Multi Threading#

Given Julia’s application in high-performance computing and its native multi-threading support, it is only natural a Julia wrapper should also allow for asynchronous execution in a similarly convenient and performant manner.

In relation to specifically a C++ <-> Julia environment, we run into a problem, however:

#include <jluna.hpp>
#include <thread>

int main()
    // initialize
    // create a lambda that does nothing
    auto noop = []() -> void {
    // execute it in a std::thread
    auto thread = std::thread(noop);
    // wait fo thread to finish
    // exit main
    return 0;

Here, we’re doing a very simple operation. We created a C++ lambda that does nothing. It executes no Julia-side code by handing jl_eval_string (the C-APIs way to execute strings as code) an empty string, after which it simply returns. We then create a C++-side thread using std::thread. In C++, once a thread is created, it starts running immediately. We wait for its execution to finish using .join().

Note that not a single jluna function was called over the runtime of this main. All functions where purely Julia C-API functions.

Running the above code, the following happens:

signal (11): Segmentation fault
in expression starting at none:0
Allocations: 782030 (Pool: 781753; Big: 277); GC: 1

Process finished with exit code 139 (interrupted by signal 11: SIGSEGV)

It segfaults without an error.

I am unsure of the exact reason for this behavior, but some have stated that it is related to the way Julia initializes thread local storage (TLS), which is the environment of code ran inside a thread. Only Julia threads themselves allocate the TLS correctly, when running Julia from a non-Julia thread, it will look for it’s allocated TLS, which isn’t there, hence the segfault.

While this was apparently fixed in Julia version 1.9, for those of us using an earlier Julia version, this makes things quite difficult for us. We cannot access the Julia state from within a C++-side thread. This has nothing to do with jluna, it is how the C-API was designed.

Given this, we can immediately throw out using any of the C++ std::thread-related multi-threading support, as well as libraries like libuv. It is important to keep this in mind, if our application already uses these frameworks, we have to take care to never execute code that interacts with Julia from within a thread. This includes any of jlunas functionalities, as it is, of course, build entirely on the Julia C-API.

All is not lost, however: jluna offers its own multi-threading framework, allowing for parallel execution of truly arbitrary C++ code - even if that code interacts with the Julia state.

Initializing the Julia Threadpool#

In Julia, we have to decide the number of threads we want to use before startup. In the Julia REPL, we would use the -threads (or -t) argument. In jluna, we instead specify the desired number of threads as an argument to jluna::initialize:

int main()
    // equivalent to `julia -t 8`
    // application here
    return 0;
[JULIA][LOG] initialization successful (8 threads).

If left unspecified, jluna will initialize Julia with exactly 1 thread. We can set the number of threads to auto by supplying the following constant to jluna::initialize:

using namespace jluna;

// equivalent to `julia -t auto`

This sets the number of threads to number of local CPUs, just like setting environment variable JULIA_NUM_THREADS to auto would do for pure Julia.

Note that any already existing JULIA_NUM_THREAD variable, in the environment the jluna executable is run in, is ignored and overridden. We can only specify the number of threads through jluna::initialize.

Creating a Task#

Owing to its status of being in-between two languages with differing vocabulary and design, jlunas thread pool architecture borrows from both C++ and Julia.

To execute a C++-side piece of code, we have to first wrap it into a C++ lambda, then wrap that lambda in a jluna::Task.

We cannot initialize a task directly, rather, we use jluna::ThreadPool::create:

using namespace jluna;

// declare lambda that prints to console and returns its argument
auto forward_arg = [](size_t in) -> size_t {
    Base["println"]("lambda called with ", in);
    return in;

// create task
auto task = ThreadPool::create<size_t(size_t)>( // signature
    forward_arg,    // function
    size_t(1234)    // function arguments

Here, ThreadPool::create takes multiple arguments:

  • its template argument is the signature of the lambda

    • Just like with as_julia_function, it expects a C-style signature. forward_arg has the signature (size_t) -> size_t, making size_t(size_t) the appropriate template argument.

  • the first argument is the function object itself

    • this can be a lambda, like forward_arg, or a std::function object.

  • any following arguments will be used as the arguments for the given function

    • In our case, because we specified size_t(1234), the thread pool will invoke our lambda forward_arg with that argument (and only that argument).

Note that create invokes the copy constructor on all its argument. If this behavior is not desired, we can wrap the argument in a std::reference_wrapper using std::ref, meaning only the reference itself will be copied, not the actual object.

Unlike with as_julia_function, the signature of lambdas used for ThreadPool::create is unrestricted - any lambda can be used.

Running a Task#

Unlike C++-threads (but much like Julia tasks), jluna::Task does not immediately start execution once it is constructed. We need to manually “start” is using .schedule().

We can wait for its execution to finish using .join(). This stalls the thread .join() was called from (usually the master thread in which main is executed) until the task completes:

// declare lambda
std::function<size_t(size_t)> forward_arg = [](size_t in) -> size_t {
    Base["println"]("lambda called with ", in);
    return in;

// create task
auto task = ThreadPool::create(forward_arg, size_t(1234));

// start task

// wait for task to finish
lambda called with 1234

Note how, even though we called the Julia function println, the task did not segfault. Using jlunas thread pool is the only way to call C++-side functions that also access the Julia state concurrently.

Managing a Tasks Lifetime#

The result of ThreadPool::create is a jluna::Task<T>, where T is the return type of the C++ function used to create it, or void. The user is responsible for keeping the task in memory. If the variable the task is bound to, goes out of scope, the task simply ends:

/// in main.cpp

// open block scope
    // declare function that counts to 9999
    std::function<void()> count_to_9999 = []() -> void
        for (size_t i = 0; i < 9999; ++i)
            std::cout << i << std::endl;

    // create task
    auto task = ThreadPool::create(count_to_9999);
    // start task

    // wait for 1 millisecond
    using namespace std::chrono_literals;
// task is destructed here

std::this_thread::sleep_for(10ms); // wait for another 10ms
return 0; // exit main

Process finished with exit code 0

C++ Hint: A block or anonymous block scope is created using { }. It acts similar to Julia’s begin end, anything declared inside the block will go out of scope when the block ends. In C++, all variables declared inside the block are local to only that block.

C++ Hint: std::chrono are C++s time-related functionalities. calling std::this_thread::sleep_for(1ms) will stall the master thread for 1 millisecond.

Here, our task is supposed to count all the way up to 9999. Instead, the task went out of scope before it could finish, only being able to count to 2611 before it was terminated.

A way to solve this is to store the task in a collection that is itself in master scope:

// task storage
std::vector<Task<void>> tasks;

    // declare lambda
    std::function<void()> print_numbers = []() -> void
        for (size_t i = 0; i < 10000; ++i)
            std::cout << i << std::endl;

    // add task to storage
    // start just pushed task
    // wait for 1ms

// wait for another 10ms
return 0;

Process finished with exit code 0

This time, because the task was not destructed prematurely, it had 10 milliseconds more time to finish. This happened to be enough for it to reach its intended count, after which it was safely destructed when main returned.

jluna has no equivalent to Julia’s Threads.@spawn. This is to force users to keep track of their tasks. To .schedule a task, they need to first cache it in a variable. This hopefully avoids situations where tasks end unexpectedly because they go out of scope.

Accessing a Tasks State#

We can check the status of any task using the following member functions:

  • is_done: is the task finished or failed, c.f. Base.istaskdone

  • is_failed: has the task failed, c.f. Base.istaskfailed

  • is_running: has the task been scheduled but not yet finished, c.f. Base.istaskstarted

Furthermore, any Julia function that works with tasks can be called directly using the jluna::Task object as an argument.

Accessing a Tasks Result#

Returning to our example from before:

// declare lambda
std::function<size_t(size_t)> forward_arg = [](size_t in) -> size_t {
    // print
    Base["println"]("lambda called with ", in);
    // return argument
    return in;

// create task, schedule, wait for it to finish
auto task = ThreadPool::create(forward_arg, size_t(1234));

Here, forward_arg does more than just print to the command line, it also returns a value.

To access the return value of a task, we use the member function .result(). This function returns an object of type jluna::Future.

A future is a thread-safe container that may or may not (yet) contain a value. The future itself is available immediately when its corresponding task is created.

auto task = ThreadPool::create(forward_arg, size_t(1234));
auto future = task.result();

Until the task has successfully completed, however, the future will be “empty”. Once the task is done, the return value will be copied into the future, after which we can access it. If we want to avoid the copying, we need to wrap the result in a std::reference_wrapper.

To get the potential value of a future, we use .get(), which returns a std::optional<T> where T is the return type of the C++ function used to create the task. Once completed, we can access the value of the optional using std::optional::value(). To check whether the value is already available, we can use jluna::Future::is_available():

// create task
auto task = ThreadPool::create(forward_arg, size_t(1234));

// get future of task (currently empty)
auto future = task.result();

std::cout << future.is_available() << std::endl; // false

// start task

// wait for task to finish

std::cout << future.is_available() << std::endl; // true

// print value of future
std::cout << future.get().value() << std::endl;

C++ Hint: Trying to access the value of a std::optional before it is available will raise an exception. See the standard libraries documentation for more ways to interact with std::optional<T>.

We can wait for the value of a future to become available by calling .wait(). This will stall the thread .wait() is called from until the value becomes accessible, after which the function will return that value. This way, we don’t necessarily need to keep track of the futures task, just having the future allows us to access the task’s result. We do still need to make sure the corresponding task stays in scope, however.

Data Race Freedom#

The user is responsible for any potential data races a jluna::Task may trigger. Useful C++-side tools for this application include the following (where their Julia-side functional equivalent is listed for reference):



C++ Documentation













Furthermore, jluna provides its own lock-like object jluna::Mutex, which is a simple wrapper around a Julia-side Base.ReentrantLock. It has the same usage and interface as std::mutex, except that it works when called both from C++ and Julia, because it is (Un)Boxable.


As a general rule, any particular part of jluna is thread-safe, as long as two threads are not modifying the same object at the same time.

For example, jluna::safe_call can be called from multiple threads, and safe_call itself will work. If both safe_call modify the same value, however, concurrency artifacts may occur.

Similarly, we can freely create unrelated proxies and modify them individually. If we create two proxies that reference the same Julia-side variable, mutating both proxies (if they are named) at the same time will possibly trigger an error or data corruption.

The user is required to ensure thread-safety in these conditions, just like they would have to in Julia. jluna has no hidden pitfalls or behind-the-scene machinery that multi-threading may throw a wrench into, all its internals (that are inaccessible to the user) are thread-safe as of version 0.9.0.

Any user-created objects are outside of jlunas responsibility, however.

Calls to C++ lambdas forwarded to the Julia state using as_julia_function are thread-safe to call, whether their behavior is thread-safe depends on the user-defined implementation.

Notably, Module::new_*, Module::create and Module::create_or_assign are thread-safe. Each jluna::Module carries exactly one lock, which allows these calls to happen safely in a multi-threaded environment. All other functions of jluna::Module do not make use of this lock. Instead, users will be required to manually “lock” an object, and manage concurrent interaction with it, themselves.

Thread-Safety in Julia#

As an example for how to make modifying an object thread-safe, let’s say we have the following variable in Main:

# in Julia
to_be_modified = []

We want to thread-safely add elements to this vector. One way to do this would be with a Julia-side Lock:

# in Julia
to_be_modified = []
to_be_modified_lock = Base.ReentrantLock()

Now, when a thread wants to modify to_be_modified, it first needs to acquire this lock:

// in cpp

// thread function
auto push_to_modify = [](size_t) -> void
    // access Base.lock
    static auto lock = Base["lock"];
    // access Base.unlock
    static auto unlock = Base["unlock"];
    // access Julia-side lock
    auto* mutex = Main["to_be_modified_lock"];
    // lock 
    // modify vector here
    // unlock

C++ Hint: Using static in block-scope declares (and potentially defines) a variable exactly once. Anytime the code is run through, afterwards, the line with static is skipped. For this example, by making lock and unlock static variables, the lookup performed by Base::operator[] is only done a single time across the entire runtime of the application, increasing performance.

See here for more information on storage classifiers.

Since push_to_modify will be executed through jluna::ThreadPool, the Julia function lock will stall the thread, allowing for safe access.

Thread-Safety in C++#

A similar approach can be taken when trying to safely modify C++-side objects. Instead of a Julia-side lock, we use a C++-side jluna::Mutex:

// C++-side variable we want to thread-safely access
jluna::Vector<size_t> to_be_modified;

// C++-side mutex
auto to_be_modified_lock = jluna::Mutex();

auto push_to_modify = [](size_t)
    // modify here

Similarly, std::mutex or std::unique_lock can be used for the same purpose.

Multi-Threading: Closing Notes#

Interacting with jluna::Task from Julia#

Internally, jluna makes accessing the Julia-state from a C++-sided, asynchronously executed function possible, by wrapping it in a Julia-side Task. jluna can then use Julia’s native thread pool, allowing for C-API functions to be safely executed. This has some side effects, most of them useful.

For example, yield, called from C++ like so:

static auto yield = Main.safe_eval("return Base.yield");

Will actually yield the thread this C++ code is executed in, letting another Julia thread take over. This applies to all Julia-side functions such as fetch, bind, etc.
Calling them from within a jluna::Task has exactly the same effect as calling them from within a Base.Task.

We can access the Julia-side object, jluna::Task is managing, using operator unsafe::Value*(), which returns a raw C-pointer to the Julia-side tasks. This allows us to create a jluna::Proxy of a jluna::Task like so:

// declare lambda
auto lambda = [](){ //...
// create task
auto task = ThreadPool::create<void()>(lambda);

// create proxy to task
auto task_proxy = Proxy(static_cast<unsafe::Value*>(task));

// all Julia-only attributes can now be accessed:
std::cout << (bool) task_proxy["sticky"] << std::endl;

Julia Hint: Threads.Tasks.sticky is a property that governs whether a task can be executed concurrently. By default, sticky is set to false, making it “stick” to the main thread, instead of “detaching” and being run on its own.

We can then use this proxy as we would use a Julia-side variable, allowing for full freedom on how to manage and schedule tasks.

Moving / Copying Tasks#

Each task holds a pointer to an internal state, which manages the task’s future among other things. To preserve this state, copying a task is made impossible (by declaring the tasks copy constructor and assignment operator as deleted):

auto f = [](){};

Task<void> original = ThreadPool::create<void()>(f);
Task<void> copy = original; // compiler error
/home/Workspace/jluna/.test/main.cpp:33:23: error: use of deleted function ‘jluna::Task<void>::Task(const jluna::Task<void>&)’
    4 |     Task<void> copy = original;
      |                       ^~~~~~~~
/home/Workspace/jluna/.src/multi_threading.inl:213:13: note: declared here
  213 |             Task(const Task&) = delete;
      |             ^~~~

However, task’s can still be move assigned / constructed. This is why we can store them in containers.

If a move is invoked explicitly, the old task’s internal state is safely transferred:

auto f = []() -> int {
    return 1234

Task<int> original = ThreadPool::create<int()>(f);
Task<int> copy = std::move(original); // no error

Users need to be aware that, after a move assignment / construction, the old task is no longer valid. Task::is_failed will return true, other member functions will simply exit at the earliest possible point. Trying to access an invalidated tasks future, however, will cause an error:

auto f = [](){};

Task<int> original = ThreadPool::create<int()>(f);
Task<int> copy = std::move(original);

auto future = original.get().value(); // despite original being now invalid
[ERROR][C++] In Task<T>::result: trying to access the future of a task that is no longer valid. Tasks are invalidated when the move assignment operator or move constructor is called, which transfers a tasks internal state into the newly constructed one.
jluna_test: /home/clem/Workspace/jluna/.src/multi_threading.inl:197: jluna::Future<Result_t>& jluna::Task<Result_t>::result() [with Result_t = int]: Assertion `this->_value != nullptr' failed.

In general, a task invalidated after a move should not be interacted with. Users should be aware of this, though during general usage it is rare to run into this error.

Do NOT use @threadcall#

Lastly, a warning: Julia has a macro called @threadcall, which purports to simply execute a ccall in a new thread. Internally, it actually uses the libuv thread pool for this, not the native Julia thread pool. Because the C-API is seemingly hardcoded to segfault any attempt at accessing the Julia state through any non-master C-side thread, using @threadcall to call arbitrary code will also trigger this segfault. Because of this, it is not recommended to use @threadcall in any circumstances. Instead, we can ccall from within a proper Base.Task, or use jlunas thread pool to execute C-side code in the first place.