1 /************************************************************************* 2 * 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * Copyright 2000, 2010 Oracle and/or its affiliates. 6 * 7 * OpenOffice.org - a multi-platform office productivity suite 8 * 9 * This file is part of OpenOffice.org. 10 * 11 * OpenOffice.org is free software: you can redistribute it and/or modify 12 * it under the terms of the GNU Lesser General Public License version 3 13 * only, as published by the Free Software Foundation. 14 * 15 * OpenOffice.org is distributed in the hope that it will be useful, 16 * but WITHOUT ANY WARRANTY; without even the implied warranty of 17 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 18 * GNU Lesser General Public License version 3 for more details 19 * (a copy is included in the LICENSE file that accompanied this code). 20 * 21 * You should have received a copy of the GNU Lesser General Public License 22 * version 3 along with OpenOffice.org. If not, see 23 * <http://www.openoffice.org/license.html> 24 * for a copy of the LGPLv3 License. 25 * 26 ************************************************************************/ 27 28 #if !defined INCLUDED_OSL_DOUBLECHECKEDLOCKING_H 29 #define INCLUDED_OSL_DOUBLECHECKEDLOCKING_H 30 31 #if defined __cplusplus 32 extern "C" { 33 #endif /* __cplusplus */ 34 35 /** A platform specific macro needed to make double-checked locking work. 36 37 See 38 <http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html> 39 for a description of double-checked locking, why it is broken, and how it 40 can be fixed. On platforms where it is necessary, this macro will expand 41 to some memory barrier instruction. On many platforms, double-checked 42 locking works as it is, though, so on those platforms this macro will be 43 empty. This is a macro instead of a (C++ inline) function to allow for 44 maximum performance in both C and C++. 45 46 If possible, use the rtl_Instance template instead of explicitly spelling 47 out the double-checked locking pattern. There are few cases where you 48 will have to spell it out explicitly (e.g., the logic of a certain 49 instance of the pattern is too complex to be mapped to the template, or 50 some compiler refuses to compile a template instantiation due to internal 51 compiler errors), though, and you should always call this macro at the 52 right places then: 53 54 static T * pInstance = 0; 55 56 T * p = pInstance; 57 if (!p) 58 { 59 Guard aGuard(aMutex); 60 p = pInstance; 61 if (!p) 62 { 63 p = ...; 64 OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER(); 65 pInstance = p; 66 } 67 } 68 else 69 OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER(); 70 return p; 71 72 One extra advantage of this macro is that it makes it easier to find all 73 places where double-checked locking is used. 74 */ 75 #define OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER() /* empty */ 76 77 #if defined __cplusplus 78 } 79 #endif /* __cplusplus */ 80 81 #endif /* INCLUDED_OSL_DOUBLECHECKEDLOCKING_H */ 82