barrier.hpp#

See Public API for a list of names and headers that are part of the public HPX API.

namespace hpx

template<typename OnCompletion = detail::empty_oncompletion> class barrier#

#include <barrier.hpp>

A barrier is a thread coordination mechanism whose lifetime consists of a sequence of barrier phases, where each phase allows at most an expected number of threads to block until the expected number of threads arrive at the barrier. [ Note: A barrier is useful for managing repeated tasks that are handled by multiple threads. - end note ] Each barrier phase consists of the following steps:

The expected count is decremented by each call to arrive or arrive_and_drop.
When the expected count reaches zero, the phase completion step is run. For the specialization with the default value of the CompletionFunction template parameter, the completion step is run as part of the call to arrive or arrive_and_drop that caused the expected count to reach zero. For other specializations, the completion step is run on one of the threads that arrived at the barrier during the phase.
When the completion step finishes, the expected count is reset to what was specified by the expected argument to the constructor, possibly adjusted by calls to arrive_and_drop, and the next phase starts.

Each phase defines a phase synchronization point. Threads that arrive at the barrier during the phase can block on the phase synchronization point by calling wait, and will remain blocked until the phase completion step is run. The phase completion step that is executed at the end of each phase has the following effects:

Invokes the completion function, equivalent to completion().
Unblocks all threads that are blocked on the phase synchronization point.

The end of the completion step strongly happens before the returns from all calls that were unblocked by the completion step. For specializations that do not have the default value of the CompletionFunction template parameter, the behavior is undefined if any of the barrier object’s member functions other than wait are called while the completion step is in progress.

Concurrent invocations of the member functions of barrier, other than its destructor, do not introduce data races. The member functions arrive and arrive_and_drop execute atomically.

CompletionFunction shall meet the Cpp17MoveConstructible (Table 28) and Cpp17Destructible (Table 32) requirements. std::is_nothrow_invocable_v<CompletionFunction&> shall be true.

The default value of the CompletionFunction template parameter is an unspecified type, such that, in addition to satisfying the requirements of CompletionFunction, it meets the Cpp17DefaultConstructible requirements (Table 27) and completion() has no effects.

barrier::arrival_token is an unspecified type, such that it meets the Cpp17MoveConstructible (Table 28), Cpp17MoveAssignable (Table 30), and Cpp17Destructible (Table 32) requirements.

Public Types

using arrival_token = bool#

Public Functions

inline explicit constexpr barrier(std::ptrdiff_t expected, OnCompletion completion = OnCompletion())#

Preconditions: expected >= 0 is true and expected <= max() is true.

Effects: Sets both the initial expected count for each barrier phase and the current expected count for the first phase to expected. Initializes completion with std::move(f). Starts the first phase. [Note: If expected is 0 this object can only be destroyed.- end note]

Throws

inline arrival_token arrive(std::ptrdiff_t update = 1)#

Preconditions: update > 0 is true, and update is less than or equal to the expected count for the current barrier phase.

Effects: Constructs an object of type arrival_token that is associated with the phase synchronization point for the current phase. Then, decrements the expected count by update.

Synchronization: The call to arrive strongly happens before the start of the phase completion step for the current phase.

Throws
Returns: : The constructed arrival_token object.

inline void wait(arrival_token &&old_phase) const#

Preconditions: arrival is associated with the phase synchronization point for the current phase or the immediately preceding phase of the same barrier object.

Effects: Blocks at the synchronization point associated with HPX_MOVE(arrival) until the phase completion step of the synchronization point’s phase is run. [ Note: If arrival is associated with the synchronization point for a previous phase, the call returns immediately. - end note ]

Throws

inline void arrive_and_wait()#: Effects: Equivalent to: wait(arrive()).

inline void arrive_and_drop()#

Preconditions: The expected count for the current barrier phase is greater than zero.

Effects: Decrements the initial expected count for all subsequent phases by one. Then decrements the expected count for the current phase by one.

Synchronization: The call to arrive_and_drop strongly happens before the start of the phase completion step for the current phase.

Throws

Public Static Functions

static inline constexpr std::ptrdiff_t() max () noexcept

Private Types

using mutex_type = hpx::spinlock#

Private Members

mutable mutex_type mtx_#

mutable hpx::lcos::local::detail::condition_variable cond_#

std::ptrdiff_t expected_#

std::ptrdiff_t arrived_#

OnCompletion completion_#

bool phase_#