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Android

Matt Styles wrote a tutorial on building SDL for Android with Visual Studio: http://trederia.blogspot.de/2017/03/building-sdl2-for-android-with-visual.html

The rest of this README covers the Android gradle style build process.

Requirements

Android SDK (version 34 or later) https://developer.android.com/sdk/index.html

Android NDK r15c or later https://developer.android.com/tools/sdk/ndk/index.html

Minimum API level supported by SDL: 21 (Android 5.0)

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: android-project/app/src/main/java/org/libsdl/app/SDLActivity.java

The Java code loads your game code, the SDL shared library, and dispatches to native functions implemented in the SDL library: src/core/android/SDL_android.c

Building an app

For simple projects you can use the script located at build-scripts/create-android-project.py

There's two ways of using it:

./create-android-project.py com.yourcompany.yourapp < sources.list
./create-android-project.py com.yourcompany.yourapp source1.c source2.c ...sourceN.c

sources.list should be a text file with a source file name in each line Filenames should be specified relative to the current directory, for example if you are in the build-scripts directory and want to create the testgles.c test, you'll run:

./create-android-project.py org.libsdl.testgles ../test/testgles.c

One limitation of this script is that all sources provided will be aggregated into a single directory, thus all your source files should have a unique name.

Once the project is complete the script will tell you how to build the project. If you want to create a signed release APK, you can use the project created by this utility to generate it.

Running the script with --help will list all available options, and their purposes.

Finally, a word of caution: re running create-android-project.py wipes any changes you may have done in the build directory for the app!

For more complex projects, follow these instructions:

  1. Get the source code for SDL and copy the 'android-project' directory located at SDL/android-project to a suitable location. Also make sure to rename it to your project name (In these examples: YOURPROJECT).

    (The 'android-project' directory can basically be seen as a sort of starting point for the android-port of your project. It contains the glue code between the Android Java 'frontend' and the SDL code 'backend'. It also contains some standard behaviour, like how events should be handled, which you will be able to change.)

  2. Move or symlink the SDL directory into the "YOURPROJECT/app/jni" directory

(This is needed as the source of SDL has to be compiled by the Android compiler)

  1. Edit "YOURPROJECT/app/jni/src/Android.mk" to include your source files.

(They should be separated by spaces after the "LOCAL_SRC_FILES := " declaration)

4a. If you want to use Android Studio, simply open your 'YOURPROJECT' directory and start building.

4b. If you want to build manually, run './gradlew installDebug' in the project directory. This compiles the .java, creates an .apk with the native code embedded, and installs it on any connected Android device

If you already have a project that uses CMake, the instructions change somewhat:

  1. Do points 1 and 2 from the instruction above.
  2. Edit "YOURPROJECT/app/build.gradle" to comment out or remove sections containing ndk-build and uncomment the cmake sections. Add arguments to the CMake invocation as needed.
  3. Edit "YOURPROJECT/app/jni/CMakeLists.txt" to include your project (it defaults to adding the "src" subdirectory). Note that you'll have SDL3 and SDL3-static as targets in your project, so you should have "target_link_libraries(yourgame SDL3)" in your CMakeLists.txt file. Also be aware that you should use add_library() instead of add_executable() for the target containing your "main" function.

If you wish to use Android Studio, you can skip the last step.

  1. Run './gradlew installDebug' or './gradlew installRelease' in the project directory. It will build and install your .apk on any connected Android device

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

android-project/app
    build.gradle            - build info including the application version and SDK
    src/main/AndroidManifest.xml	- package manifest. Among others, it contains the class name of the main Activity and the package name of the application.
    jni/			- directory holding native code
    jni/Application.mk	- Application JNI settings, including target platform and STL library
    jni/Android.mk		- Android makefile that can call recursively the Android.mk files in all subdirectories
    jni/CMakeLists.txt	- Top-level CMake project that adds SDL as a subproject
    jni/SDL/		- (symlink to) directory holding the SDL library files
    jni/SDL/Android.mk	- Android makefile for creating the SDL shared library
    jni/src/		- directory holding your C/C++ source
    jni/src/Android.mk	- Android makefile that you should customize to include your source code and any library references
    jni/src/CMakeLists.txt	- CMake file that you may customize to include your source code and any library references
    src/main/assets/	- directory holding asset files for your application
    src/main/res/		- directory holding resources for your application
    src/main/res/mipmap-*	- directories holding icons for different phone hardware
    src/main/res/values/strings.xml	- strings used in your application, including the application name
    src/main/java/org/libsdl/app/SDLActivity.java - the Java class handling the initialization and binding to SDL. Be very careful changing this, as the SDL library relies on this implementation. You should instead subclass this for your application.

Using the SDL3 Android Archive (.aar)

The create-android-project.py script can ./create-android-project.py com.yourcompany.yourapp < sources.list

The Android archive allows use of SDL3 in your Android project, without needing to copy any SDL c or java source. For integration with CMake/ndk-build, it uses prefab.

Copy the archive to a app/libs directory of your project and add the following to app/gradle.build:

android {
    /* ... */
    buildFeatures {
        prefab true
    }
}
dependencies {
    implementation files('libs/@PROJECT_NAME@-@PROJECT_VERSION@.aar')
    /* ... */
}

If you're using CMake, add the following to your CMakeLists.txt:

find_package(@PROJECT_NAME@ REQUIRED CONFIG)
target_link_libraries(yourgame PRIVATE @PROJECT_NAME@::@PROJECT_NAME@)

If you're using ndk-build, add the following somewhere after LOCAL_MODULE := yourgame to your Android.mk or Application.mk:

# https://google.github.io/prefab/build-systems.html

# Add the prefab modules to the import path.
$(call import-add-path,/out)

# Import @PROJECT_NAME@ so we can depend on it.
$(call import-module,prefab/@PROJECT_NAME@)

If you want to avoid adding the complete SDL source base as a subproject, or adding the Java sources of the bindings to your Android project

Customizing your application name

To customize your application name, edit AndroidManifest.xml and replace "org.libsdl.app" 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.

src/com/gamemaker/game/MyGame.java

Here's an example of a minimal class file:

--- MyGame.java --------------------------
package com.gamemaker.game;

import org.libsdl.app.SDLActivity;

/**
 * A sample wrapper class that just calls SDLActivity
 */

public class MyGame extends SDLActivity { }

------------------------------------------

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 "ic_launcher.png" files in the drawable directories under the res directory. There are several directories for different screen sizes.

Loading assets

Any files you put in the "app/src/main/assets" directory of your project directory will get bundled into the application package and you can load them using the standard functions in SDL_iostream.h.

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

  • SDL_GetAndroidInternalStoragePath()
  • SDL_GetAndroidExternalStorageState()
  • SDL_GetAndroidExternalStoragePath()
  • SDL_GetAndroidCachePath()

See SDL_system.h for more details on these functions.

The asset packaging system will, by default, compress certain file extensions. SDL includes two asset file access mechanisms, the preferred one is the so called "File Descriptor" method, which is faster and doesn't involve the Dalvik GC, but given this method does not work on compressed assets, there is also the "Input Stream" method, which is automatically used as a fall back by SDL. You may want to keep this fact in mind when building your APK, specially when large files are involved. For more information on which extensions get compressed by default and how to disable this behaviour, see for example:

http://ponystyle.com/blog/2010/03/26/dealing-with-asset-compression-in-android-apps/

Activity lifecycle

On Android the application goes through a fixed life cycle and you will get notifications of state changes via application events. When these events are delivered you must handle them in an event callback because the OS may not give you any processing time after the events are delivered.

e.g.

int HandleAppEvents(void *userdata, SDL_Event *event)
{
    switch (event->type)
    {
    case SDL_EVENT_TERMINATING:
        /* Terminate the app.
           Shut everything down before returning from this function.
        */
        return 0;
    case SDL_EVENT_LOW_MEMORY:
        /* You will get this when your app is paused and iOS wants more memory.
           Release as much memory as possible.
        */
        return 0;
    case SDL_EVENT_WILL_ENTER_BACKGROUND:
        /* Prepare your app to go into the background.  Stop loops, etc.
           This gets called when the user hits the home button, or gets a call.

           You should not make any OpenGL graphics calls or use the rendering API,
           in addition, you should set the render target to NULL, if you're using
           it, e.g. call SDL_SetRenderTarget(renderer, NULL).
        */
        return 0;
    case SDL_EVENT_DID_ENTER_BACKGROUND:
        /* Your app is NOT active at this point. */
        return 0;
    case SDL_EVENT_WILL_ENTER_FOREGROUND:
        /* This call happens when your app is coming back to the foreground.
           Restore all your state here.
        */
        return 0;
    case SDL_EVENT_DID_ENTER_FOREGROUND:
        /* Restart your loops here.
           Your app is interactive and getting CPU again.

           You have access to the OpenGL context or rendering API at this point.
           However, there's a chance (on older hardware, or on systems under heavy load),
           where the graphics context can not be restored. You should listen for the
           event SDL_EVENT_RENDER_DEVICE_RESET and recreate your OpenGL context and
           restore your textures when you get it, or quit the app.
        */
        return 0;
    default:
        /* No special processing, add it to the event queue */
        return 1;
    }
}

int main(int argc, char *argv[])
{
    SDL_SetEventFilter(HandleAppEvents, NULL);

    ... run your main loop

    return 0;
}

Note that if you are using main callbacks instead of a standard C main() function, your SDL_AppEvent() callback will run as these events arrive and you do not need to use SDL_SetEventFilter.

If SDL_HINT_ANDROID_BLOCK_ON_PAUSE hint is set (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).

You can control activity re-creation (eg. onCreate()) behaviour. This allows you to choose whether to keep or re-initialize java and native static datas, see SDL_HINT_ANDROID_ALLOW_RECREATE_ACTIVITY in SDL_hints.h.

Mouse / Touch events

In some case, SDL generates synthetic mouse (resp. touch) events for touch (resp. mouse) devices. To enable/disable this behavior, see SDL_hints.h:

  • SDL_HINT_TOUCH_MOUSE_EVENTS
  • SDL_HINT_MOUSE_TOUCH_EVENTS

Misc

For some device, it appears to works better setting explicitly GL attributes before creating a window: SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 5); SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 6); SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 5);

Threads and the Java VM

For a quick tour on how Linux native threads interoperate with the Java VM, take a look here: https://developer.android.com/guide/practices/jni.html

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.

If you ever want to use JNI in a native thread (created by "SDL_CreateThread()"), it won't be able to find your java class and method because of the java class loader which is different for native threads, than for java threads (eg your "main()").

the work-around is to find class/method, in you "main()" thread, and to use them in your native thread.

see: https://developer.android.com/training/articles/perf-jni#faq:-why-didnt-findclass-find-my-class

Using STL

You can use STL in your project by creating an Application.mk file in the jni folder and adding the following line:

APP_STL := c++_shared

For more information go here: https://developer.android.com/ndk/guides/cpp-support

Using the emulator

There are some good tips and tricks for getting the most out of the emulator here: https://developer.android.com/tools/devices/emulator.html

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

Notice that this software emulator is incredibly slow and needs a lot of disk space. Using a real device works better.

Troubleshooting

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 remove the library files of your project (and not the SDL lib files) with:

ndk-build clean

You can do a build with the following command:

ndk-build

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 ndk-stack to get a symbolic stack trace: https://developer.android.com/ndk/guides/ndk-stack

If you want to go through the process manually, 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/libc.so
I/DEBUG   (   31):          #01  pc 0001ccf6  /system/lib/libc.so
I/DEBUG   (   31):          #02  pc 000014bc  /data/data/org.libsdl.app/lib/libmain.so
I/DEBUG   (   31):          #03  pc 00001506  /data/data/org.libsdl.app/lib/libmain.so

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/libmain.so

and then paste in the number after "pc" in the call stack, from the line that I care about: 000014bc

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 "Application.mk" 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:

git clone https://sourceware.org/git/valgrind.git

... and follow the instructions in the file README.android to build it.

One thing I needed to do on macOS 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 org.libsdl.app to your package identifier:

--- start_valgrind_app -------------------
#!/system/bin/sh
export TMPDIR=/data/data/org.libsdl.app
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 wrap.org.libsdl.app "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 wrap.org.libsdl.app ""

Graphics debugging

If you are developing on a compatible Tegra-based tablet, NVidia provides Tegra Graphics Debugger at their website. Because SDL3 dynamically loads EGL and GLES libraries, you must follow their instructions for installing the interposer library on a rooted device. The non-rooted instructions are not compatible with applications that use SDL3 for video.

The Tegra Graphics Debugger is available from NVidia here: https://developer.nvidia.com/tegra-graphics-debugger

Why is API level 19 the minimum required?

The latest NDK toolchain doesn't support targeting earlier than API level 19. As of this writing, according to https://www.composables.com/tools/distribution-chart about 99.7% of the Android devices accessing Google Play support API level 19 or higher (August 2023).

A note regarding the use of the "dirty rectangles" rendering technique

If your app uses a variation of the "dirty rectangles" rendering technique, where you only update a portion of the screen on each frame, you may notice a variety of visual glitches on Android, that are not present on other platforms. This is caused by SDL's use of EGL as the support system to handle OpenGL ES/ES2 contexts, in particular the use of the eglSwapBuffers function. As stated in the documentation for the function "The contents of ancillary buffers are always undefined after calling eglSwapBuffers". Setting the EGL_SWAP_BEHAVIOR attribute of the surface to EGL_BUFFER_PRESERVED is not possible for SDL as it requires EGL 1.4, available only on the API level 17+, so the only workaround available on this platform is to redraw the entire screen each frame.

Reference: http://www.khronos.org/registry/egl/specs/EGLTechNote0001.html

Ending your application

Two legitimate ways:

  • return from your main() function. Java side will automatically terminate the Activity by calling Activity.finish().

  • Android OS can decide to terminate your application by calling onDestroy() (see Activity life cycle). Your application will receive an SDL_EVENT_QUIT you can handle to save things and quit.

Don't call exit() as it stops the activity badly.

NB: "Back button" can be handled as a SDL_EVENT_KEY_DOWN/UP events, with Keycode SDLK_AC_BACK, for any purpose.

Known issues

  • The number of buttons reported for each joystick is hardcoded to be 36, which is the current maximum number of buttons Android can report.

Building the SDL tests

SDL's CMake build system can create APK's for the tests. It can build all tests with a single command without a dependency on gradle or Android Studio. The APK's are signed with a debug certificate. The only caveat is that the APK's support a single architecture.

Requirements

  • SDL source tree
  • CMake
  • ninja or make
  • Android Platform SDK
  • Android NDK
  • Android Build tools
  • Java JDK (version should be compatible with Android)
  • keytool (usually provided with the Java JDK), used for generating a debug certificate
  • zip

CMake configuration

When configuring the CMake project, you need to use the Android NDK CMake toolchain, and pass the Android home path through SDL_ANDROID_HOME.

cmake .. -DCMAKE_TOOLCHAIN_FILE=<path/to/android.toolchain.cmake> -DANDROID_ABI=<android-abi> -DSDL_ANDROID_HOME=<path-to-android-sdk-home> -DANDROID_PLATFORM=23 -DSDL_TESTS=ON

Remarks:

  • android.toolchain.cmake can usually be found at $ANDROID_HOME/ndk/x.y.z/build/cmake/android.toolchain.cmake
  • ANDROID_ABI should be one of arm64-v8a, armeabi-v7a, x86 or x86_64.
  • When CMake is unable to find required paths, use cmake-gui to override required SDL_ANDROID_ CMake cache variables.

Building the APK's

For the testsprite executable, the testsprite-apk target will build the associated APK:

cmake --build . --target testsprite-apk

APK's of all tests can be built with the sdl-test-apks target:

cmake --build . --target sdl-test-apks

Installation/removal of the tests

testsprite.apk APK can be installed on your Android machine using the install-testsprite target:

cmake --build . --target install-testsprite

APK's of all tests can be installed with the install-sdl-test-apks target:

cmake --build . --target install-sdl-test-apks

All SDL tests can be uninstalled with the uninstall-sdl-test-apks target:

cmake --build . --target uninstall-sdl-test-apks

Starting the tests

After installation, the tests can be started using the Android Launcher GUI. Alternatively, they can also be started using CMake targets.

This command will start the testsprite executable:

cmake --build . --target start-testsprite

There is also a convenience target which will build, install and start a test:

cmake --build . --target build-install-start-testsprite

Not all tests provide a GUI. For those, you can use adb logcat to read the output of stdout.