CODEWITHSUNDEEP
c++c++11concurrencystd
Andrew Spott
Andrew Spott

Reputation: 3637

signaling parent threads in c++11

I have a function that I want to run in different threads. The function populates a data structure, for example:

per_thread(int start_value, std::vector<SomeStruct>& reference)
{
    for ( size_t i = 0; i < 500; i++ )
    {
        reference.push_back(func(i));
        if (i == 2)
            send_signal_back();
    }
}

However, after this is done going through the loop some number of times, I want to start another thread, using this as the start value. Unfortunately, I don't understand how to send a signal back to the parent thread.

So I want something like this:

for( size_t j = 0; j < 5000; j += num_threads)
{
    for (size_t i = 0; i < num_threads; i++)
    {
        std::async(per_thread(foo(j+i), std::ref(vec));
        //wait for signal
    }
}

How do I send such a signal?

Upvotes: 1

Views: 731

Answers (1)

Kerrek SB
Kerrek SB

Reputation: 477120

I wouldn't use async, because that's too high-level and does something else. (Here's a little rant of mine that touches on async.)

It looks like you really just want threads and control them manually.

Try this:

#include <vector>
#include <thread>

std::vector<std::thread> threads;

for (std::size_t j = 0; j < 5000; j += num_threads)
{
    for (std::size_t i = 0; i != num_threads; ++i)
    {
         threads.emplace_back(per_thread, foo(i + j), std::ref(vec));
    }
}

for (auto & t: threads)
{
    t.join();
}

This will finish once the longest-running thread has finished. (The "long-tail" effect.)

Upvotes: 4

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