The Esterel v5 21 System Manual - Courses

3.3. C CODE FOR DATA HANDLING 35
Assignment Function
A reference to the assignment procedure is generated in the following cases:
• There is an explicit assignment or variable initialization of type T, i.e.
the assignment symbol “:=” is used somewhere for a variable of type
T.
• There is a valued signal of type T, either in the main module or in
one of its submodules. In the case of valued signals, assignment is
necessary to handle signal emissions.
• An exec statement calls a task with a reference parameter of type T.
For assignments to be correctly generated, the user must write an assignment
function for each user type T. This C function will be automatically called
to execute assignments. It takes two arguments, the first one of type T*, the
second one of type T. Since the result of an assignment function is never used,
the result type should be void. For example, the source Esterel assignment
X := exp
where both X and exp are of type T generates the following C code:
_T(&__PROG_Vxx, exp)
WARNING: Assignment must always be done by full copy or
be equivalent to a full copy (or bitwise copy). Arbitrarily strange
behaviors can appear otherwise.
If C supports assignment by bitwise copy on type T, then one can just define
T in the following way:
#define _T(x,y) (*(x)=y)
The parentheses are needed to avoid priority conflicts. Notice that a source
assignment X := exp generates the C assignment
(*(&__PROG_Vxx) = exp)
which is immediately translated back into PROG Vxx = exp by the C compiler.
There is no loss of efficiency.