std::mutex

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

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

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Member functions
mutex::mutex
mutex::~mutex
Locking
mutex::lock
mutex::try_lock
mutex::unlock
Native handle
mutex::native_handle

std::mutex

Member types

Member functions

Locking

Native handle

Notes

Example