How to check if a file is being used by another application?

Question

I am using the following code to check if a file is being used by another application:

HANDLE fh = CreateFile("D:\\1.txt", GENERIC_READ, 0, NULL, OPEN_EXISTING, 0, NULL);
if (fh == INVALID_HANDLE_VALUE)
{
    MessageBox(NULL, "The file is in use", "Error", 0);
}

If the file is being used by another application, the message box is displayed. However, the message box is also displayed if the file does not exists!

So what should I do to solve this problem, should I also check if the file exists (using another function), or can the parameters of CreateFile() be changed to only return INVALID_HANDLE_VALUE if the file is in use and does exists?

Answer 1

If you wish to find out, which process has a file open, use the Restart Manager . The procedure consists of the following steps (as outlined in Raymond Chen's blog entry How do I find out which process has a file open? ):

1. Create a Restart Manager session ( RmStartSession ).
2. Add a file resource to the session ( RmRegisterResource ).
3. Ask for a list of all processes affected by that resource ( RmGetList ).
4. Close the session ( RmEndSession ).

---

Sample code:

 #include <Windows.h> #include <RestartManager.h> #pragma comment(lib, "Rstrtmgr.lib") bool IsFileLocked( const wchar_t* PathName ) { bool isFileLocked = false; DWORD dwSession = 0x0; wchar_t szSessionKey[CCH_RM_SESSION_KEY + 1] = { 0 }; if ( RmStartSession( &dwSession, 0x0, szSessionKey ) == ERROR_SUCCESS ) { if ( RmRegisterResources( dwSession, 1, &PathName, 0, NULL, 0, NULL ) == ERROR_SUCCESS ) { DWORD dwReason = 0x0; UINT nProcInfoNeeded = 0; UINT nProcInfo = 0; if ( RmGetList( dwSession, &nProcInfoNeeded, &nProcInfo, NULL, &dwReason ) == ERROR_MORE_DATA ) { isFileLocked = ( nProcInfoNeeded != 0 ); } } RmEndSession( dwSession ); } return isFileLocked; }

Answer 2

You need to use GetLastError() to know why CreateFile() failed, eg:

// this is requesting exclusive access to the file, so it will
// fail if the file is already open for any reason. That condition
// is detected by a sharing violation error due to conflicting
// sharing rights...

HANDLE fh = CreateFile("D:\\1.txt", GENERIC_READ, 0, NULL, OPEN_EXISTING, 0, NULL);
if (fh == INVALID_HANDLE_VALUE)
{
    switch (GetLastError())
    {
        case ERROR_PATH_NOT_FOUND:
        case ERROR_FILE_NOT_FOUND:
            MessageBox(NULL, "The file does not exist", "Error", 0);
            break;

        case ERROR_SHARING_VIOLATION:
            MessageBox(NULL, "The file is in use", "Error", 0);
            break;

        //...

        default:
            MessageBox(NULL, "Error opening the file", "Error", 0);
            break;
    }
}
else
{
    // the file exists and was not in use.
    // don't forget to close the handle...
    CloseHandle(fh);
}