| /************************************************************** |
| * |
| * Licensed to the Apache Software Foundation (ASF) under one |
| * or more contributor license agreements. See the NOTICE file |
| * distributed with this work for additional information |
| * regarding copyright ownership. The ASF licenses this file |
| * to you under the Apache License, Version 2.0 (the |
| * "License"); you may not use this file except in compliance |
| * with the License. You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, |
| * software distributed under the License is distributed on an |
| * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
| * KIND, either express or implied. See the License for the |
| * specific language governing permissions and limitations |
| * under the License. |
| * |
| *************************************************************/ |
| |
| |
| |
| #if !defined INCLUDED_OSL_DOUBLECHECKEDLOCKING_H |
| #define INCLUDED_OSL_DOUBLECHECKEDLOCKING_H |
| |
| #if defined __cplusplus |
| extern "C" { |
| #endif /* __cplusplus */ |
| |
| /** A platform specific macro needed to make double-checked locking work. |
| |
| See |
| <http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html> |
| for a description of double-checked locking, why it is broken, and how it |
| can be fixed. On platforms where it is necessary, this macro will expand |
| to some memory barrier instruction. On many platforms, double-checked |
| locking works as it is, though, so on those platforms this macro will be |
| empty. This is a macro instead of a (C++ inline) function to allow for |
| maximum performance in both C and C++. |
| |
| If possible, use the rtl_Instance template instead of explicitly spelling |
| out the double-checked locking pattern. There are few cases where you |
| will have to spell it out explicitly (e.g., the logic of a certain |
| instance of the pattern is too complex to be mapped to the template, or |
| some compiler refuses to compile a template instantiation due to internal |
| compiler errors), though, and you should always call this macro at the |
| right places then: |
| |
| static T * pInstance = 0; |
| |
| T * p = pInstance; |
| if (!p) |
| { |
| Guard aGuard(aMutex); |
| p = pInstance; |
| if (!p) |
| { |
| p = ...; |
| OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER(); |
| pInstance = p; |
| } |
| } |
| else |
| OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER(); |
| return p; |
| |
| One extra advantage of this macro is that it makes it easier to find all |
| places where double-checked locking is used. |
| */ |
| #define OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER() /* empty */ |
| |
| #if defined __cplusplus |
| } |
| #endif /* __cplusplus */ |
| |
| #endif /* INCLUDED_OSL_DOUBLECHECKEDLOCKING_H */ |