std::notify_all_at_thread_exit(3)

NAME
std::notify_all_at_thread_exit - std::notify_all_at_thread_exit

Synopsis
   Defined in header <condition_variable>
   void notify_all_at_thread_exit( std::condition_variable& cond, (since C++11)
                        std::unique_lock<std::mutex> lk );
   notify_all_at_thread_exit provides a mechanism to notify other threads that a given
   thread has completely finished, including destroying all thread_local objects. It
   operates as follows:
     * Ownership of the previously acquired lock lk is transferred to internal storage.
     * The execution environment is modified such that when the current thread exits,
       the condition variable cond is notified as if by lk.unlock();
       cond.notify_all();.
   The implied lk.unlock() is sequenced after the destruction of all objects with
   thread local storage duration associated with the current thread.
   If any of the following conditions is satisfied, the behavior is undefined:
     * lk is not locked by the calling thread.
     * If some other threads are also waiting on cond, lk.mutex() is different from the
       mutex unlocked by the waiting functions (wait, wait_for and wait_until) called
       on cond by those threads.

Notes
   An equivalent effect may be achieved with the facilities provided by std::promise or
   std::packaged_task.
   The supplied lock lk is held until the thread exits. Once this function has been
   called, no more threads may acquire the same lock in order to wait on cond. If some
   threads are waiting on this condition variable, ensure that the condition being
   waited for is satisfied while holding the lock on lk, and that this lock is not
   released and reacquired prior to calling notify_all_at_thread_exit to avoid
   confusion from spurious wakeups in other threads.
   In typical use cases, this function is the last thing called by a detached thread.

Parameters
   cond - the condition variable to notify at thread exit
   lk - the lock associated with the condition variable cond

Return value
   (none)

Example
   This partial code fragment illustrates how notify_all_at_thread_exit can be used to
   avoid accessing data that depends on thread locals while those thread locals are in
   the process of being destructed: // Run this code
 #include <cassert>
 #include <condition_variable>
 #include <mutex>
 #include <string>
 #include <thread>
 std::mutex m;
 std::condition_variable cv;
 bool ready = false;
 std::string result; // some arbitrary type
 void thread_func()
 {
     thread_local std::string thread_local_data = "42";
     std::unique_lock<std::mutex> lk(m);
     // assign a value to result using thread_local data
     result = thread_local_data;
     ready = true;
     std::notify_all_at_thread_exit(cv, std::move(lk));
 } // 1. destroy thread_locals;
     // 2. unlock mutex;
     // 3. notify cv.
 int main()
 {
     std::thread t(thread_func);
     t.detach();
     // do other work
     // ...
     // wait for the detached thread
     std::unique_lock<std::mutex> lk(m);
     cv.wait(lk, []{ return ready; });
     // result is ready and thread_local destructors have finished, no UB
     assert(result == "42");
 }
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
                       the call to notify_all_at_thread_exit updated the
   LWG 2140 C++11 synchronized with calls to functions waiting synchronization
                       on cond requirement

See also
                        sets the result to specific value while delivering the
   set_value_at_thread_exit notification only at thread exit
                        (public member function of std::promise<R>)
                        executes the function ensuring that the result is ready
   make_ready_at_thread_exit only once the current thread exits
                        (public member function of std::packaged_task<R(Args...)>)