Explanation of why backtrace(3) and stack overflow don't mix.

stackoverflow.m

#import <Foundation/Foundation.h>

#import <pthread.h>
#import <execinfo.h>
#import <string.h>
#import <stdint.h>

/*
 * A signal handler that is broken in two ways:
 * 1) The handler calls APIs that are not declared to be async-safe and may deadlock or otherwise fail.
 * 2) The handler can not capture a backtrace should the stack overflow.
 */
static void unsafe_signal_handler (int signo) {
    signal(signo, SIG_DFL);

    /* Attempt to use backtrace(3) to fetch the backtrace. This requires that we be
     * operating on the crashed thread's stack, but that stack just overflowed,
     * and our signal handler will simply crash.
     *
     * However, if we used sigaltstack() to correctly run on a different stack, our
     * backtrace would be empty! This is not limited to backtrace(3) -- [NSThread callStackReturnAddresses]
     * will exhibit the same behavior.
     *
     * This is one reason why backtraces must be implemented by implementing your own stack walking
     * using the ucontext_t from the SA_SIGINFO prototype.
     */
    void *callstack[128];
    int count = backtrace(callstack, 128);
    backtrace_symbols_fd(callstack, count, STDERR_FILENO);
}

int main(int argc, char *argv[]) {
    NSAutoreleasePool *pool = [NSAutoreleasePool new];

    /* Set up the signal handler. */
    signal(SIGABRT, unsafe_signal_handler);
    signal(SIGSEGV, unsafe_signal_handler);
    
    /* Run some buggy code in an unusual code path. A small typo will trigger infinite recursion ... */
    NSArray *resultMessages = [NSMutableArray arrayWithObject: @"Error message!"];
    NSMutableArray *results = [[NSMutableArray alloc] init];

    for (NSObject *result in resultMessages)
        [results addObject: results]; // Whoops!

    NSLog(@"Some strange unexpected error we never see occured with results: %@", results);

    [pool drain];
    return 0;
}