Where is Android.mk supposed to be?

Question

In the documentation for Android NDK, the following statement is present:

The Android.mk file resides in a subdirectory of your project\'s jni/ directory

Answer 1

Expected project structure

/
 |
 +-- jni/
 |    |
 |    +-- Android.mk
 |    +-- [Application.mk] (optionally)
 |    +-- main.c
 |
 +-- AndroidManifest.xml

According to a short guide from here: https://developer.android.com/ndk/guides/concepts

8. Create an Android.mk file in the jni/ directory of your project
9. ... compile your native code using the ndk-build

   $ cd //
   $ /ndk-build
   

However, there is a way to define custom NDK project structures using ndk-build with arguments.

Plain structure

/
 |
 +-- Android.mk
 +-- [Application.mk]
 +-- main.c

$ cd 
$ ndk-build NDK_PROJECT_PATH=. APP_BUILD_SCRIPT=Android.mk

Android.mk vs Application.mk

why there is a need for multiple makefiles when Android.mk will contain the definitions for all modules anyway-that could just as well be placed in Application.mk.

1. Android.mk is mandatory and Application.mk is not.
2. Android.mk contains module definitions and Application.mk describes architecture, compiler options, and other "global" options.
3. Android.mk is orthogonal to Application.mk by design. If Android.mk contains 3 modules, and Application.mk 2 ABIs (e.g. arm and x86), then NDK will build (3 * 2) artifacts.
4. Variables from Application.mk appears in Android.mk, so you can use, for instance, APP_ABI in Android.mk to link architecture dependent libraries.
5. There are complex projects with a lot of makefiles and keeping common options in Application.mk is a clean way.
6. But still Application.mk is just a design decision, it could've been done differently.