Product Documentation

Variables

Aug 31, 2016

Variables hold values that may change during extension execution. Because of dynamic typing, any variable may hold values of any type. There are no type declarations for variables. Instead, a variable's type is determined at run time. In fact, the type of a variable's value may change during execution, although this is not a recommended practice. A variable initially has the value nil.

Variable names are identifiers, so are strings of letters, digits, and underscores not beginning in a digit. Examples: headers, combined_headers.

Global Variables

In Lua, variables that are not otherwise declared are global within the program. However, global variables are not allowed in policy extension functions, because there are multiple Packet Engines in which a function can be executed, and each Packet Engine has its own memory.

If you use a global variable in your extension, you will get a runtime error: attempt to update or create a global reported in /var/log/ns.log.

Typos in variable names are a potential problem, because the variable with the typo will be interpreted as another, global variable, and will not cause a syntax error as in language like C or Java. As noted above, you will get a runtime error instead.

Local Variables

A variable may be declared to be local to a block of statements, such as a function. This is done by local variable-name. The variable will be scoped to the block, that is, it will only exist within the block. The local declaration may optionally assign a value to the variable.

Examples:

local headers = {}
local combined_headers = {}