task_scheduler_handle

[scheduler.task_scheduler_handle]

The oneapi::tbb::task_scheduler_handle class and the oneapi::tbb::finalize function allow user to wait for completion of worker threads.

When the oneapi::tbb::finalize function is called with an oneapi::tbb::task_scheduler_handle instance, it blocks the calling thread until the completion of all worker threads that were implicitly created by the library.

// Defined in header <oneapi/tbb/global_control.h>

namespace oneapi {
    namespace tbb {

        class task_scheduler_handle {
        public:
            task_scheduler_handle() = default;
            task_scheduler_handle(oneapi::tbb::attach);
            ~task_scheduler_handle();

            task_scheduler_handle(const task_scheduler_handle& other) = delete;
            task_scheduler_handle(task_scheduler_handle&& other) noexcept;
            task_scheduler_handle& operator=(const task_scheduler_handle& other) = delete;
            task_scheduler_handle& operator=(task_scheduler_handle&& other) noexcept;

            explicit operator bool() const noexcept;

            void release();
        };

        void finalize(task_scheduler_handle& handle);
        bool finalize(task_scheduler_handle& handle, const std::nothrow_t&) noexcept;

    } // namespace tbb
} // namespace oneapi

Member Functions

task_scheduler_handle()

Effects: Creates an empty instance of the task_scheduler_handle class that does not contain any references to the task scheduler.

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task_scheduler_handle(oneapi::tbb::attach)

Effects: Creates an instance of the task_scheduler_handle class that holds a reference to the task scheduler preventing its premature destruction.

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~task_scheduler_handle()

Effects: Destroys an instance of the task_scheduler_handle class. If not empty, releases a reference to the task scheduler and deactivates an instance of the task_scheduler_handle class.

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task_scheduler_handle(task_scheduler_handle &&other) noexcept

Effects: Creates an instance of the task_scheduler_handle class that references the task scheduler referenced by other. In turn, other releases its reference to the task scheduler.

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task_scheduler_handle &operator=(task_scheduler_handle &&other) noexcept

Effects: If not empty, releases a reference to the task scheduler referenced by this. Adds a reference to the task scheduler referenced by other. In turn, other releases its reference to the task scheduler. Returns: A reference to *this.

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explicit operator bool() const noexcept

Returns: true if this is not empty and refers to some task scheduler; false otherwise.

---

void release()

Effects: If not empty, releases a reference to the task scheduler and deactivates an instance of the task_scheduler_handle class; otherwise, does nothing. Non-blocking method.

Non-member Functions

void finalize(task_scheduler_handle &handle)

Effects: If handle is not empty, blocks the program execution until all worker threads have been completed; otherwise, does nothing. Throws the oneapi::tbb::unsafe_wait exception if it is not safe to wait for the completion of the worker threads.

The following conditions should be met for finalization to succeed:

• No active, not yet terminated, instances of task_arena class exist in the whole program.

• task_scheduler_handle::release is called for each other active instance of task_scheduler_handle class, possibly by different application threads.

Under these conditions, it is guaranteed that at least one finalize call succeeds, at which point all worker threads have been completed. If calls are performed simultaneously, more than one call might succeed.

Note

If user knows how many active task_scheduler_handle instances exist in the program, it is necessary to release all but the last one, then call finalize for the last instance.

Caution

The method always fails if called within a task, a parallel algorithm, or a flow graph node.

---

bool finalize(task_scheduler_handle &handle, const std::nothrow_t&) noexcept

Effects: If handle is not empty, blocks the program execution until all worker threads have been completed; otherwise, does nothing. The behavior is the same as finalize(handle) however, false is returned instead of exception or true if no exception.

Examples

#include <oneapi/tbb/global_control.h>
#include <oneapi/tbb/parallel_for.h>

#include <iostream>

int main() {
    oneapi::tbb::task_scheduler_handle handle;

    handle = oneapi::tbb::task_scheduler_handle{oneapi::tbb::attach{}};

    // Do some parallel work here, e.g.
    oneapi::tbb::parallel_for(0, 10000, [](int){});
    try {
        oneapi::tbb::finalize(handle);
        // oneTBB worker threads are terminated at this point.
    } catch (const oneapi::tbb::unsafe_wait&) {
        std::cerr << "Failed to terminate the worker threads." << std::endl;
    }
    return 0;
}

See also:

• attach