lock — locks the mutex, blocks if the mutex is not available

    void lock() {
        if (_Mtx_lock(_Mymtx()) != _Thrd_result::_Success) {
            // undefined behavior, only occurs for plain mutexes (N4950 [thread.mutex.requirements.mutex.general]/6)
            _STD _Throw_Cpp_error(_RESOURCE_DEADLOCK_WOULD_OCCUR);
    ...

fn(...)
        std::unique_lock guard(sync);
    ...
    MessageBox(...)
    fn(...)
            std::unique_lock guard(sync);

fn(...)
pva>        std::unique_lock guard(sync);
pva>    ...
pva>    MessageBox(...)
pva>    fn(...)
pva>            std::unique_lock guard(sync);
pva>

LRESULT fufelHandler(blah-blah-blah)
{
   static bool bWeAreInFunction = false;
   if (!bWeAreInFunction)
   {
      bWeAreInFunction = true;
      ...
      doSomething();
      ...
      bWeAreInFunction = false;
   }
   return 0;
}