1 /************************************************************** 2 * 3 * Licensed to the Apache Software Foundation (ASF) under one 4 * or more contributor license agreements. See the NOTICE file 5 * distributed with this work for additional information 6 * regarding copyright ownership. The ASF licenses this file 7 * to you under the Apache License, Version 2.0 (the 8 * "License"); you may not use this file except in compliance 9 * with the License. You may obtain a copy of the License at 10 * 11 * http://www.apache.org/licenses/LICENSE-2.0 12 * 13 * Unless required by applicable law or agreed to in writing, 14 * software distributed under the License is distributed on an 15 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY 16 * KIND, either express or implied. See the License for the 17 * specific language governing permissions and limitations 18 * under the License. 19 * 20 *************************************************************/ 21 22 23 24 #if !defined INCLUDED_OSL_DOUBLECHECKEDLOCKING_H 25 #define INCLUDED_OSL_DOUBLECHECKEDLOCKING_H 26 27 #if defined __cplusplus 28 extern "C" { 29 #endif /* __cplusplus */ 30 31 /** A platform specific macro needed to make double-checked locking work. 32 33 See 34 <http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html> 35 for a description of double-checked locking, why it is broken, and how it 36 can be fixed. On platforms where it is necessary, this macro will expand 37 to some memory barrier instruction. On many platforms, double-checked 38 locking works as it is, though, so on those platforms this macro will be 39 empty. This is a macro instead of a (C++ inline) function to allow for 40 maximum performance in both C and C++. 41 42 If possible, use the rtl_Instance template instead of explicitly spelling 43 out the double-checked locking pattern. There are few cases where you 44 will have to spell it out explicitly (e.g., the logic of a certain 45 instance of the pattern is too complex to be mapped to the template, or 46 some compiler refuses to compile a template instantiation due to internal 47 compiler errors), though, and you should always call this macro at the 48 right places then: 49 50 static T * pInstance = 0; 51 52 T * p = pInstance; 53 if (!p) 54 { 55 Guard aGuard(aMutex); 56 p = pInstance; 57 if (!p) 58 { 59 p = ...; 60 OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER(); 61 pInstance = p; 62 } 63 } 64 else 65 OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER(); 66 return p; 67 68 One extra advantage of this macro is that it makes it easier to find all 69 places where double-checked locking is used. 70 */ 71 #define OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER() /* empty */ 72 73 #if defined __cplusplus 74 } 75 #endif /* __cplusplus */ 76 77 #endif /* INCLUDED_OSL_DOUBLECHECKEDLOCKING_H */ 78