# When to use which `#define` Using `#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__` If 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__`. ## `__LP32__` and `__LP64__` If your code depends on "bitness" -- whether `long` and pointers are 32- or 64-bit -- use these macros to conditionally compile.