boost::condition_variable and lock

Question

boost::condition_variable cond;
boost::mutex mut;

//thread1
{
"read_socket()"
cond.notify_one();
}

//thread2
{
for(;;)
{
...
boost::unique_lock<boost::mutex> lock(mut);
cond.wait(lock);
}
}

versus

boost::condition_variable cond;
boost::mutex mut;

//thread1
{
"read_socket()"
boost::unique_lock<boost::mutex> lock(mut);
cond.notify_one();
}

//thread2
{
for(;;)
{
...
boost::unique_lock<boost::mutex> lock(mut);
cond.wait(lock);
}

Is there an impact if I omit the lock before calling cond.notify_one() ?

Answer 1

The C++11 standard does not state any requirement for notify_one and notify_all; so not holding the lock when you signal a condition_variable is fine. However, it's often necessary for the signaling thread to hold the lock until it sets the condition checked by the waiting thread after it's woken up. If it does not, the program may contain races. For an example, see this SO question: Boost synchronization.