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It was definitely realloc, in the high priority thread of an audio callback. I fixed the problem (no more crashes) by disabling garbage collection at the start of any Lua calls in this thread, and re- enabling at the end (only 2 C functions in my case), and using a pthread mutex lock to safely call the garbage collector periodically from a safe, low-priority thread.

Some example code follows. I use two Lua states and a mutex in order to share Lua data safely between two threads (it was a requirement for my application). Of course, the way I'm doing this might not be ideal, and if anyone wants to point out a glaring error I make, please do!

lua_State * L = lua_open();
lua_State * Lpriority = lua_newthread(Lidle);
pthread_mutex_t	Lmutex;
pthread_mutex_init(&Lmutex, NULL);

// idle low priority thread function:
void lua_idle()
pthread_mutex_lock(&Lmutex); // lock can block, but that's ok in the idle thread

	//.. do stuff with L
	// 	keep it short, to minimize mutex lock time
	//	if lengthy activity is needed, probably a third temporary lua_State
// could be used instead, and results xmove'd to L within the mutex lock


// high priority thread function:
void lua_priority()
lua_gc(Lpriority, LUA_GCSTOP, 0); // disable gc during this function call

	if (pthread_mutex_trylock(&Lmutex)) 			// trylock does not block
lua_xmove(L, Lpriority, lua_gettop(L)); // copy stack from L to Lpriority
		pthread_mutex_unlock(&Lmutex);			// finished working with L

		//.. do stuff with Lpriority,
		//	safely knowing that it will not collect garbage
// and that the lua_State will not be corrupted by calls to lua_idle()

	lua_gc(Lpriority, LUA_GCRESTART, 0);		// re-enable gc

On Dec 13, 2006, at 11:59 AM, Peter Pimley wrote:

"avoid using free/realloc etc, which I assume is the cause of the


What I'd do first is to write a wrapper that just calls free/realloc,
but logs* each call and alerts* you if it fails.  At least then you'd
know for sure that this is where the trouble lies, rather than assuming
(* How you do these will depend on your hardware of course)

The example function (
could be modified pretty easily.

The thing is; if realloc IS failing then you're in trouble whichever way
you look at it.  Are you sure it's not some other part of your program
leaking or fragmenting memory?


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