your problem can be solved with the producer consumer pattern.
I got inspired from Wikipedia so here is the link if you want some more details.
https://en.wikipedia.org/wiki/Producer%E2%80%93consumer_problem
I used a random number generator to generate the volatile variable but you can change that part.
Here is the code: it can be improved in terms of style (using C++11 for random numbers) but it produces what you expect.
#include <iostream>
#include <sstream>
#include <vector>
#include <stack>
#include <thread>
#include <mutex>
#include <atomic>
#include <condition_variable>
#include <chrono>
#include <stdlib.h> /* srand, rand */
using namespace std;
//random number generation
std::mutex mutRand;//mutex for random number generation (given that the random generator is not thread safe).
int GenerateNumber()
{
std::lock_guard<std::mutex> lk(mutRand);
return rand() % 3;
}
// print function for "thread safe" printing using a stringstream
void print(ostream& s) { cout << s.rdbuf(); cout.flush(); s.clear(); }
// Constants
//
const int num_producers = 3; //the three producers of random numbers
const int num_consumers = 1; //the only consumer
const int producer_delay_to_produce = 10; // in miliseconds
const int consumer_delay_to_consume = 30; // in miliseconds
const int consumer_max_wait_time = 200; // in miliseconds - max time that a consumer can wait for a product to be produced.
const int max_production = 1; // When producers has produced this quantity they will stop to produce
const int max_products = 1; // Maximum number of products that can be stored
//
// Variables
//
atomic<int> num_producers_working(0); // When there's no producer working the consumers will stop, and the program will stop.
stack<int> products; // The products stack, here we will store our products
mutex xmutex; // Our mutex, without this mutex our program will cry
condition_variable is_not_full; // to indicate that our stack is not full between the thread operations
condition_variable is_not_empty; // to indicate that our stack is not empty between the thread operations
//
// Functions
//
// Produce function, producer_id will produce a product
void produce(int producer_id)
{
while (true)
{
unique_lock<mutex> lock(xmutex);
int product;
is_not_full.wait(lock, [] { return products.size() != max_products; });
product = GenerateNumber();
products.push(product);
print(stringstream() << "Producer " << producer_id << " produced " << product << "\n");
is_not_empty.notify_all();
}
}
// Consume function, consumer_id will consume a product
void consume(int consumer_id)
{
while (true)
{
unique_lock<mutex> lock(xmutex);
int product;
if(is_not_empty.wait_for(lock, chrono::milliseconds(consumer_max_wait_time),
[] { return products.size() > 0; }))
{
product = products.top();
products.pop();
print(stringstream() << "Consumer " << consumer_id << " consumed " << product << "\n");
is_not_full.notify_all();
}
}
}
// Producer function, this is the body of a producer thread
void producer(int id)
{
++num_producers_working;
for(int i = 0; i < max_production; ++i)
{
produce(id);
this_thread::sleep_for(chrono::milliseconds(producer_delay_to_produce));
}
print(stringstream() << "Producer " << id << " has exited\n");
--num_producers_working;
}
// Consumer function, this is the body of a consumer thread
void consumer(int id)
{
// Wait until there is any producer working
while(num_producers_working == 0) this_thread::yield();
while(num_producers_working != 0 || products.size() > 0)
{
consume(id);
this_thread::sleep_for(chrono::milliseconds(consumer_delay_to_consume));
}
print(stringstream() << "Consumer " << id << " has exited\n");
}
//
// Main
//
int main()
{
vector<thread> producers_and_consumers;
// Create producers
for(int i = 0; i < num_producers; ++i)
producers_and_consumers.push_back(thread(producer, i));
// Create consumers
for(int i = 0; i < num_consumers; ++i)
producers_and_consumers.push_back(thread(consumer, i));
// Wait for consumers and producers to finish
for(auto& t : producers_and_consumers)
t.join();
return 0;
}
Hope that helps, tell me if you need more info or if you disagree with something :-)
And Good Bastille Day to all French people!
atomic? – Enwreathe