process

#process

##Syntax A processDeclaration is:

process id [ ( [ paramDeclaration {,paramDeclaration } ] )] statementsAndDeclarations end id

##Description A process declaration is much like a procedure declaration, but is activated by a fork statement rather than by a call. The fork statement starts concurrent (parallel) execution of the process while the statements following the fork continue to execute.

##Example This program initiates (forks) two concurrent processes, one of which repeatedly outputs Hi and the other Ho. The resulting output is an unpredictable sequence of Hi's and Ho's as greetings executes twice concurrently, one instance with word set to Hi and the other with word set to Ho.

    process greetings ( word : string )
        loop
            put word
        end loop
    end greetings
    
    fork greetings ( "Hi" )
    fork greetings ( "Ho" )

##Details The process declaration creates a template for a process (a concurrent activity), which is activated by a fork statement.

A process declaration can appear wherever a module declaration is allowed except that a process declaration is not allowed in a class. The declarations and statements in a process declaration are the same as those in a procedure.

See paramDeclaration for details about parameters. There is an anomaly in parameters to processes, that can lead to errors. In particular, non-var parameters that are non-scalars (such as strings and arrays) are passed by reference. The result is that the target of the reference may change value while the process is executing, which in turn means that the seemingly constant parameter is not really constant. For example, if the string variable s were passed to the greetings process and subsequently changed, the value of greetings' formal parameter would change.

The syntax of a processDeclaration presented above has been simplified by leaving out the optional stack size (compileTimeExpn), import list, pre condition, init clause, post condition and exception handler.

The full syntax is:

See pervasive for information on pervasive processes. The optional compileTimeExpn following the parameter list (if any) is used to specify the number of bytes for the process' stack.

    process [ pervasive ] id
            [ ( [ paramDeclaration {,paramDeclaration } ] )]
            [ : compileTimeExpn ]
        [ pre trueFalseExpn ]
        [ init id := expn {, id := expn } ]
        [ post trueFalseExpn ]
        [ exceptionHandler ]
        statementsAndDeclarations
    end id

##See also import.html list, pre.html condition.html, init.html clause, post.html condition.html and exceptionHandler for explanations of these additional features.