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Mutexes Mutex objects are used to protect shared data structures from being concurrently accessed. If a mutex object is destroyed while a thread is blocked waiting for that mutexthe lock, critical sections and shared data are no longer protected.

The C++ Standard, [thread.mutex.class], paragraph 5 [ISO/IEC 14882-2014], states the following:

The behavior of a program is undefined if it destroys a mutex object owned by any thread or a thread terminates while owning a mutex object.

This statement implies Similar wording exists for std::recursive_mutex, std::timed_mutex, std::recursive_timed_mutex, and std::shared_timed_mutex. These statements imply that destroying a mutex object while a thread is waiting on it is undefined behavior.

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Unfortunately, this code contains a race condition, allowing the mutex to be destroyed before while it is unlockedstill owned, because start_threads() may invoke the lockmutex's destructor before all of the threads have finished using the lock. This behavior is undefined.exited.

#include <mutex>
#include <thread>

const size_t max_threadsmaxThreads = 10;

void do_work(size_t i, std::mutex *pm) {
  std::lock_guard<std::mutex> guardlk(*pm);

  // Access data protected by the lock.
}

void start_threads() {
  std::thread threads[max_threadsmaxThreads];
  std::mutex m;

  for (size_t i = 0; i < max_threadsmaxThreads; ++i) {
    threads[i] = std::thread(do_work, i, &m);
  }
}

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This compliant solution eliminates the race condition by extending the lifetime of the lock:mutex.

#include <mutex>
#include <thread>

const size_t max_threadsmaxThreads = 10;

void do_work(size_t i, std::mutex *pm) {
  std::lock_guard<std::mutex> guardlk(*pm);

  // Access data protected by the lock.
}

std::mutex m;

void start_threads() {
  std::thread threads[max_threadsmaxThreads];

  for (size_t i = 0; i < max_threadsmaxThreads; ++i) {
    threads[i] = std::thread(do_work, i, &m);
  }
}

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This compliant solution eliminates the race condition by joining the threads before the mutex's destructor is invoked:.

#include <mutex>
#include <thread>

const size_t max_threadsmaxThreads = 10;

void do_work(size_t i, std::mutex *pm) {
  std::lock_guard<std::mutex> guardlk(*pm);

  // Access data protected by the lock.
}
void run_threads() {
  std::thread threads[max_threadsmaxThreads];
  std::mutex m;

  for (size_t i = 0; i < max_threadsmaxThreads; ++i) {
    threads[i] = std::thread(do_work, i, &m);
  }

  for (size_t i = 0; i < max_threadsmaxThreads; ++i) {
    threads[i].join();
  }
}

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Destroying a mutex while it is locked may result in invalid control flow and data corruption.

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