Lua states, also known as realms in Garry's Mod, are separate instances of the Lua language that can only interact and communicate with one another through indirect means such as the net and file libraries. As they are separate Lua states, global variables in one state cannot be retrieved in another without using indirect communication methods.

States load their custom functions and callbacks through Lua files included by the engine. All other files except those explicitly listed under each state below must be included from Lua to be used.

There are 3 Lua states in Garry's Mod:

Client

The client state is the Lua state representing the game client. It takes player input and sends it to the server while receiving data about other entities and players, then uses all of this information for Prediction. The client simulates entities in sync with the server's tickrate, but will perform rendering every frame.

The client state can interact and communicate with server state via the net library and running serverside concommands.

Lua code can detect if it is running in the client state by checking if the CLIENT global is true.

Engine Lua Files - Client

• lua/includes/init.lua
• lua/derma/init.lua
• lua/gamemodes/base/gamemode/cl_init.lua (this is always loaded regardless of the set gamemode)
• lua/autorun/*.lua
• lua/autorun/client/*.lua
• lua/postprocess/*.lua
• lua/vgui/*.lua
• lua/matproxy/*.lua
• lua/skins/default.lua
• lua/gamemodes/<gamemode_name>/gamemode/cl_init.lua
• lua/gamemodes/base/entities/entities/*.lua
• lua/gamemodes/base/entities/entities/*/cl_init.lua
• lua/gamemodes/base/entities/weapons/*.lua
• lua/gamemodes/base/entities/weapons/*/cl_init.lua
• lua/entities/*.lua
• lua/entities/*/cl_init.lua
• lua/weapons/*.lua
• lua/weapons/*/cl_init.lua
• lua/gamemodes/<gamemode_name>/entities/entities/*.lua
• lua/gamemodes/<gamemode_name>/entities/entities/*/cl_init.lua
• lua/gamemodes/<gamemode_name>/entities/weapons/*.lua
• lua/gamemodes/<gamemode_name>/entities/weapons/*/cl_init.lua

Server

The server state is the Lua state representing the game server. It can either be on the same system as the game client through a Listen Server, or on a separate system as a Dedicated Server. It takes input from players, performs physics and entity simulation at a static tickrate, then networks the result to all connected players.

The server state can interact and communicate with the client state via the net library (and formerly via the umsg library).

Lua code can detect if it is running in the server state by checking if the SERVER global is true.

Engine Lua Files - Server

• lua/includes/init.lua
• lua/gamemodes/base/gamemode/init.lua (this is always loaded regardless of the set gamemode)
• lua/autorun/*.lua
• lua/autorun/server/*.lua
• lua/gamemodes/<gamemode_name>/gamemode/init.lua
• lua/gamemodes/base/entities/entities/*.lua
• lua/gamemodes/base/entities/entities/*/init.lua
• lua/gamemodes/base/entities/weapons/*.lua
• lua/gamemodes/base/entities/weapons/*/init.lua
• lua/entities/*.lua
• lua/entities/*/init.lua
• lua/weapons/*.lua
• lua/weapons/*/init.lua
• lua/gamemodes/<gamemode_name>/entities/entities/*.lua
• lua/gamemodes/<gamemode_name>/entities/entities/*/init.lua
• lua/gamemodes/<gamemode_name>/entities/weapons/*.lua
• lua/gamemodes/<gamemode_name>/entities/weapons/*/init.lua

Menu

The menu state is an isolated internal Lua state on the game client, used solely for the main menu. It has extra functions missing from the client state. Menu modifications and addons are not officially supported, thus running menu state code requires overriding one of the engine's Lua files. For this reason, you won't need to worry about making your code work on the menu, unless you're deliberately overwriting something.

The menu state cannot interact with the client or server states; that is, the menu state cannot run any function that will make a callback occur on the client or server. The client and menu states can indirectly communicate with each other through the file library as they always share a common filesystem.

Lua code can detect if it is running in the menu state by checking if the MENU_DLL global is true.

Engine Lua Files

• lua/includes/init_menu.lua
• lua/derma/init.lua
• lua/menu/menu.lua
• lua/vgui/*.lua

Other

These are not actual states; rather, they signify that the function or hook they represent can be executed in either one of their specified states:

• Shared (Client and Server)

• Client and Menu

• Shared and Menu (all states - Client, Server and Menu)

These don't necessarily mean the function/hook will return the same values in the different states, or that it does the same thing. For example, the function Entity:GetPos can be called on either Server or Client, so it is considered "shared." undo.GetTable is also a shared function, but it works differently on Client and Server, as explained in its description. On the other hand, libraries like math and string have identical functionality in all 3 states.

"Shared" isn't a state

It's important to remember that the Server and Client states (known colloquially as "Shared") are completely separate at runtime, and they cannot coexist. Meaning, something like this:

makes no sense, because the condition SERVER and CLIENT will always return false due to both states' mutual exclusivity. Only one of the two can be true at any given point.

So, don't be fooled by the "Shared state"; it's not really a state at all. "Shared" does not mean that CLIENT and SERVER will both be true at the same time - it's just a shorthand way to communicate that a piece of code can be run on either the Server or Client at any point, but not both "at once."