backtrace miss the function that caused a crash - ios

Question

I use backtrace and backtrace_symbols to collect the call stack when my app crashed.
void InstallSignalHandler(void)
{
signal(SIGHUP, SignalExceptionHandler);
signal(SIGINT, SignalExceptionHandler);
signal(SIGQUIT, SignalExceptionHandler);
signal(SIGABRT, SignalExceptionHandler);
signal(SIGILL, SignalExceptionHandler);
signal(SIGSEGV, SignalExceptionHandler);
signal(SIGFPE, SignalExceptionHandler);
signal(SIGBUS, SignalExceptionHandler);
signal(SIGPIPE, SignalExceptionHandler);
}
void SignalExceptionHandler(int signal)
{
NSMutableString *mstr = [[NSMutableString alloc] init];
[mstr appendString:#"Stack:\n"];
void* callstack[128];
int i, frames = backtrace(callstack, 128);
char** strs = backtrace_symbols(callstack, frames);
for (i = 0; i <frames; ++i) {
[mstr appendFormat:#"%s\n", strs[i]];
}
NSLog(#"%#", mstr);
free(strs);
}
When I check the log in console, I just find the log miss the function that caused the crash.The function is:
+ (void)testCrash
{
int *nullPointer = NULL;
*nullPointer = 2019;
}
And the log in console is:
0 TestApp 0x0000000101d1e040 SignalExceptionHandler + 160
1 libsystem_platform.dylib 0x000000011002bb5d _sigtramp + 29
2 ??? 0x0000000000000000 0x0 + 0
3 TestApp 0x00000001019bbc6f __39+[MyCrashTesting showInViewController:]_block_invoke + 303
4 UIKit 0x000000010b09a559 -[UIAlertController _invokeHandlersForAction:] + 105
5 UIKit 0x000000010b09af5e __103-[UIAlertController _dismissAnimated:triggeringAction:triggeredByPopoverDimmingView:dismissCompletion:]_block_invoke.461 + 16
6 UIKit 0x000000010ae42ca2 -[UIPresentationController transitionDidFinish:] + 1346
7 UIKit 0x000000010ae46b12 __56-[UIPresentationController runTransitionForCurrentState]_block_invoke.436 + 183
8 UIKit 0x000000010ba2a3b4 -[_UIViewControllerTransitionContext c
I have thought the function name "testCrash" should be in the top of the log. Does I do something wrong?

Answer 1

Since this is a very simple method with no arguments and no ARC heavylifting inside clang the Objective-C compiler is able to do an optimization (-O1 is enough to achieve that) of removing the message runtime call.
You can prevent this behavior with:
+ (void)testCrash __attribute__((optnone))
{
int *nullPointer = NULL;
*nullPointer = 2019;
}
The lesson learnt here is you should never rely on particular stacktrace to achieve something during your program execution. Diagnostics is fine as long as you understand the optimizing principles you've just encountered.