Windows Resource – Access Denied

In the Windows PC industry it is very common for applications to attached resources to an executable. Resources can be anything from bitmaps, UI components, string translations and copyright information. These resources are then used by the executable at runtime and also by Windows Explorer (for icons and properties information).

Resources can be modified post compilation by using three methods provided by MSDN: BeginUpdateResource, UpdateResource and EndUpdateResource. I recently had to use these methods and painfully discovered a nasty race condition between my resource updater application and Windows Explorer. I found information on these random failures to be scarce, hence the blog post…

When you are modifying the resource of an executable or DLL and Windows Explorer is open and showing the directory of the file being updated (regardless of visibility), a file access race condition exists that produces one of the following two errors:

The system cannot open the device or file specified.

Access denied.

The numbers I collected show an average file access failure rate of 2.3% when updating a resource that Explorer is also showing. If you are experiencing these errors simply close Windows Explorer and your resource updater application will function as expected every time. Given that Windows is closed source I can’t confirm exactly what Explorer is doing but my hypotheses is that Explorer temporarily opens and reads the resources of each file in the directory (most likely so it can determine which icon to show) and refreshes that resource when it notices a change to the file. If your updater application requests a file handle during this time you will see one of the above errors.