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std::lock

From cppreference.com
< cpp‎ | thread
Defined in header <mutex>
template< class Lockable1, class Lockable2, class LockableN... >
void lock( Lockable1& lock1, Lockable2& lock2, LockableN& lockn... );
(since C++11)

Locks the given Lockable objects lock1, lock2, ..., lockn using a deadlock avoidance algorithm to avoid deadlock.

The objects are locked by an unspecified series of calls to lock, try_lock, unlock. If a call to lock or unlock results in an exception, unlock is called for any locked objects before rethrowing.

Contents

[edit] Parameters

lock1, lock2, ... , lockn - the Lockable objects to lock

[edit] Return value

(none)

[edit] Notes

Boost provides a version of this function that takes a sequence of Lockable objects defined by a pair of iterators.

[edit] Example

The following example uses std::lock to lock pairs of mutexes without deadlock.

#include <mutex>
#include <thread>
#include <iostream>
#include <vector>
#include <functional>
#include <chrono>
 
struct Employee {
    Employee(std::string id) : id(id) {}
    std::string id;
    std::vector<std::string> lunch_partners;
    std::mutex m;
    std::string output() const
    {
        std::string ret = "Employee " + id + " has lunch partners: ";
        for( const auto& partner : lunch_partners )
            ret += partner + " ";
        return ret;
    }
};
 
void send_mail(Employee &, Employee &)
{
    // simulate a time-consuming messaging operation
    std::this_thread::sleep_for(std::chrono::seconds(1));
}
 
void assign_lunch_partner(Employee &e1, Employee &e2)
{
    {
        static std::mutex io_mutex;
        std::lock_guard<std::mutex> lk(io_mutex);
        std::cout << e1.id << " and " << e2.id << " are waiting for locks" << std::endl;
    }
 
    // use std::lock to acquire two locks without worrying about 
    // other calls to assign_lunch_partner deadlocking us
    {
        std::unique_lock<std::mutex> lk1(e1.m, std::defer_lock);
        std::unique_lock<std::mutex> lk2(e2.m, std::defer_lock);
        std::lock(lk1, lk2);
        std::cout << e1.id << " and " << e2.id << " got locks" << std::endl;
        e1.lunch_partners.push_back(e2.id);
        e2.lunch_partners.push_back(e1.id);
    }
    send_mail(e1, e2);
    send_mail(e2, e1);
}
 
int main()
{
    Employee alice("alice"), bob("bob"), christina("christina"), dave("dave");
 
    // assign in parallel threads because mailing users about lunch assignments
    // takes a long time
    std::vector<std::thread> threads;
    threads.emplace_back(assign_lunch_partner, std::ref(alice), std::ref(bob));
    threads.emplace_back(assign_lunch_partner, std::ref(christina), std::ref(bob));
    threads.emplace_back(assign_lunch_partner, std::ref(christina), std::ref(alice));
    threads.emplace_back(assign_lunch_partner, std::ref(dave), std::ref(bob));
 
    for (auto &thread : threads) thread.join();
    std::cout << alice.output() << '\n'  << bob.output() << '\n'
              << christina.output() << '\n' << dave.output() << '\n';
}

Possible output:

alice and bob are waiting for locks
alice and bob got locks
christina and bob are waiting for locks
christina and bob got locks
christina and alice are waiting for locks
christina and alice got locks
dave and bob are waiting for locks
dave and bob got locks
Employee alice has lunch partners: bob christina 
Employee bob has lunch partners: alice christina dave 
Employee christina has lunch partners: bob alice 
Employee dave has lunch partners: bob

[edit] See also

(C++11)
attempts to obtain ownership of mutexes via repeated calls to try_lock
(function template) [edit]