The symbol __rt_entry is the starting point for a program using the ARM® C library.
Control passes to __rt_entry after all scatter-loaded regions have been relocated to their execution addresses.
The default implementation of __rt_entry:
Sets up the heap and stack.
Initializes the C library by calling __rt_lib_init.
Shuts down the C library, by calling __rt_lib_shutdown.
__rt_entry must end with a call to one of the following functions:
atexit()-registered functions and shuts down the library.
Shuts down the library but does not call
Exits directly to the execution environment. It does not shut down the library and does not call