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Posted 10 Feb 2005

An AutoUpdate Trick

, 10 Feb 2005
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This article provides a simple autoupdate class that can be added to any application.

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The company I work for, AutoUpdate+, creates software that manages updates for Win32-based software, which is great for mid to large scale users who need such a solution, but is a bit of overkill for the small developer. Many small developers only require a simple update solution, and frequently, it is only required to manage the update of the main project executable. This article will describe an autoupdate class I created, which is very simple to add to a project and which runs a little trick to update the main project executable.

Using the code

A class called CAutoUpdater was designed, with the goals that it is easy to add to any application, fully manages the update process, and correctly handles any errors. To use this class, simply call the CheckForUpdate method passing in the URL of your update server.

void CMFCUpdaterDlg::OnUpdatecheck() 
    CAutoUpdater updater;
    if (CAutoUpdater::Success == 
        // ... Notify user of success

The update process consists of the following steps:

  1. Ensure that an internet connection is available.
  2. Retrieve the config file from the update server.
  3. Check the version in the config file. Compare to the existing application version.
  4. If an update is required, then download the new application from the update server.
  5. Switch in the latest application version with the existing version.

The first, and frequently overlooked step, is to ensure that an internet connection is available. This is essential to do because otherwise you may trigger an automatic dial-up to the internet, which is likely to make most of your application users unhappy.

// Important step - ensure we have an internet
// connection. We don't want to force a dial-up.
DWORD dwType;
if (!InternetGetConnectedState(&dwType, 0))
    return false;

Retrieving files from an HTTP server is a simple task using InternetOpenUrl, but it can fail if the URL supplied is not canonicalized. This simply means that bad characters, such as spaces, must be converted into escape characters, which is done using InternetCanonicalizeUrl. The GetSession method manages this process, returning a handle to a file on the HTTP server for a given URL. To check for updates, we must define an update config file on the server which lets us know what application version to expect. In our case, the update config file is called update.txt and contains the version of our application update. You can see it here. After getting a session handle to the update config file, we can retrieve the data into a buffer using the DownloadConfig method. We now have the update version available for our use.

HINTERNET CAutoUpdater::GetSession(CString &URL)
    // Canonicalization of the URL converts
    // unsafe characters into escape character equivalents
    TCHAR canonicalURL[1024];
    DWORD nSize = 1024;
    InternetCanonicalizeUrl(URL, canonicalURL, &nSize, ICU_BROWSER_MODE);

    HINTERNET hSession = InternetOpenUrl(hInternet, 
                         canonicalURL, NULL, NULL, options, 0);
    URL = canonicalURL;

    return hSession;

bool CAutoUpdater::DownloadConfig(HINTERNET hSession, 
                           BYTE *pBuf, DWORD bufSize)
    DWORD    dwReadSizeOut;
    InternetReadFile(hSession, pBuf, 
                     bufSize, &dwReadSizeOut);
    if (dwReadSizeOut <= 0)
        return false;

    return true;

At this point, we have retrieved the update config file and must now figure out if an update is required. The text in the update config file is a simple version tag, in our case, the update version This must be compared against the version of the executable we are currently running. Surprisingly, getting the version number from the executable and running a version number comparison is not a trivial task and can get a bit messy. If it turns out that our existing application version is out-of-date, we should notify the user and then proceed to download the latest application executable from the update server. Note that our new executable will be downloaded to a temporary user directory given by GetTempPath.

Once the latest executable version is downloaded, it must somehow be switched in to replace the existing out-of-date application executable. However, a problem exists! The current executable is locked, as it is in use, and hence cannot be copied over. Nevertheless, a simple Windows trick does exist. Files in use can still be renamed, leaving space open for the update executable to be switched in to the application directory. Note that this update trick may not always be available as some files can even get locked from renaming. In this case, the MoveFileEx with MOVE_DELAY_UNTIL_REBOOT option can be used, which delays renaming until a reboot occurs to free the file.

bool CAutoUpdater::Switch(CString executable, 
                     CString update, bool WaitForReboot)
    int type = (WaitForReboot) ? 

    const TCHAR *backup = _T("OldExecutable.bak");
    CString directory = executable.Left(executable.ReverseFind(_T('\\')));
    CString backupFile = directory + _T('\\') + CString(backup);

    if (!MoveFileEx(executable, backupFile, type)) 
        return false;

    BOOL bMoveOK = (MoveFileEx(update, executable, type) == TRUE);
    int i = GetLastError();

    return bMoveOK;

Points of Interest

The class provided manages updates for a simple application, but has a few drawbacks:

  • The update check is a synchronous task which does not provide any user feedback as to its progress.
  • Having the update check within the application limits its capabilities. It is wiser to have the update check run from an external executable.
  • A program that appears trivial quickly becomes complex. The current check cannot manage complex situations, such as multiple file updates.


  • 10 Feb, 2005 - First release.


The provided source code shows a very simple update mechanism. For a more sophisticated solution, take a look at the product AutoUpdate+ at


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


About the Author

Simon Ellis
United States United States
No Biography provided

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Comments and Discussions

Questionhow can we check if a file exist Pin
Hassan Iftikhar5-Apr-07 6:36
memberHassan Iftikhar5-Apr-07 6:36 
AnswerRe: how can we check if a file exist Pin
lxnyce26-Apr-07 5:03
memberlxnyce26-Apr-07 5:03 
NewsNTLM proxy Pin
Shog91-Nov-05 8:45
sitebuilderShog91-Nov-05 8:45 
GeneralHelp Pin
Piccinano21-Feb-05 7:21
memberPiccinano21-Feb-05 7:21 
GeneralRe: Help Pin
temp0x00715-Nov-09 19:02
membertemp0x00715-Nov-09 19:02 
QuestionGreat article, but why make it a separate executable? Pin
Bill Trake12-Feb-05 16:02
sussBill Trake12-Feb-05 16:02 
AnswerRe: Great article, but why make it a separate executable? Pin
Ron Bernolds12-Feb-05 16:24
sussRon Bernolds12-Feb-05 16:24 
GeneralInternetGetConnectedState problems Pin
Neville Franks12-Feb-05 12:14
memberNeville Franks12-Feb-05 12:14 
In my experience with InternetGetConnectedState() if "Always dial" is set then InternetGetConnectedState() will fail even though we may have a good LAN connection.

See: MSDN Article ID: Q242558[^]

Neville Franks, Author of ED for Windows and Surfulater "Save what you Surf"
GeneralRe: InternetGetConnectedState problems Pin
Nishant S12-Feb-05 17:19
staffNishant S12-Feb-05 17:19 
GeneralI'm glad somebody wrote this. Pin
Jason De Arte11-Feb-05 14:52
memberJason De Arte11-Feb-05 14:52 
GeneralMoveFileEx Pin
Anatoly Ivasyuk11-Feb-05 4:36
memberAnatoly Ivasyuk11-Feb-05 4:36 
GeneralRe: MoveFileEx Pin
Blake Miller11-Feb-05 11:23
memberBlake Miller11-Feb-05 11:23 
QuestionWhat about updating an executable on a shared network drive? Pin
Don Clugston10-Feb-05 19:50
memberDon Clugston10-Feb-05 19:50 

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