Question

Consider the following main() method which you would find most iPhone applications:

int main(int argc, char *argv[]) {     NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];     int retVal = UIApplicationMain(argc, argv, nil, nil);     [pool release];     return retVal; }

In every iPhone app that I’ve run in Simulator with these (including several sample projects provided by Apple), the thread never exits UIApplicationMain() and any remaining code in main() is never executed. Is this expected behavior?

I have verified that statements after UIApplicationMain() never run by stepping through the code with a debugger. When the user stops an application (by hitting the ‘Home’ button, for example), the resulting stack trace shows that [UIApplication _terminateWithStatus:] is eventually called. This function calls your application delegate’s applicationWillTerminate: method. Once that finishes, [UIApplication _terminateWithStatus:] seems to kill/exit the thread.

Can someone confirm that this is how main() is supposed to work, or at least confirm the same behavior on their machine?

Answer 1

The original question was: ‘Why doesn’t an iPhone app’s main() function ever get a chance to finish?‘

Short Answer: Because UIApplicationMain() is coded such that it never returns.

After doing several tests in Simulator and on the device, and asking another developer to do the same tests, I have confirmed that UIApplicationMain never returns. When the user terminates an application normally by hitting the Home button, The program ultimately terminates inside an unpublished UIApplication method called _terminateWithStatus. This method calls exit(0).

This behavior matches that of the NSApplicationMain function (which is AppKit/Cocoa version of the UIApplicationMain function). The documentation for NSApplicationMain() clearly states that it will never return.

I have submitted a bug (6600198) to Apple requesting that the official documentation (and Xcode template for main.m) be corrected to state that UIApplicationMain() never returns. While this is not a functional problem, the current template and docs are misleading.

Thanks to everyone for all the input and brainstorming!