sal/inc/osl/doublecheckedlocking.h

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  30
  31 #if !defined INCLUDED_OSL_DOUBLECHECKEDLOCKING_H
  32 #define INCLUDED_OSL_DOUBLECHECKEDLOCKING_H
  33
  34 #if defined __cplusplus
  35 extern "C" {
  36 #endif /* __cplusplus */
  37
  38 /** A platform specific macro needed to make double-checked locking work.
  39
  40     See
  41     <http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html>
  42     for a description of double-checked locking, why it is broken, and how it
  43     can be fixed.  On platforms where it is necessary, this macro will expand
  44     to some memory barrier instruction.  On many platforms, double-checked
  45     locking works as it is, though, so on those platforms this macro will be
  46     empty.  This is a macro instead of a (C++ inline) function to allow for
  47     maximum performance in both C and C++.
  48
  49     If possible, use the rtl_Instance template instead of explicitly spelling
  50     out the double-checked locking pattern.  There are few cases where you
  51     will have to spell it out explicitly (e.g., the logic of a certain
  52     instance of the pattern is too complex to be mapped to the template, or
  53     some compiler refuses to compile a template instantiation due to internal
  54     compiler errors), though, and you should always call this macro at the
  55     right places then:
  56
  57       static T * pInstance = 0;
  58
  59       T * p = pInstance;
  60       if (!p)
  61       {
  62           Guard aGuard(aMutex);
  63           p = pInstance;
  64           if (!p)
  65           {
  66               p = ...;
  67               OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER();
  68               pInstance = p;
  69           }
  70       }
  71       else
  72           OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER();
  73       return p;
  74
  75     One extra advantage of this macro is that it makes it easier to find all
  76     places where double-checked locking is used.
  77  */
  78 #define OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER() /* empty */
  79
  80 #if defined __cplusplus
  81 }
  82 #endif /* __cplusplus */
  83
  84 #endif /* INCLUDED_OSL_DOUBLECHECKEDLOCKING_H */