C++ Core Guidelines: Sharing Data between Threads


If you want to have fun with threads, you should share mutable data between them. In order to get no data race and, therefore, undefined behaviour, you have to think about the synchronisation of your threads.



The three rules in this post are maybe quite obvious for the experienced multithreading developer but very crucial for the novice in the multithreading domain. Here are they:

 Let's start with the most obvious rule.

CP.20: Use RAII, never plain lock()/unlock()

No naked mutex! Put your mutex always in a lock. The lock will automatically release (unlock) the mutex if it goes out of scope. RAII stands for Resource Acquisition Is Initialization and means that you bind the lifetime of a resource to the lifetime of a local variable. C++ automatically manages the lifetime of locals. 

std::lock_guard, std::unique_lock, std::shared_lock (C++14), or std::std::scoped_lock (C++17) implement this pattern but also the smart pointers std::unique_ptr, and std::shared_ptr. My previous post Garbage Collection - No Thanks  explains the details to RAII.

What does this mean for your multithreading code?

std::mutex mtx;

void do_stuff()
    // ... do stuff ... (1)


It doesn't matter if an exception occurs in (1) or you just forgot to unlock the mtx; in both cases, you will get a deadlock if another thread wants to acquire (lock) the std::mutex mtx. The rescue is quite obvious.

std::mutex mtx;

void do_stuff()
    std::lock_guard<std::mutex> lck {mtx};
    // ... do stuff ...
}                 // (1)


Put the mutex into a lock and the mutex will automatically be unlocked at (1) because the lck goes out of scope.

CP.21: Use std::lock() or std::scoped_lock to acquire multiple mutexes

If a thread needs more than one mutex, you have to be extremely careful that you lock the mutexes always in the same sequence. If not, a bad interleaving of threads may cause a deadlock. The following program causes a deadlock.

// lockGuardDeadlock.cpp

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

struct CriticalData{
  std::mutex mut;

void deadLock(CriticalData& a, CriticalData& b){

  std::lock_guard<std::mutex>guard1(a.mut);           // (2)        
  std::cout << "Thread: " << std::this_thread::get_id() <<  std::endl;

  std::lock_guard<std::mutex>guard2(b.mut);          // (2)
  std::cout << "Thread: " << std::this_thread::get_id() <<  std::endl;
  // do something with a and b (critical region)        (3)

int main(){

  std::cout << std::endl;

  CriticalData c1;
  CriticalData c2;

  std::thread t1([&]{deadLock(c1, c2);});            // (1)
  std::thread t2([&]{deadLock(c2, c1);});            // (1)


  std::cout << std::endl;



Thread t1 and t2 need two resources CriticalData to perform their job (3). CriticalData has its own mutex mut to synchronise the access. Unfortunately, both invoke the function deadlock with the arguments c1 and c2 in a different sequence (1). Now we have a race condition. If thread t1 can lock the first mutex a.mut but not the second one b.mut because in the meantime thread t2 locks the second one, we will get a deadlock (2).


The easiest way to solve the deadlock is to lock both mutexes atomically.

With C++11, you can use an std::unique_lock together with std::lock. std::unique_lock you can defer the locking of its mutex. The function std::lock, which can lock an arbitrary number of mutexes in an atomic way, does the locking finally.

void deadLock(CriticalData& a, CriticalData& b){
    std::unique_lock<mutex> guard1(a.mut, std::defer_lock);
    std::unique_lock<mutex> guard2(b.mut, std::defer_lock);
    std::lock(guard1, guard2);
    // do something with a and b (critical region)


With C++17, an std::scoped_lock can lock an arbitrary number of mutex in one atomic operation.

void deadLock(CriticalData& a, CriticalData& b){
    std::scoped_lock(a.mut, b.mut);
    // do something with a and b (critical region


CP.22: Never call unknown code while holding a lock (e.g., a callback)

Why is this code snippet really bad?

std::mutex m;
{ std::lock_guard<std::mutex> lockGuard(m); sharedVariable = unknownFunction(); }


I can only speculate about the unknownFunction. If unknownFunction

  • tries to lock the mutex m, that will be undefined behaviour. Most of the times, you will get a deadlock.
  • starts a new thread that tries to lock the mutex m, you will get a deadlock.
  • locks another mutex m2 you may get a deadlock because you lock the two mutexes m and m2 at the same time. Now it can happen that another thread locks the same mutexes in a different sequence.
  • will not directly or indirectly try to lock the mutex m; all seems to be fine. “Seems” because your coworker can modify the function or the function is dynamically linked, and you get a different version. All bets are open to what may happen.
  • work as expected you may have a performance problem because you don’t know how long the function unknownFunction would take. What meant to be a multithreaded program may become a single-threaded program.

To solve these issues, use a local variable:

std::mutex m;
auto tempVar = unknownFunction(); { std::lock_guard<std::mutex> lockGuard(m); sharedVariable = tempVar; }


This additional indirection solves all issues. tempVar is a local variable and can, therefore, not be a victim of a data race. This means that you can invoke unknownFunction without a synchronisation mechanism. Additionally, the time for holding a lock is reduced to its bare minimum: assigning the value of tempVar to sharedVariable.

What's next?

If you don't call join or detach on your created thread child, the child will throw an std::terminate exception in its destructor. std::terminatecalls per default std::abort. To overcome this issue, the guidelines support library has a gsl::joining_thread which calls join at the end of its scope. I will have a closer look at gsl::joining_thread in my next post.




Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, Marko, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, espkk, Louis St-Amour, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Neil Wang, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Tobi Heideman, Daniel Hufschläger, Red Trip, Alexander Schwarz, Tornike Porchxidze, Alessandro Pezzato, Evangelos Denaxas, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Dimitrov Tsvetomir, Leo Goodstadt, Eduardo Velasquez, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, Robin Furness, and Michael Young.


Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, and Bhushan Ivatury.



My special thanks to Embarcadero CBUIDER STUDIO FINAL ICONS 1024 Small



I'm happy to give online seminars or face-to-face seminars worldwide. Please call me if you have any questions.

Bookable (Online)


Standard Seminars (English/German)

Here is a compilation of my standard seminars. These seminars are only meant to give you a first orientation.


Contact Me

Modernes C++,





Tags: lock, mutex

My Newest E-Books

Course: Modern C++ Concurrency in Practice

Course: C++ Standard Library including C++14 & C++17

Course: Embedded Programming with Modern C++

Course: Generic Programming (Templates)

Course: C++ Fundamentals for Professionals

Interactive Course: The All-in-One Guide to C++20

Subscribe to the newsletter (+ pdf bundle)

Blog archive

Source Code


Today 2961

Yesterday 5806

Week 2961

Month 172378

All 7440218

Currently are 182 guests and no members online

Kubik-Rubik Joomla! Extensions

Latest comments