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std::mutex

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Revision as of 19:01, 24 March 2013 by Cubbi (Talk | contribs)

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.

Contents

Member types

Member type Definition
native_handle_type implementation-defined

Member functions

Template:cpp/thread/mutex/dcl list constructorTemplate:cpp/thread/mutex/dcl list lockTemplate:cpp/thread/mutex/dcl list try lockTemplate:cpp/thread/mutex/dcl list unlockTemplate:cpp/thread/mutex/dcl list native handle
Locking
Native handle

Example

This example shows how a mutex can be used to protect a std::map shared between two threads.

#include <iostream>
#include <chrono>
#include <thread>
#include <mutex>
#include <map>
#include <string>
 
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