std::mutex

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

Member functions

Notes

std::mutex is usually not accessed directly: std::unique_lock, std::lock_guard, or std::scoped_lock (since C++17)

Example

#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';
}

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

See also