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 here at 17:10 - "exceptions are a huge pain in the neck to implement".