ExceptionHandling

This RTS has the restriction No_Exception_Propagation.

This means that any unhandled exception in a subprogram will immediately result in a call to Last_Chance_Handler, which won't return.

Also, you can't use the raise statement to do partial handling, as in

declare
   Err : exception;
begin
   begin
      raise Err with "test";
   exception
      when Err =>
         --  partial cleanup
         raise;                  -- this is not permitted
   end;
exception
   when Err => null;             -- this is OK
end;

The RTS's last chance exception handler is written as

__attribute__((weak))
void __gnat_last_chance_handler(const char *message, int line) {
  taskDISABLE_INTERRUPTS();
  vTaskSuspendAll();
  // Loop indefinitely: use the debugger to examine the backtrace.
  while (1) {}
}

and the weak attribute means that you can write your own version to do something more immediately useful, provided it matches the spec

procedure Last_Chance_Handler
  (Message : System.Address; Line : Integer)
with
  Export,
  Convention => C,
  External_Name => "__gnat_last_chance_handler";

(but it's much easier to read the exception message in the debugger if you use the C version)

Note that at the moment the Line parameter is always 0 (this may be affected by the binder argument -E).

What about full exception handling?

I've made several attempts to support exception handling; unfortunately, it's spread throughout the RTS and requires introducing and modifying many packages. I take some heart from the late Robert Dewar's remarks starting at 16:20 here - "exceptions are a huge pain in the neck to implement".