include/osl/doublecheckedlocking.h

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  19
  20 #ifndef INCLUDED_OSL_DOUBLECHECKEDLOCKING_H
  21 #define INCLUDED_OSL_DOUBLECHECKEDLOCKING_H
  22
  23 #if defined __cplusplus
  24 extern "C" {
  25 #endif /* __cplusplus */
  26
  27 /** A platform specific macro needed to make double-checked locking work.
  28
  29     See
  30     <http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html>
  31     for a description of double-checked locking, why it is broken, and how it
  32     can be fixed.  On platforms where it is necessary, this macro will expand
  33     to some memory barrier instruction.  On many platforms, double-checked
  34     locking works as it is, though, so on those platforms this macro will be
  35     empty.  This is a macro instead of a (C++ inline) function to allow for
  36     maximum performance in both C and C++.
  37
  38     If possible, use the rtl_Instance template instead of explicitly spelling
  39     out the double-checked locking pattern.  There are few cases where you
  40     will have to spell it out explicitly (e.g., the logic of a certain
  41     instance of the pattern is too complex to be mapped to the template, or
  42     some compiler refuses to compile a template instantiation due to internal
  43     compiler errors), though, and you should always call this macro at the
  44     right places then:
  45
  46     @code{.cpp}
  47       static T * pInstance = 0;
  48
  49       T * p = pInstance;
  50       if (!p)
  51       {
  52           Guard aGuard(aMutex);
  53           p = pInstance;
  54           if (!p)
  55           {
  56               p = ...;
  57               OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER();
  58               pInstance = p;
  59           }
  60       }
  61       else
  62           OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER();
  63       return p;
  64     @endcode
  65
  66     One extra advantage of this macro is that it makes it easier to find all
  67     places where double-checked locking is used.
  68  */
  69 #define OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER() /* empty */
  70
  71 #if defined __cplusplus
  72 }
  73 #endif /* __cplusplus */
  74
  75 #endif /* INCLUDED_OSL_DOUBLECHECKEDLOCKING_H */
  76
  77 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */