Shared Variables

Until now, variables declared in a program are local to that program. This means a program cannot access variables from other programs:

program Prog1() {
    print(x); // x does not exist in this process
}
main {
    let x = 0;
    run Prog1();
}

danger

The code above will return an error: the Prog1 program cannot access the x variable declared in the main program.

Declaring Shared Variables

To allow multiple processes to access the same variable, you must declare it as a shared variable. A shared variable is a variable that can be read and modified by multiple processes. Here's how to declare a shared variable:

shared {
    let X: int;
    let Y = false;
    const A = 42;
}

warning

A shared variable's name must always start with an uppercase letter.

tip

Declarations in the shared block work like classic declarations: they can be constant or mutable, have any type, and can be assigned a value. Only declarations are possible in the shared block.

Executing Processes with Shared Variables

During execution, the shared block is executed in one go before the processes. Shared variables are thus accessible and modifiable by all processes.

shared {
    let X : int;
}
program Prog1() {
    X++;
    wait X == 2;
}
main {
    run Prog1();
    run Prog1();
}

note

In this example, both Prog1 processes increment the X variable by 1. The first process then waits for X to be equal to 2 before continuing.