author Sam Lantinga <>
Wed, 07 Nov 2012 20:17:27 -0800
changeset 6664 2deb17aefbaf
parent 6657 35de500cc918
child 6678 afb9be20f8d2
permissions -rw-r--r--
Improved default behavior for pause/resume on Android

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 or symlink this SDL directory into the <project>/jni directory
3. Edit <project>/jni/src/ to include your source files
4. 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.

5. Edit <project>/ to point to the Android SDK directory
6. Run 'ant debug' in android/project. This compiles the .java and eventually 
creates a .apk with the native code embedded
7. 'ant debug 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, customize this for your app	- 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.

 Customizing your application name

To customize your application name, edit AndroidManifest.xml and replace
"" with an identifier for your product package.

Then create a Java class extending SDLActivity and place it in a directory
under src matching your package, e.g.

Here's an example of a minimal class file:
--- --------------------------

import android.os.*; 

 * A sample wrapper class that just calls SDLActivity 

public class MyGame extends SDLActivity { 
    protected void onCreate(Bundle savedInstanceState) { 
    protected void onDestroy() { 

Then replace "SDLActivity" in AndroidManifest.xml with the name of your
class, .e.g. "MyGame"

 Customizing your application icon

Conceptually changing your icon is just replacing the icon.png files in the
drawable directories under the res directory.

The easiest way to create a set of icons for your project is to remove all
the existing icon.png files, and then use the Eclipse IDE to create a dummy
project.  During the process of doing this Eclipse will prompt you to create
an icon. Then just copy the drawable directories it creates over to your
res directory.

You may need to change the name of your icon in AndroidManifest.xml to match
the filename used by Eclipse.

 Loading assets

Any files you put in the "assets" directory of your android-project directory
will get bundled into the application package and you can load them using the
standard functions in SDL_rwops.h.

There are also a few Android specific functions that allow you to get other
useful paths for saving and loading data:

See SDL_system.h for more details on these functions.

 Pause / Resume behaviour

If SDL is compiled with SDL_ANDROID_BLOCK_ON_PAUSE defined (the default),
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.

 Using STL

You can use STL in your project by creating an file in the jni
folder and adding the following line:
APP_STL := stlport_static

For more information check out CPLUSPLUS-SUPPORT.html in the NDK documentation.

 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

 Using the emulator

There are some good tips and tricks for getting the most out of the
emulator here:

Especially useful is the info on setting up OpenGL ES 2.0 emulation.


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

 Memory debugging

The best (and slowest) way to debug memory issues on Android is valgrind.
Valgrind has support for Android out of the box, just grab code using:
	svn co svn:// valgrind
... and follow the instructions in the file to build it.

One thing I needed to do on Mac OS X was change the path to the toolchain,
and add ranlib to the environment variables:
export RANLIB=$NDKROOT/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin/arm-linux-androideabi-ranlib

Once valgrind is built, you can create a wrapper script to launch your
application with it, changing to your package identifier:
--- start_valgrind_app -------------------
export TMPDIR=/data/data/
exec /data/local/Inst/bin/valgrind --log-file=/sdcard/valgrind.log --error-limit=no $*

Then push it to the device:
	adb push start_valgrind_app /data/local

and make it executable:
	adb shell chmod 755 /data/local/start_valgrind_app

and tell Android to use the script to launch your application:
	adb shell setprop "logwrapper /data/local/start_valgrind_app"

If the setprop command says "could not set property", it's likely that
your package name is too long and you should make it shorter by changing
AndroidManifest.xml and the path to your class file in android-project/src

You can then launch your application normally and waaaaaaaiiittt for it.
You can monitor the startup process with the logcat command above, and
when it's done (or even while it's running) you can grab the valgrind
output file:
	adb pull /sdcard/valgrind.log

When you're done instrumenting with valgrind, you can disable the wrapper:
	adb shell setprop ""

 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