Lecture Notes in Computer Science 3472

478 George Din
altstep Default() runs on PTCType {
[] httpPort.receive {
localTimer.stop;
setverdict (fail);
}
[] localTimer.timeout {
setverdict (fail);
}
}
Program Statements A program element (test case, function or altstep) may
contain different types of statements and operations:
• program statements: expressions, assignments, loop constructs
• behavioral statements: sequential behavior, alternative behavior, interleaving,
defaults
• communication operations: send, receive, call, getcall
• timer operations: start, stop, running
• verdict handling: set
Snapshot Semantic One of the most important features of TTCN-3 is the
snapshot mechanism which allows the definition of complex interleaved or parallel
behaviors. Alternatives are complex behaviors whose sequences of statements
are defined as multiple alternatives. To define a branch in an alternative
behavior, the alt statement is used.
The execution environment will execute the alternatives by using snapshots.
Each branch of an alternative has an associated guard. The guard represents one
or more conditions (boolean expressions) that must be fulfilled in order to enter
a branch. A snapshot is a state of the test system that includes all information
necessary to evaluate the guard of an alt branch. A snapshot is taken when
executing an alt statement; that means that all relevant information is collected
and used for evaluation of guards. This information contains the states of the
components, the timer events and the port enqueued messages. The test system
may change its configuration, but the alt statement is evaluated accordingly to
the snapshot. The execution environment selects only the branches whose guard
conditions are fulfilled. If there is more than one valid guard, then the first one
is chosen. If none is valid, then the alt is reevaluated with a new snapshot.
Communication Operations The communication operations are some of the
most important operations used to specify test behaviors. TTCN-3 supports
message-based and procedure-based communication. The communication operations
can be grouped into two parts: stimuli which send information to SUT
(send, call, reply raise) and responses used to describe the reaction of the SUT
(receive, getreply, trigger, catch).