Rings: Multiple Lua States

Introduction

Rings is a library which provides a way to create new Lua states from within Lua. It also offers a simple way to communicate between the creator (master) and the created (slave) states.

Rings is free software and uses the same license as Lua 5.1.

Rings also offers Stable, a very simple API to manage a shared table at the master state.

Building

Rings works with Lua 5.1 only. In order to build it the language library and header files for the desired Lua version must be installed properly.

Rings comprises a single C source file. The distribution provides a Makefile prepared to compile the library and install it. The file config should be edited to suit the particularities of the target platform before running make. This file has some definitions like paths to the external libraries, compiler options and the like. One important definition is the Lua version, which is not obtained from the installed software.

Installation

If you are using LuaRocks, just type

luarocks install rings

If you prefer to install manually, the compiled binary file should be copied to a directory in your C path. The file stable.lua should be copied to a directory in your Lua path.

Reference

Master functions

Rings offers a single function which creates a new Lua state and returns an object representing it. The state which creates other states is called the master and the created ones are called slaves. The master can execute code in any of its slaves but each slave only has direct access to its master (or its own slaves).

All standard Lua libraries are opened automatically in a new state; other libraries have to be loaded explicitly.

The object representing a slave state has a method (dostring) which can execute Lua code in the corresponding state. This method can receive arguments (only numbers, strings, booleans and userdata, which are converted to lightuserdata) and always returns a boolean indicating whether the code executed correctly or not, followed by eventual return values or an error message.

rings.new (env): Returns a newly created Lua state. Takes an optional environment to be used by remotedostring. If the environment is nil, it defaults to the master_M or _G tables.
state:close (): Closes the state.
state:dostring (string, ...): Executes a string in the slave state. The arguments could be accessed exactly as in a vararg function. Valid types of arguments and return values are: number, string, boolean, nil and userdata (which are converted to lightuserdata).
Returns a boolean indicating the status of the operation, followed by the returned values or an error message in case of error.

Slave function

The following function is registered in the newly created slave state.

remotedostring (string, ...): Executes a string in the master state. Behaves exactly as the method dostring except that it acts in the master state.

Stable

Stable is a simple API which provides a way for a slave state to store and retrieve data to and from its master state. This library is not opened automatically in a slave state.

stable.get (key): Returns the value of a given key.
stable.set (key, value): Stores a value associated to a key. Returns nothing.

Examples

The following sample shows how to execute code in another state passing arguments and returning values:

require"rings"
S = rings.new ()

data = { 12, 13, 14, }
print (S:dostring ([[
aux = {}
for i, v in ipairs ({...}) do
   table.insert (aux, 1, v)
end
return unpack (aux)]], unpack (data))) -- true, 14, 13, 12

S:close ()

The following example uses Stable to store a value in the master state:

require"rings"

local init_cmd = [[
require"stable"]]

local count_cmd = [[
count = stable.get"shared_counter" or 0
stable.set ("shared_counter", count + 1)
return count
]]

S = rings.new () -- new state
assert(S:dostring (init_cmd))
print (S:dostring (count_cmd)) -- true, 0
print (S:dostring (count_cmd)) -- true, 1
S:close ()

S = rings.new () -- another new state
assert (S:dostring (init_cmd))
print (S:dostring (count_cmd)) -- true, 2
S:close ()