std::lock

Since C++11

Header: <mutex>

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

# Declarations

template< class Lockable1, class Lockable2, class... LockableN >
void lock( Lockable1& lock1, Lockable2& lock2, LockableN&... lockn );

(since C++11)

# Parameters

# Return value

(none)

# Notes

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

std::scoped_lock offers a RAII wrapper for this function, and is generally preferred to a naked call to std::lock.

# Example

#include <chrono>
#include <functional>
#include <iostream>
#include <mutex>
#include <string>
#include <thread>
#include <vector>
 
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 (auto n{lunch_partners.size()}; const auto& partner : lunch_partners)
            ret += partner + (--n ? ", " : "");
        return ret;
    }
};
 
void send_mail(Employee&, Employee&)
{
    // Simulate a time-consuming messaging operation
    std::this_thread::sleep_for(std::chrono::milliseconds(696));
}
 
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::lock(e1.m, e2.m);
        std::lock_guard<std::mutex> lk1(e1.m, std::adopt_lock);
        std::lock_guard<std::mutex> lk2(e2.m, std::adopt_lock);
    // Equivalent code (if unique_locks are needed, e.g. for condition variables)
    //  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);
    // Superior solution available in C++17
    //  std::scoped_lock lk(e1.m, e2.m);
        {
            std::lock_guard<std::mutex> lk(io_mutex);
            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';
}

# See also