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(not always present) implementation-defined

Member functions¶

constructor constructs the mutex
(public member function)
destructor destroys the mutex
(public member function)
operator= not copy-assignable
[deleted] (public member function)

Locking¶

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

Native handle¶

native_handle returns the underlying implementation-defined native handle object
(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 an 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

See also¶

recursive_mutex provides mutual exclusion facility which can be locked
(C++11) recursively by the same thread
(class)
lock_guard implements a strictly scope-based mutex ownership wrapper
(C++11) (class template)
unique_lock implements movable mutex ownership wrapper
(C++11) (class template)
scoped_lock deadlock-avoiding RAII wrapper for multiple mutexes
(C++17) (class template)
condition_variable provides a condition variable associated with a std::unique_lock
(C++11) (class)