4.2 Default Action

A clause can contain any number of actions and variable manipulations. If a clause is left empty, the default action is taken. The default action is to trace the enabled probe identifier (EPID) to the principal buffer. The EPID identifies a particular enabling of a particular probe with a particular predicate and actions. From the EPID, DTrace consumers can determine the probe that induced the action. Indeed, whenever any data is traced, it must be accompanied by the EPID to enable the consumer to make sense of the data. Therefore, the default action is to trace the EPID and nothing else.

Using the default action allows for simple use of dtrace. For example, the following command enables all probes in the vmlinux module with the default action:

# dtrace -m vmlinux

The preceding command might produce output similar to the following example:

# dtrace -m vmlinux
dtrace: description 'vmlinux' matched 35 probes
CPU     ID                    FUNCTION:NAME
  0     42                 __schedule:sleep 
  0     34             dequeue_task:dequeue 
  0     40               __schedule:off-cpu 
  0     23        finish_task_switch:on-cpu 
  0     24             enqueue_task:enqueue 
  0     41               __schedule:preempt 
  0     40               __schedule:off-cpu 
  0     23        finish_task_switch:on-cpu 
  0     11        update_process_times:tick 
  0     42                 __schedule:sleep 
  0     34             dequeue_task:dequeue 
  0     40               __schedule:off-cpu 
  0     23        finish_task_switch:on-cpu 
  0     24             enqueue_task:enqueue 
  0     41               __schedule:preempt 
  0     40               __schedule:off-cpu 
  0     23        finish_task_switch:on-cpu 
  0     11        update_process_times:tick 
  0     12            try_to_wake_up:wakeup 
  0     42                 __schedule:sleep 
...