Single producer / multiple consumer deadlock - c++

Question

The following code reasults in a deadlock. The problem is that I cannot figure out how unlock the consumers waiting on the condition variable. The consumer should loop and consume from the stack when a certain condition is met. I've tried exiting when the stack is empty but of course it doesn't work.
Stack.h
class Stack {
private:
std::stack<int> stack;
std::mutex mutex;
std::condition_variable is_empty;
bool done;
public:
Stack();
void push(int);
void pop();
void print();
bool isDone() const;
~Stack();
};
Stack.cpp
#include <iostream>
#include <sstream>
#include <thread>
#include "Stack.h"
void Stack::push(int x) {
std::lock_guard lock(mutex);
std::stringstream msg1;
msg1 << "producer " << std::this_thread::get_id() << " pushing " << x << std::endl;
std::cout << msg1.str();
stack.push(x);
std::stringstream msg;
msg << "producer " << std::this_thread::get_id() << ": " << x << " pushed" << std::endl;
std::cout << msg.str();
is_empty.notify_all();
}
void Stack::pop() {
std::unique_lock lock(mutex);
std::stringstream msg;
msg << "consumer " << std::this_thread::get_id() << " waiting to consume" << std::endl;
std::cout << msg.str();
is_empty.wait(lock, [this] { return !stack.empty(); });
if (!stack.empty()) {
stack.pop();
std::stringstream msg1;
msg1 << "consumer " << std::this_thread::get_id() << " popped" << std::endl;
std::cout << msg1.str();
} else {
done = true;
is_empty.notify_all();
}
}
void Stack::print() {
std::lock_guard lock(mutex);
for (int i = 0; i < stack.size(); i++) {
std::cout << "\t" << stack.top() << std::endl;
}
}
Stack::~Stack() {
}
bool Stack::isDone() const {
return done;
}
Stack::Stack() : done(false) {}
main.cpp
#include <thread>
#include <vector>
#include <iostream>
#include "Stack.h"
int main() {
Stack stack;
std::vector<std::thread> producer;
std::vector<std::thread> consumer;
for (int i = 0; i < 10; i++) {
consumer.emplace_back([&stack]{
while (!stack.isDone()) {
stack.pop();
}
});
}
for (int i = 0; i < 1; i++) {
producer.emplace_back([&stack]{
for (int j = 0; j < 5; ++j) {
stack.push(random());
}
});
}
for (int k = 0; k < producer.size(); k++) {
producer[k].join();
std::cout << producer[k].get_id() << " joined" << std::endl;
stack.print();
}
for (int j = 0; j < consumer.size(); j++) {
consumer[j].join();
std::cout << consumer[j].get_id() << " joined" << std::endl;
stack.print();
}
return 0;
}

Answer 1

Your code is not deadlocked but your threads are waiting for more input because you haven't configure the value of done properly.
There is no way that the else condition is invoked here
is_empty.wait(lock, [this] { return !stack.empty(); });
if (!stack.empty()) {
stack.pop();
std::stringstream msg1;
msg1 << "consumer " << std::this_thread::get_id() << " popped" << std::endl;
std::cout << msg1.str();
} else {
done = true;
is_empty.notify_all();
}
Looking from the code it seems like what you want is that after the producer stops producing the consumer should wake up and empty. But this is not the way to implement it. After the producer has pushed 5 elements you should set done =true from there.
Also as answered by madducci you need to change the location of notify_all();
This is something which worked for me
is_empty.wait(lock, [&] { return stack.size()>0 || done; });
if (!stack.empty()) {
int val=stack.top();
stack.pop();
std::stringstream msg1;
msg1 << "consumer " << std::this_thread::get_id() << " popped " <<val<<std::endl;
std::cout << msg1.str();
}

Answer 2

Looks like you have a logic error in your pop function: you never call notify_all() in case you pop an element from the stack.
The correct way should be this one:
void Stack::pop() {
std::unique_lock lock(mutex);
std::stringstream msg;
msg << "consumer " << std::this_thread::get_id() << " waiting to consume" << std::endl;
std::cout << msg.str();
is_empty.wait(lock, [this] { return !stack.empty(); });
if (!stack.empty()) {
stack.pop();
std::stringstream msg1;
msg1 << "consumer " << std::this_thread::get_id() << " popped" << std::endl;
std::cout << msg1.str();
} else {
done = true;
}
is_empty.notify_all();
}
You also invoke pop() before push() in your main