Implementing Double Buffering using Futures and Promises using c++11 - c++

Question

I started learning multi-threading and came across futures and promises for synchronizing threads over shared resources. So, I thought of implementing a famous Double Buffering problem using Futures and Promises( single producer and single consumer).
The basic methodology what I have thought is :
ProducerThread:
loop:
locks_buffer1_mutex
fills_buffer1
unlocks_buffer1_mutex
passes number 1 to Consumer thread using promise.setvalue()
locks_buffer2_mutex
fills_buffer2
unlocks_buffer2_mutex
passes number 2 to Consumer thread using promise.setvalue()
back_to_loop
ConsumerThread :
loop:
wait_for_value_from_promise
switch
case 1:
lock_buffer1_mutex
process(buffer1)
unlock_buffer1_mutex
print_values
break
case 2:
lock_buffer2_mutex
process(buffer2)
unlock_buffer2_mutex
print_values
break
back_to_loop
Here is the code:
#include <iostream>
#include <thread>
#include <vector>
#include <future>
#include <mutex>
#include <iterator>
std::mutex print_mutex;
std::mutex buffer1_mutex;
std::mutex buffer2_mutex;
std::vector<int> buffer1;
std::vector<int> buffer2;
bool notify;
void DataAcquisition(std::promise<int> &p)
{
std::this_thread::sleep_for(std::chrono::seconds(2));
while(true)
{
{
std::lock_guard<std::mutex> buff1_lock(buffer1_mutex);
for(int i=0;i<200;i++)
{
buffer1.push_back(i);
}
}
p.set_value(1);
{
std::lock_guard<std::mutex> buff2_lock(buffer2_mutex);
for(int i=0;i<200;i++)
{
buffer2.push_back(199-i);
}
}
p.set_value(2);
}
}
void DataExtraction(std::future<int> &f)
{
std::vector<int>::const_iterator first,last;
std::vector<int> new_vector;
std::ostream_iterator<int> outit(std::cout, " ");
while(true)
{
int i = f.get();
std::cout << "The value of i is :" << i << std::endl;
switch(i)
{
case 1:
{
std::lock_guard<std::mutex> buff1_lock(buffer1_mutex);
first = buffer1.begin();
last = first + 10;
}
new_vector = std::vector<int>(first,last);
{
std::lock_guard<std::mutex> print_lock(print_mutex);
std::copy(new_vector.begin(),new_vector.end(),outit);
}
break;
case 2:
{
std::lock_guard<std::mutex> buff2_lock(buffer2_mutex);
first = buffer2.begin();
last = first + 10;
}
new_vector = std::vector<int>(first,last);
{
std::lock_guard<std::mutex> print_lock(print_mutex);
std::copy(new_vector.begin(),new_vector.end(),outit);
}
break;
}
}
}
int main()
{
std::promise<int> p;
std::future<int> f = p.get_future();
std::thread thread1(DataAcquisition,std::ref(p));
std::thread thread2(DataExtraction,std::ref(f));
thread1.join();
thread2.join();
return 0;
}
When I execute this code I came across through his giagntic problem, which I am fully unaware of
terminate called after throwing an instance of 'std::future_error' terminate called recursively
what(): 0 1 2 3 4 5 6 7 8 9 Promise already satisfied
Press <RETURN> to close the window
I have googled about this error, it is suggested to link -lpthread switch during linking and compile time. but could n't resolve the issue.
Please help me, Where am i going wrong..

Answer 1

You can't call set_value for a promise more that once, which is illustrated by the following code:
#include <future>
int main() {
std::promise<int> p;
p.set_value(1);
p.set_value(2); // Promise already satisfied
}
You have to look for another approach. For example, you could use two std::condition_variables - set them in producer, and wait for them in consumer.