Double buffering implementation using futures and promises using C ++ 11
I started looking into multithreading and came across futures and promises for synchronizing threads across shared resources. So, I thought about implementing the known double buffering problem using Futures and Promises (one producer and one consumer). The main methodology I thought of was:
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 am executing this code, I came across his gymagnet problem, which I am completely 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 a bug in this error, it is advised to bind the -lpthread switch at link time and compile time. but couldn't solve the problem.
Please help me where I am going wrong.
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You can not call set_value for promise
more than 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 must look for a different approach. For example, you can use two std :: condition_variables - set them in the producer and wait for them on the consumer.
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