Error in breaking a loop during its execution - c++

Question

Hello I was trying to break the loop in middle of its execution.The loop shows a countdown of seconds from 5-1 .I want to break the loop in middle using any keystroke entered by the user.I researched for some time and created a thread. I set a global variable and updated it in the second thread. I gave a condition in the main function using that global variable but the loop doesn't break.
Here is the code.
Please help.
#include<iostream>
#include<windows.h>
#include<stdlib.h>
#include<iomanip>
#include<stdio.h>
using namespace std;
bool stop=false;
DWORD WINAPI thread1(LPVOID pm)
{
//check the getchar value
int a = getchar();
while (a != '0'){
a = getchar();
}
stop = true;
return 0;
}
int main()
{
HANDLE handle = CreateThread(NULL, 0, thread1, NULL, 0, NULL);
for(int i=5;i>0 && !stop;i--)
{
cout<<"\n\n\n\n\n\n";
cout<<setw(35);
cout<<i;
Sleep(1000);
system("CLS");
}
system("PAUSE");
}
The program counts down and in middle of the count down i tried to break the loop.thread1 function takes an input and modifies stop(global variable). But the loop in main function doesn't break(it should).Loop goes on decreasing loop variable, becomes zero and loop ends.

Answer 1

You have to declare stop as volatile
volatile bool stop
It informs compiler not optimized (by cache) access to variable, because another thread can modify it.
Also, take care about read and write access on global variable by many thread : in most of case, you have to protect them using mutex. (I think in your case it's not necessary according basic small type and basic access, but take care)
EDIT
As ask in comments, this is my design when inverting thread :
#include<iostream>
#include<windows.h>
#include<stdlib.h>
#include<iomanip>
#include<stdio.h>
using namespace std;
volatile bool stop = false;
DWORD WINAPI thread1(LPVOID pm)
{
for(int i=5;i>0 && !stop;i--)
{
cout<<"\n\n\n\n\n\n";
cout<<setw(35);
cout<<i;
Sleep(1000);
system("CLS");
}
return 0;
}
int main()
{
HANDLE handle = CreateThread(NULL, 0, thread1, NULL, 0, NULL);
//check the getchar value
int a = getchar();
while (a != '0'){
a = getchar();
}
stop = true;
WaitForSingleObject(handle, INFINITE);
system("PAUSE");
}
With this solution, it will stop after waiting 1s. If you want to terminate immediatly, you can use TerminateThread but read this before : https://msdn.microsoft.com/en-us/library/windows/desktop/ms686717%28v=vs.85%29.aspx

Answer 2

Found it. getchar() was waiting for carriage return. So i used _getch() from conio library. Like this.
#include<iostream>
#include<Windows.h>
#include<conio.h>
#include<stdlib.h>
#include<iomanip>
using namespace std;
volatile bool stop = false;
DWORD WINAPI thread1(LPVOID pm)
{
int a = 0;
while (a==0)
{
a = _getch();
}
stop = true;
return 0;
}
int main()
{
HANDLE handle = CreateThread(NULL, 0, thread1, NULL, 0, NULL);
int i;
for ( i = 5; i > 0 && !stop; i--)
{
cout << "\n\n\n\n\n\n";
cout << setw(35);
cout << i;
Sleep(1000);
system("CLS");
}
if (i != 0)
cout << "Loop broken sucessflly.\n";
else
cout << "Attempt failed\n";
system("PAUSE");
}