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

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?):
Create a Restart Manager session (RmStartSession).
Add a file resource to the session (RmRegisterResource).
Ask for a list of all processes affected by that resource (RmGetList).
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);
}