std::mutex

 

C++

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Thread support library

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thread (C++11)
this_thread namespace
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mutex (C++11) timed_mutex (C++11)	recursive_mutex (C++11) recursive_timed_mutex (C++11)
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lock_guard (C++11) unique_lock (C++11) defer_lock_t try_to_lock_t adopt_lock_t (C++11) (C++11) (C++11)	lock (C++11) try_lock (C++11) defer_lock try_to_lock adopt_lock (C++11) (C++11) (C++11)
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promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11)	launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

 

std::mutex

Member functions

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 some thread. The mutex class is non-copyable.

Member types

Member type	Definition
native_handle_type	implementation-defined

Member functions

(constructor)	constructs the mutex (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 thread handle (public member function)

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";
 
    g_pages_mutex.lock();
    g_pages[url] = result;
    g_pages_mutex.unlock();
}
 
int main() 
{
    std::thread t1(save_page, "http://foo");
    std::thread t2(save_page, "http://bar");
    t1.join();
    t2.join();
 
    g_pages_mutex.lock(); // not necessary as the threads are joined, but good style
    for (const auto &pair : g_pages) {
        std::cout << pair.first << " => " << pair.second << '\n';
    }
    g_pages_mutex.unlock();
}

Output:

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