Creating N number of instances of MFC application - windows

Question

Can someone please tell me how to go about creating a maximum of N instances of an application in MFC?
Also, if N instances are running, and one instance gets closed, then one new instance can be created (but no more than N instances can run at any one time).
Thank you in advance.
a.

Answer 1

You can create a global semaphore that up to n process instances can enter. The n+1 th instance of your process will fail to enter the semaphore. Of course you should select a short timeout for the locking operation so you can present a meaningful feedback to the user.
For the semaphore stuff you can take a look at MSDN.

Answer 2

I'd use lock files. In your CMyApp::InitInstance() add:
CString Path;
// better get the path to the global app data or local user app data folder,
// depending on if you want to allow the three instances machine-wide or per user.
// Windows' file system virtualization will make GetModuleFileName() per user:
DWORD dw = GetModuleFileName(m_hInstance,
Path.GetBuffer(MAX_PATH), MAX_PATH);
Path.ReleaseBuffer();
// strip "exe" from filename and replace it with "lock"
Path = Path.Left(Path.GetLength()-3) + T("lock");
int i;
// better have the locking file in your class and do a clean Close on ExitInstance()!
CFile *pLockingFile = NULL;
for (i = 0; i < 3; i++) // restrict to three instances
{
CString Counter;
Counter.Format(T("%d"), i);
TRY
{
pLockingFile = new CFile(Path + Counter,
CFile::modeCreate|CFile::modeWrite|CFile::shareExclusive);
pLockingFile.Close();
break; // got an instance slot
}
CATCH( CFileException, e )
{
// maybe do something else here, if file open fails
}
END_CATCH
if (i >= 3)
return TRUE; // close instance, no slot available
}
Edit: To lock the software machine-wide, retrieve the common app folder using the following function instead of calling GetModuleFileName().
#pragma warning(disable: 4996) // no risk, no fun
BOOL GetCommonAppDataPath(char *path)
{
*path = '\0';
if (SHGetSpecialFolderPath(NULL, path, CSIDL_COMMON_APPDATA, TRUE))
{
strcat(path, T("\\MyApplication")); // usually found under C:\ProgramData\MyApplication
DWORD dwFileStat = GetFileAttributes(path);
if (dwFileStat == 0xffffffff) // no MyApplication directory yet?
CreateDirectory(path, NULL); // create it
dwFileStat = GetFileAttributes(path); // 2nd try, just to be sure
if (dwFileStat == 0xffffffff || !(dwFileStat & FILE_ATTRIBUTE_DIRECTORY))
return FALSE;
return TRUE;
}
return FALSE;
}
Note: This will only work from Vista on. If you have XP, writing to a global directory is an easy task, e.g. C:\Windows\Temp. I have put the function in a helper dll I only load if the OS is Vista or higher. Otherwise your software won't start because of unresolved references in system dlls.