When compiling a program, the compiler adds some optimizations that may cause your application to misbehave. For example consider the following code:

 // To avoid threads waiting on the critical section in vainif (m_instance == NULL){EnterCiriticalSection(pcs);if (m_instance == NULL)m_instance = new MyInstance();}

The compiler may cache the second condition (m_instance == NULL) and not update the content of m_instance if it has been changed by another thread. The solution is to declare the instance with the volatile keyword. This tells the compiler to get the content of m_instance every time it is used and not cache its content.

The declaration is:

 volatile MyInstance* m_instance;