Project:	ISO JTC1/SC22/WG21: Programming Language C++
Number:	P0260R7
Date:	2023-06-15
Audience	LEWG, SG1
Revises:	P0260R6
Author:	Lawrence Crowl, Chris Mysen, Detlef Vollmann, Gor Nishanov
Contact	[email protected]

C++ Concurrent Queues

Lawrence Crowl, Chris Mysen, Detlef Vollmann, Gor Nishanov

Abstract

Concurrent queues are a fundamental structuring tool for concurrent programs. We propose a concurrent queue concept and a concrete implementation (in P1958). We propose a set of communication types that enable loosely bound program components to dynamically construct and safely share concurrent queues.

Revision History
Introduction
    Target Vehicle
Existing Practice
    Concept of a Bounded Queue
    Bounded Queues with C++ Interface
Conceptual Interface
    Basic Operations
    Non-Waiting Operations
    Closed Queues
    Empty and Full Queues
    Element Type Requirements
    Exception Handling
Concrete Queues
Response to Feedback by LEWGI at Prague 2020 meeting
Implementation
Historic Contents
   Proposed Wording
       ?.? Concurrent queues [conqueues]
       ?.?.1 General [conqueues.general]
       ?.?.2 Header <conqueue> synopsis [conqueues.syn]
       ?.?.3 Operation status [conqueues.status]
       ?.?.4 Concepts [conqueues.concepts]
       ?.?.4.1 Element requirements [conqueues.concept.elemreq]
       ?.?.4.2 Element type naming [conqueues.concept.elemtype]
       ?.?.4.3 Lock-free attribute operations [conqueues.concept.lockfree]
       ?.?.4.4 Synchronization [conqueues.concept.sync]
       ?.?.4.4 State operations [conqueues.concept.state]
       ?.?.4.5 Waiting operations [conqueues.concept.wait]
       ?.?.4.6 Non-waiting operations [conqueues.concept.nonwait]
       ?.?.4.7 Type concepts [conqueues.concept.type]
       ?.?.5 Concrete queues [conqueues.concrete]
       ?.?.6 Tools [conqueues.tools]
       ?.?.6.1 Ends and Iterators [conqueues.tools.ends]
       ?.?.6.1.1 Class template generic_queue_back [conqueues.tools.back]
       ?.?.6.1.2 Class template generic_queue_front [conqueues.tools.front]
       ?.?.6.2 Binary interfaces [conqueues.tools.binary]
       ?.?.6.2.1 Class template queue_wrapper [conqueues.tools.wrapper]
       ?.?.6.2.2 Binary ends [conqueues.tools.binends]
       ?.?.6.3 Managed Ends [conqueues.tools.managed]
       ?.?.6.3.1 Class template shared_queue_back [conqueues.tools.sharedback]
       ?.?.6.3.2 Class template shared_queue_front [conqueues.tools.front]
       ?.?.6.3.3 Function template share_queue_ends [conqueues.tools.shareendsfront]
   Abandoned Interfaces
       Non-Blocking Operations
       Push Front Operations
       Queue Names
       Lock-Free Buffer Queue
       Storage Iterators
       Queue Ordering
       Lock-Free Implementations
   Concrete Queues
       Locking Buffer Queue
   Abandoned Additional Conceptual Tools
       Fronts and Backs
       Streaming Iterators
       Binary Interfaces
       Managed Indirection

Revision History

This paper revises P0260R6 - 2023-06-16 as follows.

Fix typos.
Implement LEWG feedback to derive conqueue_errc from system_error
Implement LEWG feedback to add range constructor and go back to InputIterator
size_t capacity() added
Added TBB concurrent_bounded_queue as existing practice
Moved discussion about pop() interface to separate paper

This paper revises P0260R5 - 2023-01-15 as follows.

Fixing typos.
Added a scope for the target TS.
Added questions to be answered by a TS.
Added asynchronous interface

P0260R5 revises P0260R4 - 2020-01-12 as follows.

Added more introductory material.
Added response to feedback by LEWGI at Prague meeting 2020.
Added section on existing practice.
Replaced value_pop with pop.
Replaced is_lock_free with is_always_lockfree.
Removed is_empty and is_full.
Added move-into parameter to try_push(Element&&)
Added note that exception thrown by the queue operations themselves are derived from std::exception.
Added a note that the wording is partly invalid.
Moved more contents into the "Abandoned" part to avoid confusion.

P0260R4 revised P0260R3 - 2019-01-20 as follows.

Remove the binding of queue_op_status::success to a value of zero.
Correct stale use of the Queue template parameter in shared_queue_front to Value.
Change the return type of share_queue_ends from a pair to a custom struct.
Move the concrete queue proposal to a separate paper, P1958R0.

P0260R3 revised P0260R2 - 2017-10-15 as follows.

Convert queue_wrapper to a function-like interface. This conversion removes the queue_base class. Thanks to Zach Lane for the approach.
Removed the requirement that element types have a default constructor. This removal implies that statically sized buffers cannot use an array implmentation and must grow a vector implementation to the maximum size.
Added a discussion of checking for output iterator end in the wording.
Fill in synopsis section.
Remove stale discussion of queue_owner.
Move all abandoned interface discussion to a new section.
Update paper header to current practice.

P0260R2 revised P0260R1 - 2017-02-05 as follows.

Emphasize that non-blocking operations were removed from the proposed changes.
Correct syntax typos for noexcept and template alias.
Remove static from is_lock_free for generic_queue_back and generic_queue_front.

P0260R1 revised P0260R0 - 2016-02-14 as follows.

Remove pure virtuals from queue_wrapper.
Correct queue::pop to value_pop.
Remove nonblocking operations.
Remove non-locking buffer queue concrete class.
Tighten up push/pop wording on closed queues.
Tighten up push/pop wording on synchronization.
Add note about possible non-FIFO behavior.
Define buffer_queue to be FIFO.
Make wording consistent across attributes.
Add a restriction on element special methods using the queue.
Make is_lock_free() for only non-waiting functions.
Make is_lock_free() static for non-indirect classes.
Make is_lock_free() noexcept.
Make has_queue() noexcept.
Make destructors noexcept.
Replace "throws nothing" with noexcept.
Make the remarks about the usefulness of is_empty() and is_full into notes.
Make the non-static member functions is_... and has_... functions const.

P0260R0 revised N3533 - 2013-03-12 as follows.

Update links to source code.
Add wording.
Leave the name facility out of the wording.
Leave the push-front facility out of the wording.
Leave the reopen facility out of the wording.
Leave the storage iterator facility out of the wording.

N3532 revised N3434 = 12-0043 - 2012-01-14 as follows.

Add more exposition.
Provide separate non-blocking operations.
Add a section on the lock-free queues.
Argue against push-back operations.
Add a cautionary note on the usefulness of is_closed().
Expand the cautionary note on the usefulness of is_empty(). Add is_full().
Add a subsection on element type requirements.
Add a subsection on exception handling.
Clarify ordering constraints on the interface.
Add a subsection on a lock-free concrete queue.
Add a section on content iterators, distinct from the existing streaming iterators section.
Swap front and back names, as requested.
General expository cleanup.
Add an 'Revision History' section.

N3434 revised N3353 = 12-0043 - 2012-01-14 as follows.

Change the inheritance-based interface to a pure conceptual interface.
Put 'try' operations into a separate subsection.
Add a subsection on non-blocking operations.
Add a subsection on push-back operations.
Add a subsection on queue ordering.
Merge the 'Binary Interface' and 'Managed Indirection' sections into a new 'Conceptual Tools' section. Expand on the topics and their rationale.
Add a subsection to 'Conceptual Tools' that provides for type erasure.
Remove the 'Synopsis' section.
Add an 'Implementation' section.

Introduction

Queues provide a mechanism for communicating data between components of a system.

The existing deque in the standard library is an inherently sequential data structure. Its reference-returning element access operations cannot synchronize access to those elements with other queue operations. So, concurrent pushes and pops on queues require a different interface to the queue structure.

Moreover, concurrency adds a new dimension for performance and semantics. Different queue implementation must trade off uncontended operation cost, contended operation cost, and element order guarantees. Some of these trade-offs will necessarily result in semantics weaker than a serial queue.

Concurrent queues come in a several different flavours, e.g.

bounded vs. unbounded
blocking vs. overwriting
single-ended vs. multi-ended
strict FIFO ordering vs. priority based ordering

The syntactic concept proposed here should be valid for all of these flavours, while the concrete semantics might differ.

Target Vehicle

This proposal targets a TS. It was originally sent to LEWG for inclusion into Concurrency TS v2. As Concurrency TS v2 will probably be published before this proposal is ready to be published, we propose to include concurrent queues into Concurrency TS v3 and publish this as soon as concurrent queues are ready. This leaves the door open for other proposal to share the same ship vehicle.

The scope for Concurrency TS v3 would be the same as that for v2:

"This document describes requirements for implementations of an interface that computer programs written in the C++ programming language may use to invoke algorithms with concurrent execution. The algorithms described by this document are realizable across a broad class of computer architectures."

Should the committee decide to restrict the scope of the TS to only contain concurrent queues, we propose a slightly different scope:

"This document describes requirements for implementations of an interface that computer programs written in the C++ programming language may use to communicate between different execution agents of algorithms with concurrent execution. The algorithms described by this document are realizable across a broad class of computer architectures."

Questions for a TS to Answer

We expect that the TS will inform future work on a variety of questions, particularly those listed below, using real-world implementation experience that cannot be obtained without a TS.

Is the proposed concept useful? Specifically, does it cover different implementations and does it work together with other concepts for concurrent queues, e.g. queues with only non-blocking functions or queues with an asynchronous interface?
Is the concrete queue useful without an asynchronous interface? Can an asynchronous interface be added without extra overhead?
What other concrete implementations should be provided?
Is a queue that is ignorant of execution contexts from std::execution still useful?

Existing Practice

Concept of a Bounded Queue

The basic concept of a bounded queue with potentially blocking push and pop operations is very old and widely used. It's generally provided as an operating system level facility, like other concurrency primitives.

POSIX 2001 has mq message queues (with priorities and timeout).

Windows ?

FreeRTOS, Mbed, vxWorks

Bounded Queues with C++ Interface

Literature

Boost

TBB has concurrent_bounded_queue (and an unbounded version concurrent_queue that has only non-blocking operations).

Conceptual Interface

We provide basic queue operations, and then extend those operations to cover other important issues.

By analogy with how future defines their errors, we introduce conque_errc enum and conqueue_error as follows:

  enum class conqueue_errc { success, empty, full, closed };

  template <>
  struct is_error_code_enum<conqueue_errc> : public true_type {};

  const error_category& conqueue_category() noexcept;

  error_code make_error_code(conqueue_errc e) noexcept;

  error_condition make_error_condition(conqueue_errc e) noexcept;

  class conqueue_error : public system_error;

These errors will be reported from concurrent queue operations as specified below.

Basic Operations

The essential solution to the problem of concurrent queuing is to shift to value-based operations, rather than reference-based operations.

The basic operations are:

void queue::push(const T& x);
void queue::push(T&& x);
bool queue::push(const T& x, std::error_code& ec);
bool queue::push(T&& x, std::error_code& ec);: Pushes x onto the queue via copy or move construction. The first version throws std::conqueue_error(conqueue_errc::closed) if the queue is closed. The second version returns true on success, and false and sets ec to error_code(conqueue_errc::closed) if the queue is closed.
T queue::pop();
std::optional<T> queue::pop(std::error_code& ec);

Pops a value from the queue via move construction into the return value. The first version throws std::conqueue_error(conqueue_errc::closed) if the queue is empty and closed; the second version, if the queue is empty and closed, returns std::nullopt and sets ec to std::error_code(conqueue_errc::closed). If queue is empty and open, the operation blocks until an element is available.

In the original buffer_queue paper, the pop function had signature T pop_value(). Subsequently, it was changed to void pop(T&) due to concern about the problem of loosing elements when an error occurs.

The exploration of different version of error reporting was moved to a separate paper P2921.

Asynchronous Operations

sender auto queue::async_push(T x);
sender auto queue::async_pop();: These operations return a sender that will push or pop the element. Senders must support cancellation and if the receiver is currently waiting on a push or pop operation and no longer interested in performing the operation, it should be removed from any waiting queues, if any, and be completed with std::execution::set_stopped.

Non-Waiting Operations

Waiting on a full or empty queue can take a while, which has an opportunity cost. Avoiding that wait enables algorithms to avoid queuing speculative work when a queue is full, to do other work rather than wait for a push on a full queue, and to do other work rather than wait for a pop on an empty queue.

bool queue::try_push(const T& x, std::error_code& ec);

bool queue::try_push(T&& x, std::error_code& ec);

If the queue is full or closed, returns false and sets the respective status in the ec. Otherwise, push the value onto the queue via copy or move construction and returns true.

REVISITED in Varna

The following version was introduced in response to LEWG-I concerns about loosing the element if an rvalue cannot be stored in the queue.

queue_op_status
      queue::try_push(T&&, T&);

However, SG1 reaffirmed the APIs above with the following rationale:

It seems that it is possible in both versions:

    T x = get_something();
    if (q.try_push(std::move(x))) ...

With two parameter version:

    T x;
    if (q.try_push(get_something(), x)) ...

Ergonomically they are roughly identical. API is slightly simpler with one argument version, therefore, we reverted to original one argument version.

optional<T> queue::try_pop(std::error_code& ec);

If the queue is empty, returns nullopt and set ec to conqueue_errc::empty. Otherwise, pop the element from the queue via move construction into the optional. Return true and set ec to conqueue_errc::success.

These operations will not wait when the queue is full or empty. They may block for mutual exclusion.

Closed Queues

Threads using a queue for communication need some mechanism to signal when the queue is no longer needed. The usual approach is add an additional out-of-band signal. However, this approach suffers from the flaw that threads waiting on either full or empty queues need to be woken up when the queue is no longer needed. To do that, you need access to the condition variables used for full/empty blocking, which considerably increases the complexity and fragility of the interface. It also leads to performance implications with additional mutexes or atomics. Rather than require an out-of-band signal, we chose to directly support such a signal in the queue itself, which considerably simplifies coding.

To achieve this signal, a thread may close a queue. Once closed, no new elements may be pushed onto the queue. Push operations on a closed queue will either return conqueue_errc::closed (when they have ec parameter) or throw conqueue_error(conqueue_errc::closed) (when they do not). Elements already on the queue may be popped off. When a queue is empty and closed, pop operations will either set ec to conqueue_errc::closed (when they have a ec parameter) or throw conqueue_error(conqueue_errc::closed) otherwise.

The additional operations are as follows. They are essentially equivalent to the basic operations except that they return a status, avoiding an exception when queues are closed.

void queue::close() noexcept;: Close the queue.
bool queue::is_closed() const noexcept;: Return true iff the queue is closed.

Element Type Requirements

The above operations require element types with copy/move constructors, and destructor. These operations may be trivial. The copy/move constructors operators may throw, but must leave the objects in a valid state for subsequent operations.

Exception Handling

push() and pop() may throw an exceptions of type conqueue_error that's derived from std::system_error and will contain a conqueue_errc.

Concurrent queues cannot completely hide the effect of exceptions thrown by the element type, in part because changes cannot be transparently undone when other threads are observing the queue.

Queues may rethrow exceptions from storage allocation, mutexes, or condition variables.

If the element type operations required do not throw exceptions, then only the exceptions above are rethrown.

When an element copy/move may throw, some queue operations have additional behavior.

Construction shall rethrow, destroying any elements allocated.
A push operation shall rethrow and the state of the queue is unaffected.
A pop operation shall rethrow and the element is popped from the queue. The value popped is effectively lost. (Doing otherwise would likely clog the queue with a bad element.)

Concrete Queues

In addition to the concept, the standard needs at least one concrete queue. P1958R0 provicdes one such concrete queue, buffer_queue.

buffer_queue is outlined below:

enum class conqueue_errc { success, empty, full, closed };

const error_category& conqueue_category() noexcept;
error_code make_error_code(conqueue_errc e) noexcept;
error_condition make_error_condition(conqueue_errc e) noexcept;

class conqueue_error : system_error { ... };

template <typename T,
  class Allocator = std::allocator<T>>
class buffer_queue
{
  buffer_queue() = delete;
  buffer_queue(const buffer_queue&) = delete;
  buffer_queue& operator =(const buffer_queue&) = delete;
public:
  typedef T value_type;

  // construct/destroy
  explicit buffer_queue(size_t max_elems, const Allocator& alloc = Allocator());
  explicit buffer_queue(std::initializer_list<T>, size_t max_elems = 0, 
                        const Allocator& alloc = Allocator());
  template <typename InputIterator>
    buffer_queue(InputIterator begin, InputIterator end, size_t max_elems = 0,
                 const Allocator& alloc = Allocator());
  template <container-compatible-range<T> R>
    buffer_queue(from_range_t, R&& rg, size_t max_elems = 0, 
                 const Allocator& alloc = Allocator());

  ~buffer_queue() noexcept;

  // observers
  size_t capacity() const noexcept;
  bool is_closed() const noexcept;
  static constexpr bool is_always_lock_free() noexcept;

  // modifiers
  void close() noexcept;

  T pop();
  optional<T> pop(std::error_code& ec);
  optional<T> try_pop(std::error_code& ec);

  void push(const T& x);
  void push(T&& x);
  bool push(const T& x, std::error_code& ec);
  bool push(T&& x, std::error_code& ec);
  bool try_push(const T& x, std::error_code& ec);
  bool try_push(T&& x, std::error_code& ec);
};

buffer_queue is only allowed to allocate in its constructor. constructors that take initializing sequence are allowed to omit size_t max_elem argument that will be then assumed to be equal to the size of the initialization sequence.

Response to Feedback by LEWGI at Prague 2020 meeting

At the Prague meeting in February 2020 LEWGI provided feedback an set some action items.

"Explore P0059 ring_buffer prior art and document it in paper.": ring_buffer is like std::queue is a sequential data structure and therefore provides a completely different interface than concurrent queues.

"Consider removing value_pop to increase consensus.": value_pop was replaced by pop that doesn't have the problem of loosing elements.

"Consider removing is_empty and is_full to increase consensus.": Done.

"Consider removing is_lock_free to increase consensus. If is_lock_free remains, add is_always_lock_free (a la atomic).": is_lock_free was dropped but is_always_lock_free was added anyways.

"Remove the maybe-consuming try_push(&&). Investigate prior art (such as TBB's concurrent_queue) and add either:: TBB's concurrent_queue doesn't have try_push. try_push(&&) now has an additional parameter that gets the element if it couldn't be pushed.

"Require buffer_queue to allocate all storage only once". "Require buffer_queue to allocate all storage during construction". "Instead of throwing queue_op_status objects, add a standard library exception type and throw that.": These requests are all valid and added (partly to P1958R1).

Implementation

An implementation is available at https://github.com/GorNishanov/conqueue.

A free, open-source implementation of an earlier version of these interfaces is avaliable at the Google Concurrency Library project at https://github.com/alasdairmackintosh/google-concurrency-library. The concrete buffer_queue is in ..../blob/master/include/buffer_queue.h. The concrete lock_free_buffer_queue is in ..../blob/master/include/lock_free_buffer_queue.h. The corresponding implementation of the conceptual tools is in ..../blob/master/include/queue_base.h.

Historic Contents

The Contents below is for historic reference only.

Proposed Wording

Note: This wording is left for general reference. It was not updated from previous proposals as first the design should be fixed. So the wording here partly contradicts the design proposed above. In these cases the design is proposed and not the wording!

The concurrent queue container definition is as follows. The section, paragraph, and table references are based on those of N4567 Working Draft, Standard for Programming Language C++, Richard Smith, November 2015.

?.? Concurrent queues [conqueues]

Add a new section.

?.?.1 General [conqueues.general]

Add a new section.

This section provides mechanisms for concurrent access to a queue. These mechanisms ease the production of race-free programs (1.10 [intro.multithread]).

?.?.2 Header <conqueue> synopsis [conqueues.syn]

Add a new section.


enum class queue_op_status { success, empty, full, closed };

template <typename Value> class buffer_queue;

template <typename Queue> class generic_queue_back;
template <typename Queue> class generic_queue_front;
template <typename Value> class queue_base;
template <typename Value>
  using queue_back = generic_queue_back< queue_base< Value > >;
template <typename Value>
  using queue_front = generic_queue_front< queue_base< Value > >;
template <typename Queue> class queue_wrapper;

template <typename Value> class shared_queue_back;
template <typename Value> class shared_queue_front;
template <typename Value> class shared_queue_ends;
template <typename Queue, typename ... Args>
  shared_queue_ends<typename Queue::value_type>;
  share_queue_ends(Args ... args);

?.?.3 Operation status [conqueues.status]

Add a new section.

Many concurrent queue operations return a status in the form of the following enumeration.

enum class queue_op_status

Enumerators:

success = 0, empty, full, closed

?.?.4 Concepts [conqueues.concepts]

Add a new section.

This section provides the conceptual operations for concurrent queues of type queue of Element types.

?.?.4.1 Element requirements [conqueues.concept.elemreq]

Add a new section:

The types of the elements of a concurrent queue must provide either or both of a copy constructor or a move constructor, either or both of a copy assignment operator or a move assignment operator, and a destructor.

Any copy/more constructor or copy/move assignment operator that throws shall leave the objects in a valid state for subsequent operations.

None of the above constructors, assignments or destructor may call any operation on a concurrent queue for which their objects may become a member. [Note: Queues may hold an internal lock while performing the above operations, and if they were to call a queue operation, deadlock would result. —end note]

?.?.4.2 Element type naming [conqueues.concept.elemtype]

Add a new section:

The queue class shall provide a typedef to its element value type.

typedef implementation-defined value_type;

?.?.4.3 Lock-free attribute operations [conqueues.concept.lockfree]

Add a new section:

A queue type provides lock-free operations (1.10 [intro.multithread], or it does not.

static bool queue::is_lock_free() noexcept;

Returns:

If the non-waiting operations of the queue type are lock-free, true. Otherwise, false.

Remark:

The function returns the same result for all instances of the type.

?.?.4.4 Synchronization [conqueues.concept.sync]

Add a new section:

For synchronization purposes, and unless otherwise stated, all queue operations appear to operate on a single memory location, all non-const queue operations appear to be sequentially consistent atomic read-modify-write operations, and all const queue operations appear to be atomic loads from this location. [Note: In particular, all queue operations appear to execute in a single global order, that is part of the total order S (29.3 [atomics.order]) of sequentially consistent operations. Each non-const queue operation A strongly happens before every operation on the same queue that follows A in S. Whether or not the queue preserves a FIFO order is a property of the concrete class. —end note]

static bool queue::is_lock_free() noexcept;

Returns:

If the non-waiting operations of the queue type are lock-free, true. Otherwise, false.

Remark:

The function returns the same result for all instances of the type.

?.?.4.4 State operations [conqueues.concept.state]

Add a new section:

Upon construction, every queue shall be in an open state. It may move to a closed state, but shall not move back to an open state.

void queue::close() noexcept;

Effects:

Closes the queue. No pushes subsequent to the close shall succeed.

bool queue::is_closed() const noexcept;

Returns:

true if the queue is closed, otherwise, false

?.?.4.5 Waiting operations [conqueues.concept.wait]

Add a new section:

void queue::push(const Element&);
void queue::push(Element&&);

Effects:

If the queue is closed, throws an exception. Otherwise, if space is available on the queue, copies or moves the element onto the queue and returns. Otherwise, waits until space is available or the queue is closed.

Throws:

Any exception from operations on storage allocation, mutexes, or condition variables. If an element copy/move operation throws, the state of the queue is unaffected and the push shall rethrow the exception. If the operation cannot otherwise complete because the queue is closed, throws queue_op_status::closed.

void queue::pop(Element&);

Effects:

If an element is available on the queue, moves the element from the queue to the parameter and returns. Otherwise, if the queue is closed, throws an exception. Otherwise, waits until an element is available or the queue is closed.

Throws:

Any exception from operations on storage allocation, mutexes, or condition variables. If an element copy/move operation throws, the state of the element is popped and the pop shall rethrow the exception. If the operation cannot otherwise complete because the queue is closed, throws queue_op_status::closed.

queue_op_status queue::wait_push(const Element&);
queue_op_status queue::wait_push(Element&&);

Effects:

If the queue is closed, returns. Otherwise, if space is available on the queue, copies or moves the element onto the queue and returns. Otherwise, waits until space is available or the queue is closed.

Returns:

If the queue was closed, queue_op_status::closed. Otherwise, the push was successful, queue_op_status::success.

Throws:

Any exception from operations on storage allocation, mutexes, or condition variables. If an element copy/move operation throws, the state of the queue is unaffected and the push shall rethrow the exception.

queue_op_status queue::wait_pop(Element&);

Effects:

If an element is available on the queue, moves the element from the queue to the parameter and returns. Otherwise, if the queue is closed, returns. Otherwise, waits until an element is available or the queue is closed.

Returns:

If the queue was closed, queue_op_status::closed. Otherwise, the pop was successful, queue_op_status::success.

Throws:

Any exception from operations on storage allocation, mutexes, or condition variables. If an element copy/move operation throws, the state of the element is popped and the pop shall rethrow the exception.

?.?.4.6 Non-waiting operations [conqueues.concept.nonwait]

Add a new section:

queue_op_status queue::try_push(const Element&);
queue_op_status queue::try_push(Element&&);

Effects:

If the queue is closed, returns. Otherwise, if space is available on the queue, copies or moves the element onto the queue and returns. Otherwise, returns.

Returns:

If the queue was closed, queue_op_status::closed. Otherwise, if the push was successful, queue_op_status::success. Otherwise, space was unavailable, queue_op_status::full.

Throws:

Any exception from operations on storage allocation, mutexes, or condition variables. If an element copy/move operation throws, the state of the queue is unaffected and the push shall rethrow the exception.

queue_op_status queue::try_pop(Element&);

Effects:

If an element is available on the queue, moves the element from the queue to the parameter and returns. Otherwise, returns.

Returns:

If the pop was successful, queue_op_status::success. Otherwise, if the queue is closed, queue_op_status::closed. Otherwise, no element was available, queue_op_status::empty.

Throws:

Any exception from operations on storage allocation, mutexes, or condition variables. If an element copy/move operation throws, the state of the element is popped and the pop shall rethrow the exception.

?.?.4.7 Type concepts [conqueues.concept.type]

Add a new section:

The WaitingConcurrentQueue concept provides all of the operations specified above.

The NonWaitingConcurrentQueue concept provides all of the operations specified above, except the waiting operations ([conqueues.concept.wait]). A NonWaitingConcurrentQueue is lock-free (1.10 [intro.multithread]) when its member function is_lock_free reports true.

The WaitingConcurrentQueueBack concept provides all of the operations specified above except the pop operations.

The WaitingConcurrentQueueFront concept provides all of the operations specified above except the push operations.

The NonWaitingConcurrentQueueBack concept provides all of the operations specified above except the pop operations and the waiting push operations. A NonWaitingConcurrentQueueBack is lock-free (1.10 [intro.multithread]) when its member function is_lock_free reports true.

The NonWaitingConcurrentQueueFront concept provides all of the operations specified above except the push operations and the waiting pop operations. A NonWaitingConcurrentQueueFront is lock-free (1.10 [intro.multithread]) when its member function is_lock_free reports true.

?.?.5 Concrete queues [conqueues.concrete]

Add a new section, with content to be provided by other papers.

?.?.6 Tools [conqueues.tools]

Add a new section:

Additional tools help to use and manage concurrent queues.

?.?.6.1 Ends and Iterators [conqueues.tools.ends]

Add a new section:

Access to only a single end of a queue is a valuable code structuring tool. A single end can also provide unambiguous begin and end operations that return iterators.

Because queues may be closed and hence accept no further pushes, output iterators must also be checked for having reached the end, ie. having been closed. [Example:
void iterate(
    generic_queue_back<buffer_queue<int>>::iterator bitr,
    generic_queue_back<buffer_queue<int>>::iterator bend,
    generic_queue_front<buffer_queue<int>>::iterator fitr,
    generic_queue_front<buffer_queue<int>>::iterator fend,
    int (*compute)( int ) )
{
    while ( fitr != fend && bitr != bend )
        *bitr++ = compute(*fitr++);
}
—end example]

?.?.6.1.1 Class template `generic_queue_back` [conqueues.tools.back]

Add a new section:

template <typename Queue>
class generic_queue_back
{
public:
  typedef typename Queue::value_type value_type;
  typedef value_type& reference;
  typedef const value_type& const_reference;

  typedef implementation-defined iterator;
  typedef const iterator const_iterator;

  generic_queue_back(Queue& queue);
  generic_queue_back(Queue* queue);
  generic_queue_back(const generic_queue_back& other) = default;
  generic_queue_back& operator =(const generic_queue_back& other) = default;

  void close() noexcept;
  bool is_closed() const noexcept;
  bool is_empty() const noexcept;
  bool is_full() const noexcept;
  bool is_lock_free() const noexcept;
  bool has_queue() const noexcept;

  iterator begin();
  iterator end();
  const iterator cbegin();
  const iterator cend();

  void push(const value_type& x);
  queue_op_status wait_push(const value_type& x);
  queue_op_status try_push(const value_type& x);

  void push(value_type&& x);
  queue_op_status wait_push(value_type&& x);
  queue_op_status try_push(value_type&& x);
};

The class template generic_queue_back implements WaitingConcurrentQueueBack

generic_queue_back(Queue& queue);
generic_queue_back(Queue* queue);

Effects:: Constructs the queue back with a pointer to the queue object given.

~generic_queue_back() noexcept;

Effects:: Destroys the queue back.

bool has_queue() const noexcept;

Returns:: true if the contained pointer is not null. false otherwise.

?.?.6.1.2 Class template `generic_queue_front` [conqueues.tools.front]

Add a new section:

template <typename Queue>
class generic_queue_front
{
public:
  typedef typename Queue::value_type value_type;
  typedef value_type& reference;
  typedef const value_type& const_reference;

  typedef implementation-defined iterator;
  typedef const iterator const_iterator;

  generic_queue_front(Queue& queue);
  generic_queue_front(Queue* queue);
  generic_queue_front(const generic_queue_front& other) = default;
  generic_queue_front& operator =(const generic_queue_front& other) = default;

  void close() noexcept;
  bool is_closed() const noexcept;
  bool is_empty() const noexcept;
  bool is_full() const noexcept;
  bool is_lock_free() const noexcept;
  bool has_queue() const noexcept;

  iterator begin();
  iterator end();
  const iterator cbegin();
  const iterator cend();

  value_type value_pop();
  queue_op_status wait_pop(value_type& x);
  queue_op_status try_pop(value_type& x);
};

The class template generic_queue_front implements WaitingConcurrentQueueFront

generic_queue_front(Queue& queue);
generic_queue_front(Queue* queue);

Effects:: Constructs the queue front with a pointer to the queue object given.

~generic_queue_front() noexcept;

Effects:: Destroys the queue front.

bool has_queue() const noexcept;

Returns:: true if the contained pointer is not null. false otherwise.

?.?.6.2 Binary interfaces [conqueues.tools.binary]

Add a new section:

Occasionally it is best to have a binary interface to any concurrent queue of a given element type. This binary interface is provided by a wrapper class that erases the type of the concrete queue class.

?.?.6.2.1 Class template `queue_wrapper` [conqueues.tools.wrapper]

Add a new section:

template<typename Value>
struct queue_wrapper
{
  using value_type = Value;

  template<typename Queue>
  queue_wrapper(Queue * arg);
  template<typename Queue>
  queue_wrapper(Queue & arg);
  ~queue_wrapper() noexcept;

  void close() noexcept;
  bool is_closed() const noexcept;
  bool is_empty() const noexcept;
  bool is_full() const noexcept;
  bool is_lock_free() const noexcept;

  void push(const value_type & x);
  queue_op_status wait_push(const value_type & x);
  queue_op_status try_push(const value_type & x);
  queue_op_status nonblocking_push(const value_type & x);

  void push(value_type && x);
  queue_op_status wait_push(value_type && x);
  queue_op_status try_push(value_type && x);
  queue_op_status nonblocking_push(value_type && x);

  value_type value_pop();
  queue_op_status wait_pop(value_type &);
  queue_op_status try_pop(value_type &);
  queue_op_status nonblocking_pop(value_type &);
};

The template type parameter Queue and the he class template queue_base shall implement the WaitingConcurrentQueue concept.

template<typename Queue> queue_wrapper(Queue* arg); template<typename Queue> queue_wrapper(Queue& arg);

Effects:: Constructs the queue wrapper, referencing the given queue.

~queue_base() noexcept;

Effects:: Destroys the queue wrapper, but not the referenced queue.

?.?.6.2.2 Binary ends [conqueues.tools.binends]

Add a new section:

In addition to binary interfaces to queues, binary interfaces to ends are also useful.
template <typename Value>
using queue_back = generic_queue_back< queue_wrapper< Value > >;
template <typename Value>
using queue_front = generic_queue_front< queue_wrapper< Value > >;

?.?.6.3 Managed Ends [conqueues.tools.managed]

Add a new section:

Automatically managing references to queues can be helpful when queues are used as a communication medium.

?.?.6.3.1 Class template `shared_queue_back` [conqueues.tools.sharedback]

Add a new section:

template <typename Value>
class shared_queue_back
{
public:
  typedef typename Value value_type;
  typedef value_type& reference;
  typedef const value_type& const_reference;

  typedef implementation-defined iterator;
  typedef const iterator const_iterator;

  shared_queue_back(const shared_queue_back& other);
  shared_queue_back& operator =(const shared_queue_back& other);

  void close() noexcept;
  bool is_closed() const noexcept;
  bool is_empty() const noexcept;
  bool is_full() const noexcept;
  bool is_lock_free() const noexcept;

  iterator begin();
  iterator end();
  const iterator cbegin();
  const iterator cend();

  void push(const value_type& x);
  queue_op_status wait_push(const value_type& x);
  queue_op_status try_push(const value_type& x);

  void push(value_type&& x);
  queue_op_status wait_push(value_type&& x);
  queue_op_status try_push(value_type&& x);
};
The class template shared_queue_back implements WaitingConcurrentQueueBack

shared_queue_back(const shared_queue_back& other);
shared_queue_back& operator =(const shared_queue_back& other) = default;

Effects:

Copy the pointer to the queue, but keep the back of the queue reference counted.

~shared_queue_back() noexcept;

Effects:

Destroys the queue back. If this is the last back reference, and there are no front references, destroy the queue. If this is the last back reference, and there are front references, close the queue.

?.?.6.3.2 Class template `shared_queue_front` [conqueues.tools.sharedfront]

Add a new section:

template <typename Value>
class shared_queue_front
{
public:
  typedef typename Value value_type;
  typedef value_type& reference;
  typedef const value_type& const_reference;

  typedef implementation-defined iterator;
  typedef const iterator const_iterator;

  shared_queue_front(Queue& queue);
  shared_queue_front(Queue* queue);
  shared_queue_front(const shared_queue_front& other) = default;
  shared_queue_front& operator =(const shared_queue_front& other) = default;

  void close() noexcept;
  bool is_closed() const noexcept;
  bool is_empty() const noexcept;
  bool is_full() const noexcept;
  bool is_lock_free() const noexcept;
  bool has_queue() const noexcept;

  iterator begin();
  iterator end();
  const iterator cbegin();
  const iterator cend();

  value_type value_pop();
  queue_op_status wait_pop(value_type& x);
  queue_op_status try_pop(value_type& x);
};
The class template shared_queue_front implements WaitingConcurrentQueueFront

shared_queue_front(const shared_queue_front& other);
shared_queue_front& operator =(const shared_queue_front& other) = default;

Effects:

Copy the pointer to the queue, but keep the front of the queue reference counted.

~shared_queue_front() noexcept;

Effects:

Destroys the queue front. If this is the last front reference, and there are no back references, destroy the queue. If this is the last front reference, and there are back references, close the queue.

?.?.6.3.3 Class template `shared_queue_ends` [conqueues.tools.shareqends]

Add a new section:

template <typename Value>
class shared_queue_ends
{
public:
  shared_queue_back<Value> back;
  shared_queue_front<Value> front;
};

?.?.6.3.4 Function template `share_queue_ends` [conqueues.tools.shareends]

Add a new section:

template <typename Queue, typename ... Args>
shared_queue_ends<typename Queue::value_type>
share_queue_ends(Args ... args);

Effects:

Constructs a Queue with the given Args. Initializes a set of reference counters for that queue.

Returns:

a shared_queue_ends consisting of one shared_queue_back and one shared_queue_front for the constructed queue.

Abandoned Interfaces

Re-opening a Queue

There are use cases for opening a queue that is closed. While we are not aware of an implementation in which the ability to reopen a queue would be a hardship, we also imagine that such an implementation could exist. Open should generally only be called if the queue is closed and empty, providing a clean synchronization point, though it is possible to call open on a non-empty queue. An open operation following a close operation is guaranteed to be visible after the close operation and the queue is guaranteed to be open upon completion of the open call. (But of course, another close call could occur immediately thereafter.)

void queue::open();: Open the queue.

Note that when is_closed() returns false, there is no assurance that any subsequent operation finds the queue closed because some other thread may close it concurrently.

If an open operation is not available, there is an assurance that once closed, a queue stays closed. So, unless the programmer takes care to ensure that all other threads will not close the queue, only a return value of true has any meaning.

Given these concerns with reopening queues, we do not propose wording to reopen a queue.

Non-Blocking Operations

For cases when blocking for mutual exclusion is undesirable, one can consider non-blocking operations. The interface is the same as the try operations but is allowed to also return queue_op_status::busy in case the operation is unable to complete without blocking.

queue_op_status queue::nonblocking_push(const Element&);
queue_op_status queue::nonblocking_push(Element&&);: If the operation would block, return queue_op_status::busy. Otherwise, if the queue is full, return queue_op_status::full. Otherwise, push the Element onto the queue. Return queue_op_status::success.
queue_op_status queue::nonblocking_pop(Element&);: If the operation would block, return queue_op_status::busy. Otherwise, if the queue is empty, return queue_op_status::empty. Otherwise, pop the Element from the queue. The element will be moved out of the queue in preference to being copied. Return queue_op_status::success.

These operations will neither wait nor block. However, they may do nothing.

The non-blocking operations highlight a terminology problem. In terms of synchronization effects, nonwaiting_push on queues is equivalent to try_lock on mutexes. And so one could conclude that the existing try_push should be renamed nonwaiting_push and nonblocking_push should be renamed try_push. However, at least Thread Building Blocks uses the existing terminology. Perhaps better is to not use try_push and instead use nonwaiting_push and nonblocking_push.

In November 2016, the Concurrency Study Group chose to defer non-blocking operations. Hence, the proposed wording does not include these functions. In addition, as these functions were the only ones that returned busy, that enumeration is also not included.

Push Front Operations

Occasionally, one may wish to return a popped item to the queue. We can provide for this with push_front operations.

void queue::push_front(const Element&);
void queue::push_front(Element&&);: Push the Element onto the back of the queue, i.e. in at the end of the queue that is normally popped. Return queue_op_status::success.
queue_op_status queue::try_push_front(const Element&);
queue_op_status queue::try_push_front(Element&&);: If the queue was full, return queue_op_status::full. Otherwise, push the Element onto the front of the queue, i.e. in at the end of the queue that is normally popped. Return queue_op_status::success.
queue_op_status queue::nonblocking_push_front(const Element&);
queue_op_status queue::nonblocking_push_front(Element&&);: If the operation would block, return queue_op_status::busy. Otherwise, if the queue is full, return queue_op_status::full. Otherwise, push the Element onto the front queue. i.e. in at the end of the queue that is normally popped. Return queue_op_status::success.

This feature was requested at the Spring 2012 meeting. However, we do not think the feature works.

The name push_front is inconsistent with existing "push back" nomenclature.
The effects of push_front are only distinguishable from a regular push when there is a strong ordering of elements. Highly concurrent queues will likely have no strong ordering.
The push_front call may fail due to full queues, closed queues, etc. In which case the operation will suffer contention, and may succeed only after interposing push and pop operations. The consequence is that the original push order is not preserved in the final pop order. So, push_front cannot be directly used as an 'undo'.
The operation implies an ability to reverse internal changes at the front of the queue. This ability implies a loss efficiency in some implementations.

In short, we do not think that in a concurrent environment push_front provides sufficient semantic value to justify its cost. Consequently, the proposed wording does not provide this feature.

Queue Names

It is sometimes desirable for queues to be able to identify themselves. This feature is particularly helpful for run-time diagnotics, particularly when 'ends' become dynamically passed around between threads. See Managed Indirection.

const char* queue::name();: Return the name string provided as a parameter to queue construction.

There is some debate on this facility, but we see no way to effectively replicate the facility. However, in recognition of that debate, the wording does not provide the name facility.

Lock-Free Buffer Queue

We provide a concrete concurrent queue in the form of a fixed-size lock_free_buffer_queue. It meets the NonWaitingConcurrentQueue concept. The queue is still under development, so details may change.

In November 2016, the Concurrency Study Group chose to defer lock-free queues. Hence, the proposed wording does not include a concrete lock-free queue.

Storage Iterators

In addition to iterators that stream data into and out of a queue, we could provide an iterator over the storage contents of a queue. Such and iterator, even when implementable, would mostly likely be valid only when the queue is otherwise quiecent. We believe such an iterator would be most useful for debugging, which may well require knowledge of the concrete class. Therefore, we do not propose wording for this feature.

Empty and Full Queues

It is sometimes desirable to know if a queue is empty.

bool queue::is_empty() const noexcept;: Return true iff the queue is empty.

This operation is useful only during intervals when the queue is known to not be subject to pushes and pops from other threads. Its primary use case is assertions on the state of the queue at the end if its lifetime, or when the system is in quiescent state (where there no outstanding pushes).

We can imagine occasional use for knowing when a queue is full, for instance in system performance polling. The motivation is significantly weaker though.

bool queue::is_full() const noexcept;: Return true iff the queue is full.

Not all queues will have a full state, and these would always return false.

Queue Ordering

The conceptual queue interface makes minimal guarantees.

The queue is not empty if there is an element that has been pushed but not popped.
A push operation synchronizes with the pop operation that obtains that element.
A close operation synchronizes with an operation that observes that the queue is closed.
There is a sequentially consistent order of operations.

In particular, the conceptual interface does not guarantee that the sequentially consistent order of element pushes matches the sequentially consistent order of pops. Concrete queues could specify more specific ordering guarantees.

Lock-Free Implementations

Lock-free queues will have some trouble waiting for the queue to be non-empty or non-full. Therefore, we propose two closely-related concepts. A full concurrent queue concept as described above, and a non-waiting concurrent queue concept that has all the operations except push, wait_push, value_pop and wait_pop. That is, it has only non-waiting operations (presumably emulated with busy wait) and non-blocking operations, but no waiting operations. We propose naming these WaitingConcurrentQueue and NonWaitingConcurrentQueue, respectively.

Note: Adopting this conceptual split requires splitting some of the facilities defined later.

For generic code it's sometimes important to know if a concurrent queue has a lock free implementation.

constexpr static bool queue::is_always_lock_free() noexcept;: Return true iff the has a lock-free implementation of the non-waiting operations.

Abandoned Additional Conceptual Tools

There are a number of tools that support use of the conceptual interface. These tools are not part of the queue interface, but provide restricted views or adapters on top of the queue useful in implementing concurrent algorithms.

Fronts and Backs

Restricting an interface to one side of a queue is a valuable code structuring tool. This restriction is accomplished with the classes generic_queue_front and generic_queue_back parameterized on the concrete queue implementation. These act as pointers with access to only the front or the back of a queue. The front of the queue is where elements are popped. The back of the queue is where elements are pushed.

void send( int number, generic_queue_back<buffer_queue<int>> arv );

These fronts and backs are also able to provide begin and end operations that unambiguously stream data into or out of a queue.

Streaming Iterators

In order to enable the use of existing algorithms streaming through concurrent queues, they need to support iterators. Output iterators will push to a queue and input iterators will pop from a queue. Stronger forms of iterators are in general not possible with concurrent queues.

Iterators implicitly require waiting for the advance, so iterators are only supportable with the WaitingConcurrentQueue concept.

void iterate(
    generic_queue_back<buffer_queue<int>>::iterator bitr,
    generic_queue_back<buffer_queue<int>>::iterator bend,
    generic_queue_front<buffer_queue<int>>::iterator fitr,
    generic_queue_front<buffer_queue<int>>::iterator fend,
    int (*compute)( int ) )
{
    while ( fitr != fend && bitr != bend )
        *bitr++ = compute(*fitr++);
}

Note that contrary to existing iterator algorithms, we check both iterators for reaching their end, as either may be closed at any time.

Note that with suitable renaming, the existing standard front insert and back insert iterators could work as is. However, there is nothing like a pop iterator adapter.

Binary Interfaces

The standard library is template based, but it is often desirable to have a binary interface that shields client from the concrete implementations. For example, std::function is a binary interface to callable object (of a given signature). We achieve this capability in queues with type erasure.

We provide a queue_base class template parameterized by the value type. Its operations are virtual. This class provides the essential independence from the queue representation.

We also provide queue_front and queue_back class templates parameterized by the value types. These are essentially generic_queue_front<queue_base<Value>> and generic_queue_front<queue_base<Value>>, respectively.

To obtain a pointer to queue_base from an non-virtual concurrent queue, construct an instance the queue_wrapper class template, which is parameterized on the queue and derived from queue_base. Upcasting a pointer to the queue_wrapper instance to a queue_base instance thus erases the concrete queue type.

extern void seq_fill( int count, queue_back<int> b );

buffer_queue<int> body( 10 /*elements*/, /*named*/ "body" );
queue_wrapper<buffer_queue<int>> wrap( body );
seq_fill( 10, wrap.back() );

Managed Indirection

Long running servers may have the need to reconfigure the relationship between queues and threads. The ability to pass 'ends' of queues between threads with automatic memory management eases programming.

To this end, we provide shared_queue_front and shared_queue_back template classes. These act as reference-counted versions of the queue_front and queue_back template classes.

The share_queue_ends(Args ... args) template function will provide a pair of shared_queue_front and shared_queue_back to a dynamically allocated queue_object instance containing an instance of the specified implementation queue. When the last of these fronts and backs are deleted, the queue itself will be deleted. Also, when the last of the fronts or the last of the backs is deleted, the queue will be closed.

auto x = share_queue_ends<buffer_queue<int>>( 10, "shared" );
shared_queue_back<int> b(x.back);
shared_queue_front<int> f(x.front);
f.push(3);
assert(3 == b.value_pop());