Re: CreateThread() question

From:

"Jochen Kalmbach [MVP]" <nospam-Jochen.Kalmbach@holzma.de>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 01 Jan 2007 22:10:01 +0100

Message-ID:

<#Fh12keLHHA.4928@TK2MSFTNGP06.phx.gbl>

Hi Igor!

Is h simply a pointer to id (or a value identical to id)?

A thread is uniquely identified by its ID, but there may be multiple
handles referring to the same thread (see e.g. DuplicateHandle). The ID
is valid system-wide, the handle is valid within a single process only.
The handle, in addition to referring to a thread, also carries access
control list specifying what operations you are allowed to perform on
it.

Just a small addition: The thread ID is only unique, if the thread is
running... if the thread has terminated, the OS will "reuse" this
thread-ID after a while.

You can verify this by creating a simple test-program:

#include <windows.h>
#include <tchar.h>
#include <hash_set>

CRITICAL_SECTION cs;
stdext::hash_set<DWORD> s_ThreadIDs;

DWORD WINAPI MyThread(LPVOID)
{
   DWORD dwThreadId = GetCurrentThreadId();
   EnterCriticalSection(&cs);
   if (s_ThreadIDs.find(dwThreadId) != s_ThreadIDs.end())
     _tprintf(_T("Duplicate ThreadID: %d\n"), dwThreadId);
   else
     s_ThreadIDs.insert(dwThreadId);
   LeaveCriticalSection(&cs);
   return dwThreadId;
}

int _tmain()
{
   InitializeCriticalSection(&cs);
   int i = 0;
   while(true)
   {
     DWORD dwThreadID;
     HANDLE hThread = CreateThread(NULL, 0, MyThread, NULL, 0, &dwThreadID);
     if (hThread != INVALID_HANDLE_VALUE)
       CloseHandle(hThread);
     else
       Sleep(1000); // more than about 2000 threads... so be patient...
     i++;
     if (( i % 100) == 0) Sleep(1);
   }
}

--
Greetings
   Jochen

    My blog about Win32 and .NET
    http://blog.kalmbachnet.de/