std::mutex (3) - Linux Man Pages

std::mutex: std::mutex

NAME

std::mutex - std::mutex

Synopsis


Defined in header <mutex>
class mutex; (since C++11)


The mutex class is a synchronization primitive that can be used to protect shared data from being simultaneously accessed by multiple threads.
mutex offers exclusive, non-recursive ownership semantics:


* A calling thread owns a mutex from the time that it successfully calls either lock or try_lock until it calls unlock.
* When a thread owns a mutex, all other threads will block (for calls to lock) or receive a false return value (for try_lock) if they attempt to claim ownership of the mutex.
* A calling thread must not own the mutex prior to calling lock or try_lock.


The behavior of a program is undefined if a mutex is destroyed while still owned by any threads, or a thread terminates while owning a mutex. The mutex class satisfies all requirements of Mutex and StandardLayoutType.
std::mutex is neither copyable nor movable.

Member types


Member type Definition
native_handle_type(optional) implementation-defined

Member functions


              constructs the mutex
constructor (public member function)
              destroys the mutex
destructor (public member function)


operator= not copy-assignable
              (public member function)
[deleted]

Locking


              locks the mutex, blocks if the mutex is not available
lock (public member function)
              tries to lock the mutex, returns if the mutex is not available
try_lock (public member function)
              unlocks the mutex
unlock (public member function)

Native handle


              returns the underlying implementation-defined native handle object
native_handle (public member function)

Notes


std::mutex is usually not accessed directly: std::unique_lock, std::lock_guard,
or std::scoped_lock
(since C++17) manage locking in a more exception-safe manner.

Example


This example shows how a mutex can be used to protect a std::map shared between two threads.
// Run this code


  #include <iostream>
  #include <map>
  #include <string>
  #include <chrono>
  #include <thread>
  #include <mutex>


  std::map<std::string, std::string> g_pages;
  std::mutex g_pages_mutex;


  void save_page(const std::string &url)
  {
      // simulate a long page fetch
      std::this_thread::sleep_for(std::chrono::seconds(2));
      std::string result = "fake content";


      std::lock_guard<std::mutex> guard(g_pages_mutex);
      g_pages[url] = result;
  }


  int main()
  {
      std::thread t1(save_page, "http://foo");
      std::thread t2(save_page, "http://bar");
      t1.join();
      t2.join();


      // safe to access g_pages without lock now, as the threads are joined
      for (const auto &pair : g_pages) {
          std::cout << pair.first << " => " << pair.second << '\n';
      }
  }

Output:


  http://bar => fake content
  http://foo => fake content