author Sam Lantinga <>
Tue, 18 Sep 2012 01:45:15 -0700
changeset 6436 29a35c72905a
parent 6387 58f0fb54bf88
child 6631 47ab7ba21530
permissions -rw-r--r--
Fixed retina display input scaling

Simple DirectMedia Layer for Android


Android SDK

Android NDK r4 or later

 How the port works

- Android applications are Java-based, optionally with parts written in C
- As SDL apps are C-based, we use a small Java shim that uses JNI to talk to 
the SDL library
- This means that your application C code must be placed inside an android 
Java project, along with some C support code that communicates with Java
- This eventually produces a standard Android .apk package

The Android Java code implements an "activity" and can be found in:

The Java code loads your game code, the SDL shared library, and
dispatches to native functions implemented in the SDL library:

Your project must include some glue code that starts your main() routine:

 Building an app

1. Copy the android-project directory wherever you want to keep your projects and rename it to the name of your project.
2. Move this SDL directory into the <project>/jni directory and then copy
SDL_config_android.h to SDL_config.h inside the include folder
3. Place your application source files in the <project>/jni/src directory
4. Edit <project>/jni/src/ to include your source files
5. Run 'ndk-build' (a script provided by the NDK). This compiles the C source

If you want to use the Eclipse IDE, skip to the Eclipse section below.

6. Edit <project>/ to point to the Android SDK directory
7. Run 'ant debug' in android/project. This compiles the .java and eventually 
creates a .apk with the native code embedded
8. 'ant install' will push the apk to the device or emulator (if connected)

Here's an explanation of the files in the Android project, so you can customize them:

	AndroidManifest.xml	- package manifest, do not modify	- empty
	build.xml		- build description file, used by ant	- holds the ABI for the application, currently android-5 which corresponds to the Android 2.0 system image	- holds the SDK path, you should change this to the path to your SDK
	jni/			- directory holding native code
	jni/		- Android makefile that includes all subdirectories
	jni/SDL/		- directory holding the SDL library files
	jni/SDL/	- Android makefile for creating the SDL shared library
	jni/src/		- directory holding your C/C++ source
	jni/src/	- Android makefile that you should customize to include your source code and any library references
	res/			- directory holding resources for your application
	res/drawable-*		- directories holding icons for different phone hardware
	res/layout/main.xml	- place holder for the main screen layout, overridden by the SDL video output
	res/values/strings.xml	- strings used in your application, including the application name shown on the phone.
	src/org/libsdl/app/	- the Java class handling the initialization and binding to SDL.  Be very careful changing this, as the SDL library relies on this implementation.

 Pause / Resume behaviour

If SDL is compiled with SDL_ANDROID_BLOCK_ON_PAUSE defined, the event loop will
block itself when the app is paused (ie, when the user returns to the main
Android dashboard). Blocking is better in terms of battery use, and it allows your
app to spring back to life instantaneously after resume (versus polling for
a resume message).
Upon resume, SDL will attempt to restore the GL context automatically.
In modern devices (Android 3.0 and up) this will most likely succeed and your
app can continue to operate as it was.
However, there's a chance (on older hardware, or on systems under heavy load),
where the GL context can not be restored. In that case you have to listen for
a specific message, (which is not yet implemented!) and restore your textures
manually or quit the app (which is actually the kind of behaviour you'll see
under iOS, if the OS can not restore your GL context it will just kill your app)

 Threads and the JAVA VM

For a quick tour on how Linux native threads interoperate with the JAVA VM, take
a look here:
If you want to use threads in your SDL app, it's strongly recommended that you
do so by creating them using SDL functions. This way, the required attach/detach
handling is managed by SDL automagically. If you have threads created by other
means and they make calls to SDL functions, make sure that you call
Android_JNI_SetupThread before doing anything else otherwise SDL will attach
your thread automatically anyway (when you make an SDL call), but it'll never
detach it.

 Additional documentation

The documentation in the NDK docs directory is very helpful in understanding the build process and how to work with native code on the Android platform.

The best place to start is with docs/OVERVIEW.TXT

 Using Eclipse

First make sure that you've installed Eclipse and the Android extensions as described here:

Once you've copied the SDL android project and customized it, you can create an Eclipse project from it:
 * File -> New -> Other
 * Select the Android -> Android Project wizard and click Next
 * Enter the name you'd like your project to have
 * Select "Create project from existing source" and browse for your project directory
 * Make sure the Build Target is set to Android 2.0
 * Click Finish

 Loading files and resources



You can create and run an emulator from the Eclipse IDE:
 * Window -> Android SDK and AVD Manager

You can see if adb can see any devices with the following command:
	adb devices

You can see the output of log messages on the default device with:
	adb logcat

You can push files to the device with:
	adb push local_file remote_path_and_file

You can push files to the SD Card at /sdcard, for example:
	adb push moose.dat /sdcard/moose.dat

You can see the files on the SD card with a shell command:
	adb shell ls /sdcard/

You can start a command shell on the default device with:
	adb shell

You can do a clean build with the following commands:
	ndk-build clean

You can see the complete command line that ndk-build is using by passing V=1 on the command line:
	ndk-build V=1

If your application crashes in native code, you can use addr2line to convert the addresses in the stack trace to lines in your code.

For example, if your crash looks like this:
I/DEBUG   (   31): signal 11 (SIGSEGV), code 2 (SEGV_ACCERR), fault addr 400085d0
I/DEBUG   (   31):  r0 00000000  r1 00001000  r2 00000003  r3 400085d4
I/DEBUG   (   31):  r4 400085d0  r5 40008000  r6 afd41504  r7 436c6a7c
I/DEBUG   (   31):  r8 436c6b30  r9 435c6fb0  10 435c6f9c  fp 4168d82c
I/DEBUG   (   31):  ip 8346aff0  sp 436c6a60  lr afd1c8ff  pc afd1c902  cpsr 60000030
I/DEBUG   (   31):          #00  pc 0001c902  /system/lib/
I/DEBUG   (   31):          #01  pc 0001ccf6  /system/lib/
I/DEBUG   (   31):          #02  pc 000014bc  /data/data/
I/DEBUG   (   31):          #03  pc 00001506  /data/data/

You can see that there's a crash in the C library being called from the main code.  I run addr2line with the debug version of my code:
	arm-eabi-addr2line -C -f -e obj/local/armeabi/
and then paste in the number after "pc" in the call stack, from the line that I care about:

I get output from addr2line showing that it's in the quit function, in testspriteminimal.c, on line 23.

You can add logging to your code to help show what's happening:

#include <android/log.h>

	__android_log_print(ANDROID_LOG_INFO, "foo", "Something happened! x = %d", x);

If you need to build without optimization turned on, you can create a file called "" in the jni directory, with the following line in it:
APP_OPTIM := debug

 Known issues

- SDL audio (although it's mostly written, just not working properly yet)
- TODO. I'm sure there's a bunch more stuff I haven't thought of