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Lua/ToLua - Memory leak

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Hi, I've recently made use of ToLua to export my Vector and Matrix classes for use in Lua scripts. After much messing around with my new toy, my swap disk started to go crazy and my game would segfault. After some testing with a system activity monitor, it revealed my game's memory size grew approx. 1MB every second. I've narrowed the problem down to ToLua, and after some Googling I think it's something to do with "tag methods". Here's some example code: Vector class (C++):

        float x;
        float y;
        float z;

        SYM_VECTOR3() { x = y = z = 0.0f; }

        SYM_VECTOR3(float _x, float _y, float _z)
            x = _x;
            y = _y;
            z = _z;


Matrix class (C++):
		SYM_MATRIX4() { Reset(); }

		SYM_MATRIX4(float *Matrix)
			memcpy(_Matrix, Matrix, 16 * sizeof(float));


SYM_VECTOR3 RotateVector(SYM_VECTOR3 Vector)
			return SYM_VECTOR3( Vector.x * get(0, 0) + Vector.y * get(1, 0) + Vector.z * get(2, 0),
                                Vector.x * get(0, 1) + Vector.y * get(1, 1) + Vector.z * get(2, 1),
                                Vector.x * get(0, 2) + Vector.y * get(1, 2) + Vector.z * get(2, 2));

inline float get(const int col, const int row) const
			return _Matrix[col * 4 + row];

//The 4x4 matrix
		float _Matrix[16];

A troublesome Lua script:
-- Experimental hovercraft script

-- Get matrix
Matrix = getmatrix("HCraft")
-- Get force direction in global space
ThrusterDir = Matrix:RotateVector(SYM_VECTOR3:new(0, 1, 0))

To generate the binding Lua script, I use this command:
tolua -o lua_matrix.cpp -H lua_matrix.h -n sym_matrix matrix.h
tolua -o lua_vector.cpp -H lua_vector.h -n sym_vector vector.h
Any help appreciated

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After some reading, I've discovered this:


User defined types

All other types that appear in the package file being processed are considered user defined types. These are mapped to tagged userdata type in Lua. Lua can only store pointers to user defined types; although, tolua automatically makes the necessary arrangement to deal with references and values. For instance, if a function or method returns a value of user defined type, tolua allocates a clone object when returning it to Lua and sets the garbage collection tag method to automatically free the allocated object when no longer in use by Lua.

How can I pass a user type value without these pointers building up? Here's a few examples of how I've tried getting it to work:

Works for testing purposes, but memory obviously builds up:

SYM_MATRIX4 *Matrix = new SYM_MATRIX4(Engine->Scene.GetGeometry(lua_tostring(LuaState, 1))->GetMatrix());
tolua_pushusertype(LuaState, Matrix, "SYM_MATRIX4");

Gives invalid results on the Lua end:

tolua_pushusertype(LuaState, (void*)&Engine->Scene.GetGeometry(lua_tostring(LuaState, 1))->GetMatrix(), "SYM_MATRIX4");

Any ideas?

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I haven't tired toLua myself, instead I'm using MLuaBind and tried this test out, it's not leaking any memory even after a million iterations of this. I suggest using MLuaBind, its a modern template Lua/C++ binding library and it's excellent! I got it working in minutes and it auto detects parameters and binding for funcitons and methods.

You'll need Loki, a support library.

Try it out!


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I believe the problem lies in calling SYM_VECTOR3:new(0, 1, 0) rather than just doing SYM_VECTOR3(0, 1, 0). Tolua sets up a collection scheme so that temporary and local objects are collected using the tag methods for new and delete, as long as they are constructed using, ie, SYM_VECTOR3() instead of explicitly calling the new function. The new function doesn't mark the created object as being managed by the garbage collector. The function SYM_VECTOR3() does something similar to this:

function create_SYM_VECTOR3(x,y,z)
local obj=SYM_VECTOR3:new(x,y,z)
return obj

The takeownership part is the key part; otherwise, it's pretty much just a raw new call, and what you are doing is tantamount to performing raw new calls in straight C++, without deleting.

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