I have the following situation for multithreading in C++:

Struct MyData
{
….
}
//Class to create a thread specific data
Class MiscData
{
    public:
        CList<CPtrArray, MyData*> myDataList;
        HANDLE m_dbHanle;
        String m_SQLCmd;
        MiscData ()
{
    m_dbHandle = DatabaseConnect(connection parameters);
}
inline FillMyDataList(MyData* data)
{
    myData* pData = allocate memory;
    pData = data;
    myDataList.Add(pData);
}
BuildSQLCmd(string str)
{
    m_SQLCmd .Format(“…”, str);
}
}

Global Methods:
Void GetMyDataFromDB(MiscData* pMiscData)
{
    pMiscData->ByuldSQLCmd(_T(“Select * from myTable”));
BuildDataFromDB(pMiscData);
//Access MyData structure from pMiscData->myDataList

}
BuildDataFromDB(MiscData* pMiscData)
{
    int nReturn;
    ExecuteSQLCMD(pMiscData, &nReturn);
    While(recordsin table)
{
    myData.x=record.columnValue
    pMiscData-> FillMyDataList(&myData);
}
}
Int ExecuteSQLCMD(MiscData* pMiscData, int* nReturn)
{
    String tablename = Extract Table name from pMiscData->m_SQLCmd;//SQL query
    Calls SQLExecDirect(pMiscData->m_SQLCmd)
    …
}

ThreadFunction(void* pParam)
{
    MiscData* pThreadData = (MiscData*) pParam;
}

main()
{
    MiscData pMiscData1 = new MiscData(); //Thread specific data
MiscData pMiscData2 = new MiscData();//Thread specific data
MiscData pMiscData3 = new MiscData();//Thread specific data

//Handle hT1  = createthread()
Unsigned uiThreadId;

Handle hT1  = _beginThreadex(NULL,
                0,
ThreadFuntion,
pMiscData1,
CREATE_SUSPENDED,
& uiThreadId);

Handle hT2  = _beginThreadex(NULL,
                0,
ThreadFuntion,
pMiscData2,
CREATE_SUSPENDED,
& uiThreadId);

Handle hT3  = _beginThreadex(NULL,
                0,
ThreadFuntion,
pMiscData3,
CREATE_SUSPENDED,
& uiThreadId);

    ResumeThread(hT1);
    ResumeThread(hT2);
    ResumeThread(hT3);
WaitForSingleObject(ht1,INFINITE);
WaitForSingleObject(ht2,INFINITE);
WaitForSingleObject(ht3,INFINITE);

}

All threads have same thread function. Thread function in turn calls different methods, which in turn calls other methods. In all the methods a thread specific data (pMiscData) is passed.
My questions are:
(1)Creating thread specific data (pMiscData1, pMiscData2, pMiscData3)and passing them separately to each thread, does it make really a thread specific data and is only available to its respective thread?

(2)As all threads can enter a method simultaneously, so threads can preempt each other, if so what would happen to the data in the thread specific data (pMiscData1)?

(3)After pre-empting, if a thread returns back, does it start from wherever it was left off?

(4)Other than creating thread specific objects in the main thread and passing them to their respective threads(while creating thread), I do not have any global or static variables, so my above program is a thread safe?

Please suggest me what I am doing wrong.

Sorry for the long message, sorry for any confusion. I am happy to answer any questions.
Thanks for your time & suggestions in advance.

Thanks again.
-BubblesM

Edited 3 Years Ago by Dani: Formatting fixed

First, use code tags when posting - it makes reading the code easier.

1) As you've coded it, yes - kind of. That is, you create each object in main(), then pass each off to a distinct thread. However, there's nothing stopping you from, after starting the thread, from modifying the data in, say "pMiscData1", since, of course, you've passed a pointer to the thread function, and the thread function (short as it is - I understand this is an example) doesn't copy the data from the object into a thread-local variable. If you had a long-running thread that was working on the data, statements further in main() could change the data in pMiscData1 and affect what is going on in the thread. But, as you've coded it, there's no way that operations in the first thread can affect the data being worked on in the second - that's good.

2) Nothing. The OS will handle that.

3) Yes. The universe would cease to exist if that did not happen.

4) As far as I can tell without running it, yes, it appears so.

What *is* going wrong with your program? That is, what symptoms is it exhibiting? Just as a guess, are you hanging at one of the WaitForSingleObject calls? If so, see this link about threads that run so fast they may not properly set the thread handle (see the "Remarks" section).

Hope this helps!

Thank you so much. Next time I will yous code tags. I had many doubts and wanted to hear some experts' opinion. Your reply has given me lot of confidence. Will update once after I complete my testing.

Thanks again.

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