predicate for condition variable

Question

I am new to multi threading. While writing multi threaded code in C++11 using condition variable , I use the following construct

while(predicate) {
    cond_var.wait(&lock);
}

However, I have been reading Deitel's third edition book on operating systems(chp 6) where the following construct is being used

if(predicate) {
    cond_var.wait(&lock);
}

So, what's the difference? Why isn't the book using while? Isn't spurious call an issue?

Answer 1

Spurious wakeup is always a potential issue. For example, look at the answers here: Do spurious wakeups actually happen?. Perhaps Deitel's code is part of a larger loop that can help them deal with the spurious wakeup? Or maybe it's just a typo.

In any case, there's never a (good) reason not to use your construct, and in fact the wait function has a variant that does it for you (http://en.cppreference.com/w/cpp/thread/condition_variable/wait).

template< class Predicate >
void wait( std::unique_lock<std::mutex>& lock, Predicate pred );

which is equivalent to:

while (!pred()) {
     wait(lock);
}