How can I remove UIApplicationMain from an iPhone application? - iphone

Question

I'm trying to port a game library over to the iPhone. Unlike SDL, this library doesn't take full control of your main() function, it's communicated with via quickly-returning functions from your own code. So, for example, obvious pseudocode:
int main() {
library_init();
// game init code here
while(we_have_not_quit_the_game) {
library_message_loop();
library_init_render();
// render stuff
library_end_render();
// update game state
}
library_shutdown();
}
iPhone makes this difficult, as it requires that you call a UIApplicationMain function that never returns. There's simply no way I could ever get back to the user code after library_init();.
I'm not convinced it's necessary - there's NSRunLoop which supposedly could be used to handle the events. I don't know if UIApplicationMain does anything else of importance, however. (Note that I have no plans to use .nib files, which is the only other thing I've found that UIApplicationMain does.)
I've got three real ideas that I can think of, but they're all a major implementation effort so I'd like to know if anyone has experience with this before I burn a day trying doomed ideas.
In Init, spawn a new thread, run UIApplicationMain in that thread. Either communicate all events across threads (ugh) or just put the UIApplicationMain thread to sleep and use a CFRunLoop in the main thread. I've heard UIApplicationMain does not like being run in a different thread, however.
Ignore UIApplicationMain entirely, just use NSRunLoop. Am I going to be missing important iPhone setup? Who knows!
Do something horrifying with longjmp() to leap out of the UIApplicationMain code after setup, pray that it doesn't do anything important during teardown.
Suggestions?

Answer 1

Looks like I'm answering my own question here! I'm not accepting my answer until I've been able to both test it on real hardware and get it into the app store. That said, I'll keep my Most Up-To-Date Info here, including which options didn't work.
Idea #1: It turns out that each NSRunLoop is thread-specific. If I create a UIApplicationMain in a separate thread, it doesn't get any messages. As a side effect this makes it impossible to determine when it's finished initializing, so if there's anything non-threadsafe it does, it just won't work. I may be able to send it a message across threads to figure out when it's finished initializing, but for now I'm calling this a dead end.
Idea #2: UIApplicationMain does a lot of subtle stuff. I'm not sure what it's restricted to, but I was unable to make anything work without involving UIApplicationMain. Idea #2 is right out.
Idea #3: Receiving the OS signals is important - you need to know if there's a phone-call overlay, or whether you're about to exit. On top of that, some of the setup messages seem vital in order to start the app properly. I was unable to find any method to keep messages being sent without being inside UIApplicationMain. The only options I came up with were NSRunLoop and CFRunLoop. Neither one worked - the messages didn't come in like I wanted. I may not be using these right, but in any case, Idea #3 is out.
Brand-new crazy Idea #4: It's possible to use setjmp/longjmp to fake coroutines in C/C++. The trick is to first set the stack pointer to some value that won't clobber anything important, then start your second routine, then jump back and forth, pretending like you have two stacks. Things get a tiny bit messy if your "second coroutine" decides to return from its main function, but luckily, UIApplicationMain never returns, so this isn't a problem.
I don't know if there's a way to set the stack pointer explicitly on real hardware, say, to a chunk of data that I allocated on the fly. Luckily, it doesn't matter. The iPhone has a 1MB stack by default, which is easily enough to fit a few coroutines in.
What I'm currently doing is using alloca() to push the stack pointer ahead by 768 kilobytes, then spawning UIApplicationMain, then using setjmp/longjmp to bounce back and forth between my "UI routine" and my "main routine". So far, this is working.
Caveats:
It's impossible to know when the "UI routine" has no messages to handle, and when it has no messages to handle, it will just block indefinitely until that's no longer the case. I'm solving this by making a timer that triggers every 0.1 milliseconds. Every time the timer triggers, I drop out to my "main routine", do a single game loop, then head back into the "UI routine" for another timer tick. Reading the documentation indicates that it won't stack up "timer calls" indefinitely. I do seem to get the "terminate" message appropriately, though I haven't managed to test it thoroughly yet, and I haven't tested any other important messages. (Luckily, there's only four messages total, and one of them is setup-related.)
Most modern OSes won't allocate the entire stack at once. The iPhone is probably one of these. What I don't know is whether bumping the stack pointer 3/4 of a meg forward will allocate everything "behind it", so to speak. If so, I may be effectively wasting 3/4 of a meg of RAM, which, on the iPhone, is significant. This could be handled by bumping the pointer forward a smaller amount, but this is really courting stack size disaster - it effectively limits your stack to however far you bump the pointer ahead, and you'll have to figure this out in advance. Some sentinel data in the stack, coupled with good monitoring and a logging system for stack size issues, can probably solve this, but it's a nontrivial issue. (Alternatively, if I can figure out how to muck with the stack pointer directly on the native hardware, I can just malloc()/new[] a few kilobytes, point the stack pointer at it, and use it as my new stack. I'll have to figure out how much space it needs but I doubt it'll be much, considering that it's not doing very much.)
This is currently untested on the actual hardware (give it a week or two, I got another project to finish first.)
I have no idea whether Apple will figure out what I'm doing and slap a giant REJECTED sticker on it when I try submitting to the app store. This is, shall we say, slightly outside their intentions for the API. Fingers crossed.
I'll keep this post updated, and officially accept it once I've verified that it, you know, works.
Late update: I got distracted by a variety of other things. Since then, I've had a few changes that make me far less interested in Apple development. My current approach showed no sign of not working, but I don't really have the motivation to keep fleshing it out. Sorry! If I ever change my mind I'll update this further, but Outlook Not So Good.

Answer 2

I know NSApplicationMain() reads Info.plist and does stuff to the app (like get the initial nib file) so I would guess that UIApplicationMain() does the same for iPhone (like get the initial status bar style, zoom the default.png image in etc.). That stuff isn't exposed anywhere else, so the functions it calls would still need to be run for the app to launch with out any side effects. You're only bet is to reverse engineer those and copy them (and hope that anything they do is in the public SDK).

Answer 3

Is the goal to take that main() function and have it work unmodified on the iPhone?
It would seem there's no way you're going to completely insulate the users of the library from thinking about the iPhone platform--they're going to have to deal with XCode for code signing and that sort of thing.
Given that, telling users they have to break their main() function up into a few pieces that you can call from applicationDidFinishLaunching and from an appropriate timer doesn't seem like it would inconvenience anybody much.

Answer 4

Why not start your game loop from within UIApplication? (Yes, you can't have it in main() but that shouldn't matter.) Some of the delegate messages are good candidates.