Re: Protect bool on concurrent access

Peter Dimov <>
Sun, 1 Jun 2008 18:44:55 CST
On Jun 1, 11:57 am, Johan Torp <> wrote:

I have a bool which will transition from false to true once. One
thread sets the bool to true (once), the other reads it every now and
then. I know C++03 and 0x has undefined behaviour, but in practice...

Is it necessary to protect such bools? Are there any known hardware/
platform/compilers which might cause problems?

Yes, this will cause problems on multi-processor/multi-core machines.

What problems? Will the program crash on any known compiler/


char storage[ sizeof(T) ]; // ignoring alignment issues
bool init = false;


new( storage ) T();
init = true;


if( init )

Without memory synchronization, there is no guarantee that T2 will see
an initialized T object. If f is virtual, it may well crash because of
an invalid vtable pointer. For proper operation the store to init
needs to have release semantics, and the load - acquire semantics.

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