Contents

task_scheduler_handle Class

Note

To enable this feature, set the TBB_PREVIEW_WAITING_FOR_WORKERS macro to 1.

Description

The oneapi::tbb::task_scheduler_handle class and the oneapi::tbb::finalize function allow 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.

API

Synopsis

namespace oneapi {
    namespace tbb {

        class task_scheduler_handle {
        public:
            task_scheduler_handle() = default;
            ~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;

            static task_scheduler_handle get();

            static void release(task_scheduler_handle& handle);
        };

        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 instance of the task_scheduler_handle class that does not contain any reference to the task scheduler.

~task_scheduler_handle()

Effects: Destroys an instance of the task_scheduler_handle class. Releases a reference to the task scheduler and deactivates an instance of the task_scheduler_handle class.

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.

task_scheduler_handle &operator=(task_scheduler_handle &&other) noexcept

Effects: 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.

explicit operator bool() const noexcept

Returns: true if this references any task scheduler; false otherwise.

task_scheduler_handle get()

Returns: An instance of the task_scheduler_handle class that holds a reference to the task scheduler preventing its premature destruction.

void release(task_scheduler_handle &handle)

Effects: Releases a reference to the task scheduler and deactivates an instance of the task_scheduler_handle class. Non-blocking method.

Non-member Functions

void finalize(task_scheduler_handle &handle)

Effects: Blocks the program execution until all worker threads have been completed. 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 class task_arena exist in the whole program;

  • task_scheduler_handle::release is called for each other active instance of class task_scheduler_handle, 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 you know 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: Blocks the program execution until all worker threads have been completed. Same as above, but returns true if all worker threads have been completed successfully, or false if it is not safe to wait for the completion of the worker threads.

Examples

#define TBB_PREVIEW_WAITING_FOR_WORKERS 1
#include <oneapi/tbb/global_control.h>
#include <oneapi/tbb/parallel_for.h>

#include <iostream>

int main() {
    oneapi::tbb::task_scheduler_handle handle = oneapi::tbb::task_scheduler_handle::get();
    // 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;
}