1# When to use which `#define` 2 3Using `#ifdef` or equivalents is common when writing portable code. Which to use 4when can be quite tricky. This document describes the most common choices 5related to Android. 6 7## `__BIONIC__` 8 9If your code is specific to Android's C library, bionic, use `__BIONIC__`. This 10is typically a good choice when you use libc API that's only in bionic, such as 11the system property functions. Common alternatives on this dimension are 12`__GLIBC__`, `__APPLE__`, or `_WIN32`. Note that although bionic is most often 13seen on Android devices, it is possible to use bionic on the host too. 14 15## `__ANDROID__` 16 17If your code is specific to Android devices, use `__ANDROID__`. This isn't 18useful as you might think, and one of the other choices on this page is usually 19more appropriate. This is typically a good choice if you have code that's part 20of the OS and needs to behave differently on the host than on the device. 21Genuine cases are quite rare, and `__BIONIC__` is often more specific (but 22remember that it is possible -- if unusual -- to use bionic on the host). 23 24## `__ANDROID_API__` 25 26If your code can be built targeting a variety of different OS versions, use 27`__ANDROID_API__` to test which version you're building against. This is 28typically useful if you can use new NDK APIs when available, but don't require 29them if not. 30 31One thing to note (if your code may also be built as part of the OS itself) is 32that for most of the year, the OS builds with this set to 10,000 rather than the 33obvious "next" API level such as 19. Once the API level has been decided, the 34value of `__ANDROID_API__` drops to that number. 35 36## `__linux__` 37 38If your code requires a Linux kernel, use `__linux__`. This is typically a good 39choice when you use Linux-specific API, such as a Linux-specific system call or 40a file in `/proc`, but aren't restricted to just Android and would work equally 41well on a desktop Linux distro, say. Common alternatives on this dimension 42are `__APPLE__` or `_WIN32`. 43 44## `__ANDROID_NDK__` 45 46If your code can be built either as part of an app _or_ as part of the OS 47itself, use `__ANDROID_NDK__` to differentiate between those two circumstances. 48This is typically a good choice when your code uses non-NDK API if it's built as 49part of the OS, but sticks to just the NDK APIs otherwise. 50 51## `__NDK_MAJOR__`, `__NDK_MINOR__`, `__NDK_BETA__`, `__NDK_BUILD__`, `__NDK_CANARY__` 52 53If your code can be built with a variety of different NDK versions, and needs to 54work around issues with some of them, use these macros to detect the versinon of 55the NDK you're being built with. Usually only `__NDK_MAJOR__` will be necessary. 56 57## `__arm__`/`__aarch64__`, `__i386__`/`__x86_64__`, `__riscv` 58 59If your code is specific to a particular processor architecture, use 60these macros to conditionally compile. Note that the ABI usually called 61`arm64` uses the macro `__aarch64__` and the ABI usually called `x86` uses 62`__i386__`. Android only supports riscv64, so `__riscv` is a sufficient 63check for Android-only code. If you need to write code portable to other 64operating systems that do support riscv32, you'll also need to check 65whether `__riscv_xlen` is 32 or 64. 66 67## `__ILP32__` and `__LP64__` 68 69If your code depends on "bitness" -- whether `long` and pointers are 32- 70or 64-bit -- use these macros to conditionally compile. Note the extra 71"I" in the 32-bit macro (since `int`, `long`, and pointers are all 32-bit 72on such systems, with `long long` being needed for a 64-bit type). 73