IBM AIX Continuous Availability Features - IBM Redbooks

► An optional predicate
The predicate, if present, identifies a condition that is to be checked at the time the probe
is triggered. The predicate must evaluate to TRUE for the probe actions of the clause to
be executed.
These elements are described in more detail in the following sections.
Probe point specification
A probe point specification identifies the code location whose execution, or the event whose
occurrence, should trigger the probe actions. Multiple probe points can be associated with the
same set of probe actions and the predicate, if any, by providing a comma-separated list of
probe specifications at the top of the Vue clause.
The format for a probe specification is probe-type specific. The probe specification is a tuple
of ordered list of fields separated by colons. It has the following general format:
@@::...::
AIX V6.1 supports the following probe types:
1. User Function Entry probes (or uft probes)
For example, a uft probe at entry into any function called foo() (in the main executable or
any of the loaded modules including libraries) in process with ID = 34568:
@@uft:34568:*:foo:entry
2. System Call Entry/Exit probes (or syscall probes)
For example, a syscall probe at the exit of a read system call:
@@syscall:*:read:exit
3. Probes that fire at specific time intervals (or interval probes)
For example, an interval probe to fire every 500 milliseconds (wall clock time):
@@interval:*:clock:500
Action block
The action block identifies the set of actions to be performed when a thread hits the probe
point. Supported actions are not restricted to the capturing and formatting of trace data; the
full power of Vue can be employed.
An action block in Vue is similar to a procedure in procedural languages. It consists of a
sequence of statements that are executed in order. The flow of execution is essentially
sequential. The only exceptions are that conditional execution is possible using the if-else
statement, and control may be returned from within the action block using the return
statement.
Unlike procedures in procedural languages, an action block in Vue does not have an output
or a return value. And it does not have inherent support for a set of input parameters. On the
other hand, the context data at the point where a probe is entered can be accessed within the
action block to regulate the actions to be performed.
Predicate
Predicates should be used when execution of clauses at probe points must be performed
conditionally.
116 IBM AIX Continuous Availability Features