< cpp‎ | thread
Concurrency support library
this_thread namespace
Mutual exclusion
Generic lock management
Condition variables
Latches and Barriers
Safe Reclamation
Hazard Pointers

Atomic types
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
Memory ordering
Free functions for atomic operations
Free functions for atomic flags
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, and unlock. If a call to lock or unlock results in an exception, unlock is called for any locked objects before rethrowing.


[edit] Parameters

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

[edit] Return value


[edit] 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.

[edit] Example

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

#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
void assign_lunch_partner(Employee& e1, Employee& e2)
    static std::mutex io_mutex;
        std::lock_guard<std::mutex> lk(io_mutex);
        std::cout << << " and " << << " 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 << << " and " << << " got locks" << std::endl;
    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)
    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
Dave and Bob are waiting for locks
Dave and Bob got locks
Christina and Alice 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

implements movable mutex ownership wrapper
(class template) [edit]
attempts to obtain ownership of mutexes via repeated calls to try_lock
(function template) [edit]
deadlock-avoiding RAII wrapper for multiple mutexes
(class template) [edit]