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Development < Lua Scripting < Lua-LuaCOB

NOTE LuaCob will merged with LuaRules in 76b2


To use Lua code in your BOS/COB files, you can either include the gadgetHandler (include it in the mod, see the CA mod or include it from the springcontent.sdz) or you create your own enviroment:

  • create a mod-level directory LuaCOB/,
  • make a LuaCOB/main.lua file. (Note: it's LuaRules/main.lua now)

Any additional files need to be included with a

include("LuaCOB/additional_file.lua" [, fenvTable])

If you don't know what a fenvTable is, don't worry, you probably don't need it anyway. More information can be found on the Lua site. It it highly recommended to read the manual and the tutorials found there. Note that Spring 76b1 used Lua version is 5.1.2, so please take note which version of the manual you're reading.

Also note that the usual Lua functions for executing code in other files (dofiles(), require()) are disabled for technical reasons.

Interfacing COB->Lua

All Lua functions that are called by COB are given three parameters:

  • unitID - ID of the unit calling the function,
  • unitDefID - ID of the unitDef,
  • teamID - self-explanatory.

You can call scripts with any (enough for all practical purpouses) amount of additional arguments.

Example functions:

 function GetNearestEnemy(unitID, unitDefID, teamID, radius)
   local px, py, pz = Spring.GetUnitPosition(unitID)
   if (px == nil) then
   local nearDist = 1.0e20
   local nearUnit
   local units = Spring.GetUnitsInCylinder(px, pz, radius)
   for _,enemyID in ipairs(units) do
     if (not Sim.IsUnitAllied(enemyID)) then             
       local ex, ey, ez = Spring.GetUnitPosition(enemyID)
       if (ex ~= nil) then
         local dx, dy, dz = (px - ex), (py - ey), (pz - ez)
         local dist = (dx * dx) + (dy * dy) + (dz + dz)
         if (dist < nearDist) then
           nearUnit = enemyID
           nearDist = dist
   if (nearUnit == nil) then
     return -1
     return nearUnit

Please note that returning tables doesn't currently work as one could expect. (Description what it does here) If you want to return multiple values from a table, you need to do it like this:

function ReturnSomeValues(u, ud, t, ...)
    return arg[1], arg[2], arg[3]  -- instead of arg

Calling Lua from BOS

To call a Lua function from BOS code, you need to declare it first:

// BOS
lua_MyFunction() { return 0; }

Then just call this script. The lua_ prefix does its magic and routes the call to Lua code instead of calling the usual BOS code. Note that arguments don't matter, they are passed anyway:

call-script lua_MyFunction(arg1, arg2);

Getting output

If you wish to print out some debugging info, please note that print and io.write don't quite work in Windows versions of Spring, since stdout isn't availible (or is well-hidden.) Use Spring.Echo("a string"), this will print a message to the Spring console and into the infolog.


// the GET/SET constants
#define LUA0 110
#define LUA1 111
#define LUA2 112
#define LUA3 113
#define LUA4 114
#define LUA5 115
#define LUA6 116
#define LUA7 117
#define LUA8 118
#define LUA9 119

get LUA0 is used to check whether call succeeded (1 if true, 0 otherwise.) LUA1..9 store return values, e.g. lua_ReturnSomeValues above would put its first argument into get LUA1, second into get LUA2, and so on.


Steps needed to call a Lua function in a BOS call-script call:

  • define a Lua function MyFunction(unitID, unitDefID, teamID, ...) in LuaCOB/main.lua or a file included by it
  • define a BOS/COB script lua_MyFunction() { return 0; } so the compiler knows about it
  • call your function with call-script lua_MyFunction()
  • return values can be found in get LUA1..9
  • get LUA0 returns 1 when call was successful (regardless of its return values) and 0 in case of failure.

Examples of code

  • Inter-unit communication (shameless plug ;p) - a library that allows passing messages between units and reading/writing of public variables exposed by them.Included is a tech demo involving modified Nanoblobs 0.64. Patch, start the game, make a sheep and see for yourself.
  • Your own!

Interfacing Lua->COB


Spring.CallCOBScript(unitID, "CustomScript", retArgCount, arg1, ..., argN)
Spring.CallCOBScriptCB(unitID, "CustomScript", retArgCount, callbackData, arg1, ..., argN)
retArgCount:  the number of arguments expected to be returned
callbackData: number passed to CobCallback() when the cob call completes


CustomScript(arg1, ..., argN) { ... }

One particularly useful COB script, since Lua doesn't seem to have a way of making get SOMETHING(args) requests (but it may provide other ways of getting the same information):

LuaGet(r, cmd, p1, p2, p3, p4)
 if (r == 0) { r = get cmd;                 return r }
 if (r == 1) { r = get cmd(p1,  0,  0,  0); return r }
 if (r == 2) { r = get cmd(p1, p2,  0,  0); return r }
 if (r == 3) { r = get cmd(p1, p2, p3,  0); return r }
 if (r == 4) { r = get cmd(p1, p2, p3, p4); return r }
 return 1234;


  1. Spring Lua Scripts forum
  2. trepan's description of Lua-COB interface - a little bit outdated, but still worth a read


trepan for doing the hard work of integrating Lua into Spring!

NOTE from trepan:

 Thanks, but there were lua startscripts before I started doing
 any Spring coding. The benefits of the new LuaCob, LuaGaia, and
 LuaRules scripts are that they can be loaded from map/mod archives,
 and that they all use the same base libraries (as does LuaUI).
 They also have significantly more features then the current
 startscript code (ex: more call-ins and call-outs, the ability to
 render graphics in several different modes, play sounds, etc...)