Understanding Linux Network Internals

Defining a Chain

The elements of the notification chain's list are of type notifier_block, whose definition is the following:

struct notifier_block
{
    int (*notifier_call)(struct notifier_block *self, unsigned long, void *);
    struct notifier_block *next;
    int priority;
};

notifier_call is the function to execute, next is used to link together the elements of the list, and priority represents the priority of the function. Functions with higher priority are executed first. But in practice, almost all registrations leave the priority out of the notifier_block definition, which means it gets the default value of 0 and execution order ends up depending only on the registration order (i.e., it is a semirandom order). The return values of notifier_call are listed in the upcoming section, "Notifying Events on a Chain."

Common names for notifier_block instances are xxx _chain, xxx _notifier_chain, and xxx _notifier_list.