Before fork(2) processing commences in the parent process. If more than one prepare handler is registered they will be called in the opposite order they were registered.
parent
After fork(2) completes in the parent process. If more than one parent handler is registered they will be called in the same order they were registered.
child
After fork(2) processing completes in the child process. If more than one child handler is registered they will be called in the same order they were registered.
If no handling is desired at one or more of these three points, a null pointer may be passed as the corresponding fork handler.