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 eitherlock
ortry_lock
until it callsunlock
. - When a thread owns a
mutex
, all other threads will block (for calls tolock
) or receive a false return value (fortry_lock
) if they attempt to claim ownership of themutex
. - A calling thread must not own the
mutex
prior to callinglock
ortry_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
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 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