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libsst-concurrency/SST_Once.c

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+/*
+	SST_Once.c
+	Author: Patrick Baggett <ptbaggett@762studios.com>
+	Created: 12/20/2011
+
+	Purpose: 
+
+	libsst-concurrency "once" functions. This requires libsst-atomic.
+
+	License:
+
+	This program is free software. It comes without any warranty, to
+	the extent permitted by applicable law. You can redistribute it
+	and/or modify it under the terms of the Do What The Fuck You Want
+	To Public License, Version 2, as published by Sam Hocevar. See
+	http://sam.zoy.org/wtfpl/COPYING for more details.
+
+*/
+
+#include <SST/SST_Once.h>
+#include <SST/SST_Atomic.h>
+#include <SST/SST_Concurrency.h>
+#include <SST/SST_MemBarrier.h>
+
+#define SST_ONCE_INPROGRESS_BIT	0x80000000 /* Highest bit */
+#define SST_ONCE_DONE_BIT		0x00000001 /* Lowest bit */
+
+#define SST_ONCE_DONE 0xd1d d0
+#ifdef _DEBUG
+#include <stdlib.h>
+#endif
+
+static void wait_done(volatile int* vonce)
+{
+	/* Spin carefully */
+	while((*vonce & SST_ONCE_DONE_BIT) == 0)
+	{
+		SST_Concurrency_YieldThread();
+		SST_Concurrency_Spin();
+	}
+}
+
+void SST_Concurrency_ExecOnce(SST_Once* once, SST_OnceFunc once_func, void* arg)
+{
+	int state;
+	volatile int* vonce = (volatile int*)once;
+	
+	/* Take a snapshot of the current state */
+	state = *vonce;
+	
+
+	/* Already done? */
+	if((state & SST_ONCE_DONE_BIT) != 0)
+		return;
+		
+	/* In progress? */
+	if((state & SST_ONCE_INPROGRESS_BIT) != 0)
+	{
+		wait_done(vonce);
+		return;
+	}
+		
+	/* Not done, not yet started -> attempt to set SST_ONCE_INPROGRESS bit.  */
+	if(SST_Atomic_CAS(vonce, SST_ONCE_INIT, SST_ONCE_INIT | SST_ONCE_INPROGRESS_BIT) == SST_ONCE_INIT)
+	{
+		/* Success: call user's once function */
+		once_func(arg);
+			
+		/* Done, so set that we are done */
+		SST_Atomic_Or(vonce, SST_ONCE_DONE_BIT);
+	}
+	else /* Already started or done, so just chill */
+		wait_done(vonce);
+
+}
+
+/*************************************************************************/
+
+void SST_Concurrency_InitOnceHandle(SST_Once* once)
+{
+	*once = SST_ONCE_INIT;
+	
+	/* This is the only tricky part -- a memory barrier. An atomic
+	operation could be used, but honestly, that's overkill for just
+	ensuring an atomic write. */
+	SST_Concurrency_MemoryBarrier();
+}
+
+