3D Shooters Episode II: Doom for Android - Pro Android Games - page 258

Original Game Changes
In order for the JNI glue to work, changes are required to the original game engine. Some are simple,
such as inserting calls to the C to Java callbacks; some are not so simple, such as removing invalid
dependencies. Table 7-6 shows the original files and the changes required. Considering that the engine
has a total of 90,000 lines of code, these changes are not that bad.

Table 7-6. Changes Required to the Original Engine to Insert the JNI Glue

File Changes

i_main.c Rename the main subroutine to doom_main .

i_system.c

In I_Error , insert jni_fatal_error .

i_sound.c Comment SDL dependencies. In I_StartSound , insert start sound callback

jni_start_sound .

s_sound.c

In S_SetMusicVolume , insert volume callback jni_set_music_volume .

i_video.c Comment SDL dependencies. Insert code to build an Android ARBG pixel array from the

video buffer. In I_SetRes , add JNI callback to initialize graphics. In I_FinishUpdate , send
pixels to Java with jni_send_pixels .

These changes are explained in more detail in the next sections.

Renaming main
Let's start with the simplest change: renaming the main() subroutine in i_main.c so it can be invoked
from the Java native Java_doom_jni_Natives_DoomMain , which will start the game from Java, as shown in
the next fragment:
// In i_main.c
int main(int argc, char **argv)
int doom_main(int argc, char **argv)

// In jni_doom.c
extern int doom_main(int argc, char **argv);

JNIEXPORT jint JNICALL Java_doom_jni_Natives_DoomMain
(JNIEnv * env, jclass class, jobjectArray jargv)

{
...
doom_main (clen, args);
...
}