mutex with waitforsingleobject not working as expected [duplicate] - c++

Question

I have written a simple singleton application.
The following is my sample main class
// ThreadsafeSingletonUsingSemaphore.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <iostream>
#include <conio.h>
#include "MySingleton.h"
using namespace std;
int i =0;
#define THREADCOUNT 100
DWORD WINAPI ThreadProc(LPVOID lParam);
HANDLE g_semaphore = NULL;
int _tmain(int argc, _TCHAR* argv[])
{
g_semaphore = CreateSemaphore(NULL,1,1,_T("TreadOne"));
HANDLE hThread[THREADCOUNT];
DWORD aThreadID;
for(int iCount = 0; iCount < THREADCOUNT ; iCount++)
{
hThread[iCount] = CreateThread(NULL, 0, ThreadProc, 0,0, &aThreadID);
if( hThread[iCount] == NULL )
{
cout<<"CreateThread error: %d" << GetLastError() << endl;
return 1;
}
}
WaitForMultipleObjects(THREADCOUNT, hThread, TRUE, INFINITE);
// Close thread and semaphore handles
for(int i=0; i < THREADCOUNT; i++ )
CloseHandle(hThread[i]);
cout << MySingleton::getInstance().getCounter() << endl ;
CloseHandle(g_semaphore);
_getch();
return 0;
}
DWORD WINAPI ThreadProc(LPVOID lpParam)
{
//DWORD result = WaitForSingleObject(g_semaphore,INFINITE);
//if(WAIT_OBJECT_0 == result)
MySingleton::getInstance().incrementCouner();
//ReleaseSemaphore(g_semaphore,1, NULL);
return TRUE;
}
This is my singleton implementation class.
#include "StdAfx.h"
#include "MySingleton.h"
MySingleton* MySingleton::m_instance = NULL;
HANDLE MySingleton::m_hSem = CreateSemaphore(NULL, 1, 1, _T("MySingleton"));
HANDLE MySingleton::m_One = CreateSemaphore(NULL, 1, 1, _T("MyOne"));
MySingleton::MySingleton(void) : m_counter(0)
{
}
MySingleton::~MySingleton(void)
{
cout << "destructor" << endl;
CloseHandle(m_hSem);
CloseHandle(m_One);
}
MySingleton& MySingleton::getInstance()
{
DWORD result = WaitForSingleObject(m_hSem, INFINITE);
if(WAIT_OBJECT_0 == result)
{
if(m_instance == NULL)
{
cout << "creating" << endl;
m_instance = new MySingleton();
}
}
ReleaseSemaphore(m_hSem,1,NULL);
return *m_instance;
}
void MySingleton::setCouner(int iCount_in)
{
m_counter = iCount_in;
}
int MySingleton::getCounter()
{
return m_counter;
}
void MySingleton::incrementCouner()
{
DWORD result = WaitForSingleObject(m_One, INFINITE);
if(WAIT_OBJECT_0 == result)
m_counter++;
ReleaseSemaphore(m_One,1,NULL);
}
This is my .h class.
#pragma once
#include <windows.h>
#include <iostream>
#include <conio.h>
using namespace std;
class MySingleton
{
private:
static HANDLE m_hSem, m_One;
HANDLE m_hCountSem;
static MySingleton* m_instance;
int m_counter;
MySingleton();
MySingleton(const MySingleton& obj_in);
MySingleton& operator=(const MySingleton& obj_in);
public:
~MySingleton(void);
static MySingleton& getInstance();
void setCouner(int iCount_in);
int getCounter();
void incrementCouner();
};
The problem is the final value of counter is never 100. can someone please explain me why and what is that i am doing wrong.I am not able to understand the problem. When I introduce sleep in the main before creating each thread, it works fine.

Answer 1

The problem is that call to WaitForMultipleObjects handles up to MAXIMUM_WAIT_OBJECTS which, at least in Visual Studio 2017, is 64.
Notice how your call to WaitForMultipleObjects to join threads returns WAIT_FAILED.
In order to wait for more objects one should, according to the documentation:
To wait on more than MAXIMUM_WAIT_OBJECTS handles, use one of the following methods:
Create a thread to wait on MAXIMUM_WAIT_OBJECTS handles, then wait on that thread plus the other handles. Use this technique to break the handles into groups of MAXIMUM_WAIT_OBJECTS.
Call RegisterWaitForSingleObject to wait on each handle. A wait thread from the thread pool waits on MAXIMUM_WAIT_OBJECTS registered objects and assigns a worker thread after the object is signaled or the time-out interval expires.

Answer 2

You don't need to write all of that code. The easiest way to implement a threadsafe singleton is to use Scott Meyer's singleton idiom:
class Singleton {
int counter;
mutable std::mutex counter_guard;
Singleton() {}
public:
Singleton(const Singleton&) = delete;
Singleton(Singleton&&) = delete;
Singleton& operator=(const Singleton&) = delete;
Singleton& operator=(Singleton&&) = delete;
static Singleton& instance() {
static Singleton theInstance;
return theInstance;
}
void setCounter(int newVal) {
std::unique_lock<std::mutex> lock(counter_guard);
counter = newVal;
}
void incrementCounter() {
std::unique_lock<std::mutex> lock(counter_guard);
++counter;
}
int getCounter() const {
std::unique_lock<std::mutex> lock(counter_guard);
return counter;
}
};
An even easier way would be to use a std::atomic<int> type for the counter member variable. Then the mutex and lock guards can be omitted at all.