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 a false return 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.
Contents |
[edit]Nested types
| Name | Definition |
native_handle_type(optional*) | implementation-defined |
[edit]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) | |
[edit]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.
[edit]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(conststd::string& url){// simulate a long page fetchstd::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 joinedfor(constauto&[url, page]: g_pages)std::cout<< url <<" => "<< page <<'\n';}
Output:
http://bar => fake content http://foo => fake content
[edit]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) |
