Cancellation

Cancelling a Thread

int pthread_cancel(pthread_t);

• Registers a cancellation event with the specified thread.
• There is no way to unregister a cancellation event.
• Does not wait for the thread to terminate.
• Joining with a cancelled thread will return get a status of PTHREAD_CANCELLED

Controlling Cancellation

• A thread must be in one of three cancellable states... disabled, deferred, or asynchronous
• A thread always starts its life in the deferred state.

int pthread_setcancelstate(int state, int * old_state);

int pthread_setcanceltype(int type, int * old_type);

Deferred Cancellation

• Deferred cancellation can only occur at certain point
• Usually a blocking syscall is a cancellation point
• Mutexes are NEVER a cancellation point

void pthread_testcancel(void);

• pthread_testcancel is a cancelation point
• It does nothing if cancellation is disabled
• There is no nondestructive test for a cancelation event.