doublecheckedlocking.h File Reference

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Macros
#define	OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER()   /* empty */
	A platform specific macro needed to make double-checked locking work. More...

Macro Definition Documentation

OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER

#define OSL_DOUBLE_CHECKED_LOCKING_MEMORY_BARRIER

(

)

/* empty */

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.