The idea is to wait on some condition - here called the guard.

But evaluating the guard must be done exclusively by only one thread!

Code using a condition variable - cond_var - would look like something like this:

pthread_mutex_lock(&mutex);
while(!guard)
{
  pthread_cond_wait(&cond_var, &mutex);
}
statement 1;
...
statement n;
pthread_mutex_unlock(&mutex);