mach/exc_server.h Kernel.framework

A catch_exception_raise function is called by exc_server as the result of a kernel message indicating that an exception occurred within a thread. The exception_port parameter specifies the port named via a previous call to thread_set_exception_ports or task_set_exception_ports as the port that responds when the thread takes an exception. The alternate message forms (the format being selected when the exception port was set) allow for selected thread state to be included.

Notes. When an exception occurs in a thread, the thread sends an exception message to its exception port, blocking in the kernel waiting for the receipt of a reply. It is assumed that some task is listening (most likely with mach_msg_server) to this port, using the exc_server function to decode the messages and then call the linked in catch_exception_raise. It is the job of catch_exception_raise to handle the exception and decide the course of action for thread. If the thread should continue from the point of exception, catch_exception_raise would return KERN_SUCCESS. This causes a reply message to be sent to the kernel, which will allow the thread to continue from the point of the exception. If some other action should be taken by thread, the following actions should be performed by catch_exception_raise: thread_suspend This keeps the thread from proceeding after the next step. thread_abort This aborts the message receive operation currently blocking the thread. thread_set_state (if using the catch_exception_raise form). Set the thread's state so that it continues doing something else. thread_resume Let the thread start running from its new state. Returning a value other than KERN_SUCCESS insures that no reply message will be sent. sent. (Actually, the kernel uses a send once right to send the exception message, which thread_abort destroys, so replying to the message is harmless.) The thread can always be destroyed with thread_terminate. A thread can have two exception ports active for it: its thread type specific exception port and the task type specific exception port. The kernel will try sending an exception message to both ports looking for a reply message with a return value of KERN_SUCCESS. The kernel tries the thread specific port first, then the task specific port. If the return value from the first exception message the kernel sends has a return value of KERN_SUCCESS, the thread continues (with a possibly modified state). If the return value is not KERN_SUCCESS, the kernel tries the second port. If that return value is KERN_SUCCESS, the thread continues; otherwise, the thread is terminated. To get the effect of a non-success return value, the server interface should return MIG_DESTROY_REQUEST. This causes exc_server and mach_msg_server to destroy the kernel's request (as opposed to sending a reply with a KERN_SUCCESS value).

The exc_server function is the MIG generated server handling function to handle messages from the kernel relating to the occurrence of an exception in a thread. Such messages are delivered to the exception port set via thread_set_exception_ports or task_set_exception_ports. When an exception occurs in a thread, the thread sends an exception message to its exception port, blocking in the kernel waiting for the receipt of a reply. The exc_server function performs all necessary argument handling for this kernel message and calls catch_exception_raise, catch_exception_raise_state or catch_exception_raise_state_identity, which should handle the exception. If the called routine returns KERN_SUCCESS, a reply message will be sent, allowing the thread to continue from the point of the exception; otherwise, no reply message is sent and the called routine must have dealt with the exception thread directly.