An exception is a synchronous event caused by the execution of a thread. These include both the machine exceptions raised by hardware (such as divide-by-zero, memory fault and illegal instruction) and machine exceptions raised by software (such as deadline overrun). The standard C++ exception mechanism is too expensive to use for this, and in any case has the wrong semantics for the exception handling in an RTOS.
The simplest, and most flexible, mechanism for exception handling is to call a function. This function needs context in which to work, so access to some working data is required. The function may also need to be handed some data about the exception raised: at least the exception number and some optional parameters.
The exception handler receives a data argument which is a value that was registered with the handler and points to context information. It also receives an exception_number which identifies the exception taken, and an error code which contains any additional information (such as a memory fault address) needed to handle the exception. Returning from the function will allow the thread to continue.
Exception handlers may be either global or per-thread, or both, depending on configuration options. If exceptions are per-thread, it is necessary to have an exception handler attached to each thread.