std::mutex

 

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std::mutex

Member functions

mutex::mutex

mutex::~mutex

Locking

mutex::lock

mutex::try_lock

mutex::unlock

Native handle

mutex::native_handle

 

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.

Nested types

Name	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 an std::map shared between two threads.

Run this code

#include <chrono>
#include <iostream>
#include <map>
#include <mutex>
#include <string>
#include <thread>
 
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& [url, page] : g_pages)
        std::cout << url << " => " << page << '\n';
}

Output:

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

See also

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