Parsing

Parsing in Spicy is centered around the unit type which in many ways looks similar to a struct type.

A unit declares an ordered list of fields which are parsed from the input.

If a unit is public it can serve as a top-level entry point for parsing.

module foo;

public type Foo = unit {
    version: uint32;

    on %done { print "The version is %s." % self.version; }
};

• The parser for Foo consists of a single parser which extracts an uint32 with the default network byte order.
• The extracted uint32 is bound to a named field to store its value in the unit.
• We added a unit hook which runs when the parser is done.

We can run that parser by using a driver which feeds it input (potentially incrementally).

$ printf '\x00\x00\x00\xFF' | spicy-driver -d hello.spicy
The version is 255.

We use spicy-driver as driver. It reads input from its stdin and feeds it to the parser, and executes hooks.

Another driver is spicy-dump which prints the unit after parsing. Zeek includes its own dedicated driver for Spicy parsers.

The major differences to struct are:

• unit fields need to have a parsable type,
• by default all unit fields are &optional, i.e., a unit value can have any or all fields unset.