The Esterel v5 21 System Manual - Courses

3.5. TASK HANDLING 45
3.4.7 Warnings and Advises
• Since assignments of initial values to valued interface signals are performed
by the reset function, it is highly mandatory to always call the
reset function before initialization if there are initialized signals.
• The relations between input signals specified in the source program
are not checked when the automaton is called in direct embedded
execution. The code may behave strangely if called with inputs which
do not satisfy the relations. However, the simulation interface does
enforce relations.
• The combination functions associated with combined signals must be
commutative and associative. Otherwise, the results of signal combinations
can be arbitrary.
• The automaton code is not reentrant and its execution must be atomic.
Therefore, during a call to the automaton, it is forbidden to call it
again and to call input C functions. Arbitrarily strange behaviors can
arise otherwise. In particular, interrupt handling routines should never
call directly input C functions or the automaton. They should instead
fill event queues to be read when the automaton call terminates. One
can also use interruption masking during the automaton execution.
• Access to uninitialized variables and uninitialized signal values are not
checked when the reaction function is called in direct execution. But
the simulation engine does enforce that check.
3.5 Task Handling
We now describe the interface of the C code generated by the Esterel compiler
for an exec statement. This code reflects concretely the way tasks are
handled abstractly in Esterel. It is organized in two layers. The low-level
layer is a direct interface to run-time C data structures that contain all the
required information about the status of exec statements. The optional
higher-level layer provides the user with a functional interface.
Our design might look heavy at first glance, but it intends to provide
the user with maximal flexibility with respect to actual task handling. The
functional interface is reasonably simple, but not fully general since other
ways of interfacing exec statements can be thought of, in particular as
far as task suspension is concerned. The low-level interface is meant to be
convenient for any user who wants to design its own fine-grain task handling.