Lua code can explicitly raise an error by calling the function. (This function never returns.)

Because Lua is an embedded extension language, Lua code starts running by a call from C code in the host program. (When you use Lua standalone, the lua application is the host program.) Usually, this call is protected; so, when an otherwise unprotected error occurs during the compilation or execution of a Lua chunk, control returns to the host, which can take appropriate measures, such as printing an error message.

When you use (or , in C) you may give a message handler to be called in case of errors. This function is called with the original error object and returns a new error object. It is called before the error unwinds the stack, so that it can gather more information about the error, for instance by inspecting the stack and creating a stack traceback. This message handler is still protected by the protected call; so, an error inside the message handler will call the message handler again. If this loop goes on for too long, Lua breaks it and returns an appropriate message. The message handler is called only for regular runtime errors. It is not called for memory-allocation errors nor for errors while running finalizers or other message handlers.