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

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

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.

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.

Noncompliant Code Example

This noncompliant code example creates several threads that each invoke the do_work() function, passing a unique number as an ID.

Unfortunately, this code contains a race condition, allowing the mutex to be destroyed while it is still owned, because start_threads() may invoke the mutex's destructor before all of the threads have exited.

#include <mutex>
#include <thread>

const size_t maxThreads = 10;

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

  // Access data protected by the lock.
}

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

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

Compliant Solution

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

#include <mutex>
#include <thread>

const size_t maxThreads = 10;

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

  // Access data protected by the lock.
}

std::mutex m;

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

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

Compliant Solution

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 maxThreads = 10;

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

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

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

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

Risk Assessment

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

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography