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diff --git a/src/tet3/apithr/api_lock.c b/src/tet3/apithr/api_lock.c
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+/*
+ *	SCCS: @(#)api_lock.c	1.9 (98/08/28)
+ *
+ *	UniSoft Ltd., London, England
+ *
+ * (C) Copyright 1996 X/Open Company Limited
+ *
+ * All rights reserved.  No part of this source code may be reproduced,
+ * stored in a retrieval system, or transmitted, in any form or by any
+ * means, electronic, mechanical, photocopying, recording or otherwise,
+ * except as stated in the end-user licence agreement, without the prior
+ * permission of the copyright owners.
+ * A copy of the end-user licence agreement is contained in the file
+ * Licence which accompanies this distribution.
+ * 
+ * X/Open and the 'X' symbol are trademarks of X/Open Company Limited in
+ * the UK and other countries.
+ */
+
+#ifndef lint
+static char sccsid[] = "@(#)api_lock.c	1.9 (98/08/28) TET3 release 3.3";
+#endif
+
+
+/************************************************************************
+
+SCCS:   	@(#)api_lock.c	1.9 98/08/28 TETware release 3.3
+NAME:		API lock/unlock function
+PRODUCT:	TETware
+AUTHOR:		Geoff Clare, UniSoft Ltd.
+DATE CREATED:	July 1996
+SYNOPSIS:
+
+	void	tet_api_lock(int getlock, char *file, int line);
+
+DESCRIPTION:
+
+	Tet_api_lock() is used to implement a top-level API mutex
+	that can be used in a nested fashion via the macros API_LOCK
+	and API_UNLOCK.  When an API function calls another the
+	underlying mutex is not unlocked at the end of the called
+	function, only at the end of the calling function.
+
+	On UNIX systems all blockable signals are blocked while the mutex
+	is held, so that the thread cannot call TET_THR_EXIT() from a signal
+	handler (leaving the mutex locked).  Any API function which needs
+	to catch a signal must be sure to unblock it.
+
+MODIFICATIONS:
+	Geoff Clare, UniSoft Ltd., Oct 1996
+	Use TET_THR_EQUAL() to compare thread IDs.
+
+	Geoff Clare, UniSoft Ltd., July 1997
+	Changes to support NT threads.
+
+	Andrew Dingwall, UniSoft Ltd., February 1998
+	Use TETware-specific macros to access threads functions and
+	data items.
+
+************************************************************************/
+
+#include <string.h>
+#include <signal.h>
+#include "dtmac.h"
+#include "error.h"
+#include "dtthr.h"
+#include "sigsafe.h"
+
+#ifndef NOTRACE
+#include "ltoa.h"
+#endif
+
+#ifdef NEEDsrcFile
+static char srcFile[] = __FILE__;
+#endif
+
+extern tet_mutex_t tet_top_mtx;
+
+void
+tet_api_lock(getlock, file, line)
+int getlock;
+char *file;
+int line;
+{
+	sigset_t tmpset;
+	static sigset_t oset;
+	static int nestlevel;
+	static tet_thread_t ownertid; /* only valid when nestlevel > 0 */
+
+	if (getlock)
+	{
+		TRACE3(tet_Ttcm, 5, "API_LOCK requested from %s, %s",
+			file, tet_i2a(line));
+
+		/* grab the top-level mutex, if not nested */
+		if (nestlevel == 0 || !TET_THR_EQUAL(ownertid, TET_THR_SELF()))
+		{
+			if (TET_THR_SIGSETMASK(SIG_BLOCK, &tet_blockable_sigs,
+				&tmpset) != 0)
+			{
+				fatal(0, "TET_THR_SIGSETMASK() failed in tet_api_lock()", (char *)0);
+			}
+			TET_MUTEX_LOCK(&tet_top_mtx);
+			ownertid = TET_THR_SELF();
+
+			/* now it's safe to store the old signal set */
+			(void) memcpy((void *)&oset, (void *)&tmpset, sizeof oset);
+		}
+		nestlevel++;
+
+		TRACE4(tet_Ttcm, 5, "API_LOCK (%s, %s) nestlevel %s",
+			file, tet_i2a(line), tet_i2a(nestlevel));
+	}
+	else
+	{
+		/* release the top-level mutex, if not nested */
+		ASSERT(nestlevel > 0);
+		ASSERT(TET_THR_EQUAL(ownertid, TET_THR_SELF()));
+		TRACE4(tet_Ttcm, 5, "API_UNLOCK (%s, %s) nestlevel %s",
+			file, tet_i2a(line), tet_i2a(nestlevel));
+		nestlevel--;
+		if (nestlevel == 0)
+		{
+			/* copy signal set to safe storage before unlocking */
+			(void) memcpy((void *)&tmpset, (void *)&oset, sizeof oset);
+
+			TET_MUTEX_UNLOCK(&tet_top_mtx);
+
+			(void) TET_THR_SIGSETMASK(SIG_SETMASK, &tmpset, (sigset_t *)0);
+		}
+	}
+}