std::lock
From cppreference.com
| Defined in header <mutex>
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| 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.
Parameters
| lock1, lock2, ... , lockn | - | the Lockable objects to lock |
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
The following example uses std::lock to lock pairs of mutexes without deadlock.
Run this code
#include <mutex> #include <thread> #include <iostream> #include <vector> #include <functional> #include <chrono> #include <string> 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::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'; }
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
See also
| (C++11) |
attempts to obtain ownership of mutexes via repeated calls to try_lock (function template) |
| (C++17) |
deadlock-avoiding RAII wrapper for multiple mutexes (class template) |