ is often more specific (but remember that it is possible -- if unusual -- to use bionic on the host).
__ANDROID_API__If your code can be built targeting a variety of different OS versions, use __ANDROID_API__ to test which version you're building against. This is typically useful if you can use new NDK APIs when available, but don't require them if not.
One thing to note (if your code may also be built as part of the OS itself) is that for most of the year, the OS builds with this set to 10,000 rather than the obvious "next" API level such as 19. Once the API level has been decided, the value of __ANDROID_API__ drops to that number.
__linux__If your code requires a Linux kernel, use __linux__. This is typically a good choice when you use Linux-specific API, such as a Linux-specific system call or a file in /proc, but aren't restricted to just Android and would work equally well on a desktop Linux distro, say. Common alternatives on this dimension are __APPLE__ or _WIN32.
__ANDROID_NDK__If your code can be built either as part of an app or as part of the OS itself, use __ANDROID_NDK__ to differentiate between those two circumstances. This is typically a good choice when your code uses non-NDK API if it's built as part of the OS, but sticks to just the NDK APIs otherwise.
__NDK_MAJOR__, __NDK_MINOR__, __NDK_BETA__, __NDK_BUILD__, __NDK_CANARY__If your code can be built with a variety of different NDK versions, and needs to work around issues with some of them, use these macros to detect the versinon of the NDK you're being built with. Usually only __NDK_MAJOR__ will be necessary.
__arm__/__aarch64__, __i386__/__x86_64__, __riscvIf your code is specific to a particular processor architecture, use these macros to conditionally compile. Note that the ABI usually called arm64 uses the macro __aarch64__ and the ABI usually called x86 uses __i386__. Android only supports riscv64, so __riscv is a sufficient check for Android-only code. If you need to write code portable to other operating systems that do support riscv32, you'll also need to check whether __riscv_xlen is 32 or 64.
__ILP32__ and __LP64__If your code depends on "bitness" -- whether long and pointers are 32- or 64-bit -- use these macros to conditionally compile. Note the extra "I" in the 32-bit macro (since int, long, and pointers are all 32-bit on such systems, with long long being needed for a 64-bit type).