include/osl/doublecheckedlocking.h

   1 /* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
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  20 /*
  21  * This file is part of LibreOffice published API.
  22  */
  23
  24 #ifndef 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     @code{.cpp}
  51       static T * pInstance = 0;
  52
  53       T * p = pInstance;
  54       if (!p)
  55       {
  56           Guard aGuard(aMutex);
  57           p = pInstance;
  58           if (!p)
  59           {
  60               p = ...;
  61               OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER();
  62               pInstance = p;
  63           }
  64       }
  65       else
  66           OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER();
  67       return p;
  68     @endcode
  69
  70     One extra advantage of this macro is that it makes it easier to find all
  71     places where double-checked locking is used.
  72  */
  73 #define OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER() /* empty */
  74
  75 #if defined __cplusplus
  76 }
  77 #endif /* __cplusplus */
  78
  79 #endif /* INCLUDED_OSL_DOUBLECHECKEDLOCKING_H */
  80
  81 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */