Attributes¶

The Zeek scripting language supports the following attributes.

Name	Description
`&redef`	Redefine a global constant or extend a type.
`&priority`	Specify priority for event handler or hook.
`&log`	Mark a record field as to be written to a log.
`&optional`	Allow a record field value to be missing.
`&default`	Specify a default value.
`&add_func`	Specify a function to call for each “redef +=”.
`&delete_func`	Same as “&add_func”, except for “redef -=”.
`&expire_func`	Specify a function to call when container element expires.
`&read_expire`	Specify a read timeout interval.
`&write_expire`	Specify a write timeout interval.
`&create_expire`	Specify a creation timeout interval.
`&on_change`	Specify a function to call on set/table changes
`&raw_output`	Open file in raw mode (chars. are not escaped).
`&error_handler`	Used internally for reporter framework events.
`&type_column`	Used by input framework for “port” type.
`&deprecated`	Marks an identifier as deprecated.

Warning

A confusing pitfall can be mistaking that attributes bind to a variable or a type, where in reality they bind to a value. Example:

global my_table: table[count] of string &create_expire=1sec;

event zeek_init()
    {
    my_table = table();
    my_table[1] = "foo";
    }

In the above, the re-assignment of my_table will also drop the original value’s &create_expire and no entries will ever be expired from my_table. The alternate way of re-assignment that creates a new table value with the expected attribute would be:

my_table = table() &create_expire=1sec;

Here is a more detailed explanation of each attribute:

&redef¶

Allows use of a redef to redefine initial values of global variables (i.e., variables declared either global or const). Example:

const clever = T &redef;
global cache_size = 256 &redef;

Note that a variable declared “global” can also have its value changed with assignment statements (doesn’t matter if it has the “&redef” attribute or not).

&priority¶

Specifies the execution priority (as a signed integer) of a hook or event handler. Higher values are executed before lower ones. The default value is 0. Example:

event zeek_init() &priority=10
    {
    print "high priority";
    }

&log¶: Writes a record field to the associated log stream.

&optional¶

Allows a record field value to be missing (i.e., neither initialized nor ever assigned a value).

In this example, the record could be instantiated with either “myrec($a=127.0.0.1)” or “myrec($a=127.0.0.1, $b=80/tcp)”:

type myrec: record { a: addr; b: port &optional; };

The ?$ operator can be used to check if a record field has a value or not (it returns a bool value of T if the field has a value, and F if not).

&default¶

Specifies a default value for a record field, container element, or a function/hook/event parameter.

In this example, the record could be instantiated with either “myrec($a=5, $c=3.14)” or “myrec($a=5, $b=53/udp, $c=3.14)”:

type myrec: record { a: count; b: port &default=80/tcp; c: double; };

In this example, the table will return the string "foo" for any attempted access to a non-existing index:

global mytable: table[count] of string &default="foo";

When used with function/hook/event parameters, all of the parameters with the “&default” attribute must come after all other parameters. For example, the following function could be called either as “myfunc(5)” or as “myfunc(5, 53/udp)”:

function myfunc(a: count, b: port &default=80/tcp)
    {
    print a, b;
    }

&add_func¶: Can be applied to an identifier with &redef to specify a function to be called any time a “redef <id> += …” declaration is parsed. The function takes two arguments of the same type as the identifier, the first being the old value of the variable and the second being the new value given after the “+=” operator in the “redef” declaration. The return value of the function will be the actual new value of the variable after the “redef” declaration is parsed.

&delete_func¶: Same as &add_func, except for redef declarations that use the “-=” operator.

&expire_func¶

Called right before a container element expires. The function’s first argument is of the same type as the container it is associated with. The function then takes a variable number of arguments equal to the number of indexes in the container. For example, for a table[string,string] of count the expire function signature is:

function(t: table[string, string] of count, s: string, s2: string): interval

The return value is an interval indicating the amount of additional time to wait before expiring the container element at the given index (which will trigger another execution of this function).

&read_expire¶: Specifies a read expiration timeout for container elements. That is, the element expires after the given amount of time since the last time it has been read. Note that a write also counts as a read.

&write_expire¶: Specifies a write expiration timeout for container elements. That is, the element expires after the given amount of time since the last time it has been written.

&create_expire¶: Specifies a creation expiration timeout for container elements. That is, the element expires after the given amount of time since it has been inserted into the container, regardless of any reads or writes.

&on_change¶

Called right after a change has been applied to a container. The function’s first argument is of the same type as the container it is associated with, followed by a TableChange record which specifies the type of change that happened. The function then takes a variable number of arguments equal to the number of indexes in the container, followed by an argument for the value of the container (if the container has a value) For example, for a table[string,string] of count the on_change function signature is:

function(t: table[string, string] of count, tpe: TableChange, s: string, s2: string, val: count)

For a set[count] the function signature is:

function(s: set[count], tpe: TableChange, c: count)

The passed value specifies the state of a value before the change, where this makes sense. In case a element is changed, removed, or expired, the passed value will be the value before the change, removal, or expiration. When an element is added, the passed value will be the value of the added element (since no old element existed).

Note that the on_change function is only changed when the container itself is modified (due to an assignment, delete operation, or expiry). When a container contains a complex element (like a record, set, or vector), changes to these complex elements are not propagated back to the parent. For example, in this example the change_function for the table will only be called once, when s is inserted - but it will not be called when s is changed:

local t: table[string] of set[string] &on_change=change_function;
local s: set[string] = set();
t["s"] = s; # change_function of t is called
add s["a"]; # change_function of t is _not_ called.

Also note that the on_change function of a container will not be called when the container is already handling on_change_function. Thus, writing a on_change function like this is supported and will not lead to a infinite loop :

local t: table[string] of set[string] &on_change=hange_function;
function change_function(t: table[string, int] of count, tpe: TableChange, idxa: string, idxb: int, val: count)
  {
  t[idxa, idxb] = val+1;
  }

&raw_output¶: Opens a file in raw mode, i.e., non-ASCII characters are not escaped.

&error_handler¶: Internally set on the events that are associated with the reporter framework: reporter_info, reporter_warning, and reporter_error. It prevents any handlers of those events from being able to generate reporter messages that go through any of those events (i.e., it prevents an infinite event recursion). Instead, such nested reporter messages are output to stderr.

&type_column¶

Used by the input framework. It can be used on columns of type port (such a column only contains the port number) and specifies the name of an additional column in the input file which specifies the protocol of the port (tcp/udp/icmp).

In the following example, the input file would contain four columns named “ip”, “srcp”, “proto”, and “msg”:

type Idx: record {
    ip: addr;
};


type Val: record {
    srcp: port &type_column = "proto";
    msg: string;
};

&deprecated¶

The associated identifier is marked as deprecated and will be removed in a future version of Zeek. Look in the NEWS file for more instructions to migrate code that uses deprecated functionality. This attribute can be assigned an optional string literal value to print along with the deprecation warning. The preferred format of this warning message should include the version number in which the identifier will be removed:

type warned: string &deprecated="This type is deprecated. Removed in x.y.z.";