Passing stack variables across threads brings exception pointers crash

In an old code of hand Q anime, the net finds a handful of crash, error code examples:

    Char str[100] = "Hello";    Dispatch_async (Dispatch_get_main_queue (), ^{        NSLog (@ "%s", str);    });

STR in the above code may be a parameter in a method, the variables in the stack at the end of the current function call, the memory of STR may be rewritten as random data, access to the memory will cause an exception.

Passing stack variables across threads brings exception pointers crash