Adding additional parser state

We might want to add additional state to parsers, e.g.,

• share or modify data outside of our parser, or
• to locally aggregate data while parsing.

Sharing state across multiple units in the same Zeek connection with unit contexts will be discussed separately in a later section.

Passing outside state into units

We might want to pass additional state into a unit, e.g., to parameterize the unit's behavior, or to give the unit access to external state. This can be accomplished with unit parameters.

type X = unit(init: uint64 = 64) {
    var sum: uint64;

    on %init { self.sum = init; }

    : uint8 { self.sum += $$; }
    : uint8 { self.sum += $$; }
    : uint8 { self.sum += $$; }

    on %done { print self.sum; }
};

A few things to note here:

• Unit parameter look a lot like function parameters to the unit.
• Unit parameters can have default values which are used if the parameter was not passed.
• We refer to unit parameters by directly using their name; self is not used.

Unit parameters can also be used to give a unit access to its parent units and their state.

public type X = unit {
    var sum: uint8;

    : (Y(self))[];
};

type Y = unit(outer: X) {
    : uint8 { outer.sum += $$; }
};

Unit variables

Unit variables allow to add additional data to units. Their data can be accessed like other unit fields.

type X = unit {
    var sum: uint8;

    : uint8 { self.sum += $$; }
    : uint8 { self.sum += $$; }
    : uint8 { self.sum += $$; }

    on %done { print self.sum; }
};

By default unit variables are initialized with the default value of the type, e.g., for a uint8 with 0.

To use with a different value, assign the variable in the unit's %init hook, e.g.,

on %init { self.sum = 100; }