std::condition_variable::wait(3)

NAME
std::condition_variable::wait - std::condition_variable::wait

Synopsis
   void wait( std::unique_lock<std::mutex>& lock ); (1) (since C++11)
   template< class Predicate > (2) (since C++11)
   void wait( std::unique_lock<std::mutex>& lock, Predicate pred );
   wait causes the current thread to block until the condition variable is notified or
   a spurious wakeup occurs. pred can be optionally provided to detect spurious wakeup.
   1) Atomically calls lock.unlock() and blocks on *this.
   The thread will be unblocked when notify_all() or notify_one() is executed. It may
   also be unblocked spuriously.
   When unblocked, calls lock.lock() (possibly blocking on the lock), then returns.
   2) Equivalent to
   while (!pred())
       wait(lock);.
   This overload may be used to ignore spurious awakenings while waiting for a specific
   condition to become true.
   If pred() is ill-formed, or
   the return value of pred() is not convertible to bool
   (until C++20)
   decltype(pred()) does not model boolean-testable
   (since C++20), the program is ill-formed.
   Right after wait returns, lock.owns_lock() is true, and lock.mutex() is locked by
   the calling thread. If these postconditions cannot be satisfied^[1], calls
   std::terminate.
   If any of the following conditions is satisfied, the behavior is undefined:
     * lock.owns_lock() is false.
     * lock.mutex() is not locked by the calling thread.
     * If some other threads are also waiting on *this, lock.mutex() is different from
       the mutex unlocked by the waiting functions (wait, wait_for and wait_until)
       called on *this by those threads.
    1. ↑ This can happen if the re-locking of the mutex throws an exception.

Parameters
   lock - an lock which must be locked by the calling thread
   pred - the predicate to check whether the waiting can be completed

Type requirements
   -
   Predicate must meet the requirements of FunctionObject.

Exceptions
   1) Throws nothing.
   2) Any exception thrown by pred.

Notes
   The effects of notify_one()/notify_all() and each of the three atomic parts of
   wait()/wait_for()/wait_until() (unlock+wait, wakeup, and lock) take place in a
   single total order that can be viewed as modification order of an atomic variable:
   the order is specific to this individual condition variable. This makes it
   impossible for notify_one() to, for example, be delayed and unblock a thread that
   started waiting just after the call to notify_one() was made.

Example
// Run this code
 #include <chrono>
 #include <condition_variable>
 #include <iostream>
 #include <thread>
 std::condition_variable cv;
 std::mutex cv_m; // This mutex is used for three purposes:
                  // 1) to synchronize accesses to i
                  // 2) to synchronize accesses to std::cerr
                  // 3) for the condition variable cv
 int i = 0;
 void waits()
 {
     std::unique_lock<std::mutex> lk(cv_m);
     std::cerr << "Waiting... \n";
     cv.wait(lk, []{ return i == 1; });
     std::cerr << "...finished waiting. i == 1\n";
 }
 void signals()
 {
     std::this_thread::sleep_for(std::chrono::seconds(1));
     {
         std::lock_guard<std::mutex> lk(cv_m);
         std::cerr << "Notifying...\n";
     }
     cv.notify_all();
     std::this_thread::sleep_for(std::chrono::seconds(1));
     {
         std::lock_guard<std::mutex> lk(cv_m);
         i = 1;
         std::cerr << "Notifying again...\n";
     }
     cv.notify_all();
 }
 int main()
 {
     std::thread t1(waits), t2(waits), t3(waits), t4(signals);
     t1.join();
     t2.join();
     t3.join();
     t4.join();
 }

Possible output:
 Waiting...
 Waiting...
 Waiting...
 Notifying...
 Notifying again...
 ...finished waiting. i == 1
 ...finished waiting. i == 1
 ...finished waiting. i == 1
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 2135 C++11 the behavior was unclear if calls std::terminate in this
                       lock.lock() throws an exception case

See also
              blocks the current thread until the condition variable is awakened or
   wait_for after the specified timeout duration
              (public member function)
              blocks the current thread until the condition variable is awakened or
   wait_until until specified time point has been reached
              (public member function)
   C documentation for
   cnd_wait

External links
   1. The Old New Thing article: Spurious wake-ups in Win32 condition variables.

Hidden category:
     * Pages with unreviewed LWG DR marker