std::mutex
From cppreference.com
| 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
mutexfrom the time that it successfully calls eitherlockortry_lockuntil it callsunlock. - When a thread owns a
mutex, all other threads will block (for calls tolock) or receive afalsereturn value (fortry_lock) if they attempt to claim ownership of themutex. - A calling thread must not own the
mutexprior to callinglockortry_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
| constructs the mutex (public member function) | |
| destroys the mutex (public member function) | |
operator= [deleted] |
not copy-assignable (public member function) |
Locking | |
| locks the mutex, blocks if the mutex is not available (public member function) | |
| tries to lock the mutex, returns if the mutex is not available (public member function) | |
| unlocks the mutex (public member function) | |
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
(C++11) |
provides mutual exclusion facility which can be locked recursively by the same thread (class) |
(C++11) |
implements a strictly scope-based mutex ownership wrapper (class template) |
(C++11) |
implements movable mutex ownership wrapper (class template) |
(C++17) |
deadlock-avoiding RAII wrapper for multiple mutexes (class template) |
(C++11) |
provides a condition variable associated with a std::unique_lock (class) |