Re: Thread Function and access

From:

"Alex Blekhman" <xfkt@oohay.moc>

Newsgroups:

microsoft.public.vc.language

Date:

Sun, 10 Dec 2006 20:30:31 +0200

Message-ID:

<#5R2HlIHHHA.1468@TK2MSFTNGP04.phx.gbl>

"Peter Schmitt" wrote:

I have a written a Thread class (CMyThread) and have a
question
concerning the Thread Safety.
The reason for using a Thread is because i need to check
the Version of
Application (multilanguage) online by downloading the
version string.
Normally this action takes several seconds.

So i decided to call the Run() method of CMyThread, for
example:
Run("Connection failed", "New Version available", "You're
using the
latest Version" ...);

For easy explanation i will use only one string in the
following class.
My question is, if it's safe to access this.m_msg in the
Run Method. Or
is there any way to tell the Thread to call a method of my
CMyThread
after (!) executiuon to display the version check result?!

Thanks for any answers and best regards

class CMyThread
{
  private string m_msg;

  bool Start(string msg)
  {
     // Save string for later access in Run Method
     this.m_msg = msg;
     _beginthreadex(NULL, 0, &Starter, this, 0,
&m_ThreadID);
  }

  // The one and only Thread Function for performing some
work
  unsigned Run()
  {
     // Is it safe to access this.m_msg here?!
     // For example:
     MessageBox(NULL, this.m_msg.c_str(), "", NULL);
     // or...
     this.m_msg = "some text";
     // ...
     return 0;
  }

  static unsigned __stdcall Starter(void* pVoid)
  {
     CMyThread *pInst = (CMyThread*)pVoid;
     unsigned iRet = pInst->Run();
     _endthreadex(iRet);

^^^^^^^^^^^^^^^^^^

No need to call `_endthreadex'. It will be called
automatically by `_beginthreadex' when thread function
returns.

return iRet;
}
};

Actually, from the code you posted it's hard to tell whether
`CMyThread' class is thread safe. The most important part is
to keep `m_msg' member intact while thread is running. For
example, this usage won't be thread safe and likely will
lead to crash:

<code>

bool Foo()
{
    CMyThread myThread;
    bool ret = myThread.Start("Hello");

    return ret;
}

</code>

Here, `CMyThread' instance is created, new thread is started
and then function exits. So, local variable `myThread' is
destroyed upon return from `Foo' while new thread can be
still running. Any access to `m_msg' or even `this' within
the thread will cause an access violation, if you're lucky,
or will silently corrupt memory.

HTH
Alex