Trying to switch between threads using condition_variables - c++

Question

I have two threads. Both get initialized, but the first thread runs first, while the second thread waits for the first thread (call_once(flagEven1, [&](){cond2.wait(lock, [&](){return zeroEvenOddFlag == 1; }); });). After a certain condition is met, the first thread wakes up the second thread and goes to sleep.
zeroEvenOddFlag = 0;
cond1.notify_all();
cond2.wait(lock, [&](){return zeroEvenOddFlag == 1; });
This process alternates until the task is finished. Right now the threads just get locked up and I don't know what's happening. The threads together should process the input string and produce two strings of zeros and non-zero numbers. I corrected for spurious wakeups by adding an integer that is shared between the threads (zeroEvenOddFlag).
#include <string>
#include <thread>
#include <future>
#include <cstdio>
#include <iostream>
#include <queue>
#include <condition_variable>
using namespace std;
mutex mu;
condition_variable cond1;
condition_variable cond2;
once_flag flagZero;
once_flag flagEven;
once_flag flagEven1;
int zeroEvenOddFlag = 0;
string printerFunc(queue<char>& input, int zeroEvenOdd, once_flag& flag){
string output = "";
function<void(string&)> f;
unique_lock<mutex> lock(mu);
call_once(flag, [&](){
if (zeroEvenOdd == 1){
f = [&](string& output){
int i = input.front() - '0';
if (i == 0){
output += to_string(i);
input.pop();
}
else {
zeroEvenOddFlag = 1;
cond2.notify_all();
cond1.wait(lock, [&](){return zeroEvenOddFlag == 0; });
}
};
}
else if (zeroEvenOdd == 2){
call_once(flagEven1, [&](){cond2.wait(lock, [&](){return zeroEvenOddFlag == 1; }); });
f = [&](string& output){
int i = input.front() - '0';
if (i != 0){
output += to_string(i);
input.pop();
}
else {
zeroEvenOddFlag = 0;
cond1.notify_all();
cond2.wait(lock, [&](){return zeroEvenOddFlag == 1; });
}
};
}
});
while(input.size()!=0){
f(output);
}
return output;
}
int _tmain(int argc, _TCHAR* argv[])
{
string input = "0102030";
queue<char> inputQueue;
for(char c : input){
inputQueue.push(c);
}
auto zeros = async(launch::async, printerFunc, ref(inputQueue), 1, ref(flagZero));
auto evens = async(launch::async, printerFunc, ref(inputQueue), 2, ref(flagEven));
string zerosString = zeros.get();
string evensString = evens.get();
cout << "these are the zeros " << zerosString << endl;
cout << "these are the evens " << evensString << endl;
this_thread::sleep_for(chrono::seconds(10));
return 0;
}

Answer 1

You need to always check for spurious wake-ups conditions. The use of cond2.wait(lock, [&](){return zeroEvenOddFlag == 1; }); and the like help check. Also, you reach an issue at the end of your loop. Which ever thread finally owns the mutex has to release it when it consumes the last bit of the queue, so that the other threads can be freed up to finish. The final code should look like this:
#include <string>
#include <thread>
#include <future>
#include <cstdio>
#include <iostream>
#include <queue>
#include <condition_variable>
using namespace std;
mutex mu;
condition_variable cond1;
condition_variable cond2;
once_flag flagZero;
once_flag flagEven;
once_flag flagEven1;
int zeroEvenOddFlag = 0;
string printerFunc(queue<char>& input, int zeroEvenOdd, once_flag& flag){
string output = "";
function<void(string&)> f;
unique_lock<mutex> lock(mu);
call_once(flag, [&](){
if (zeroEvenOdd == 1){
f = [&](string& output){
int i = input.front() - '0';
if (i == 0){
output += to_string(i);
input.pop();
if (input.size() == 0){
zeroEvenOddFlag = 1;
cond2.notify_all();
}
}
else {
zeroEvenOddFlag = 1;
cond2.notify_all();
cond1.wait(lock, [&](){return zeroEvenOddFlag == 0; });
}
};
}
else if (zeroEvenOdd == 2){
call_once(flagEven1, [&](){cond2.wait(lock, [&](){return zeroEvenOddFlag == 1; }); });
f = [&](string& output){
int i = input.front() - '0';
if (i != 0){
output += to_string(i);
input.pop();
if (input.size() == 0){
zeroEvenOddFlag = 0;
cond1.notify_all();
}
}
else {
zeroEvenOddFlag = 0;
cond1.notify_all();
cond2.wait(lock, [&](){return zeroEvenOddFlag == 1; });
}
};
}
});
while(input.size()!=0){
f(output);
}
return output;
}
int _tmain(int argc, _TCHAR* argv[])
{
string input = "0102030";
queue<char> inputQueue;
for(char c : input){
inputQueue.push(c);
}
auto zeros = async(launch::async, printerFunc, ref(inputQueue), 1, ref(flagZero));
auto evens = async(launch::async, printerFunc, ref(inputQueue), 2, ref(flagEven));
string zerosString = zeros.get();
string evensString = evens.get();
cout << "these are the zeros " << zerosString << endl;
cout << "these are the evens " << evensString << endl;
this_thread::sleep_for(chrono::seconds(10));
return 0;
}