diff --git a/Sandboxie/core/dll/file.c b/Sandboxie/core/dll/file.c
index ff99d86f..68f33585 100644
--- a/Sandboxie/core/dll/file.c
+++ b/Sandboxie/core/dll/file.c
@@ -381,6 +381,23 @@ static ULONG File_Snapshot_Count = 0;
 #include "file_init.c"
 
 
+//---------------------------------------------------------------------------
+// File_FindBoxPrefixLength
+//---------------------------------------------------------------------------
+
+
+_FX ULONG File_FindBoxPrefixLength(const WCHAR* CopyPath)
+{
+	ULONG length = wcslen(CopyPath);
+	ULONG prefixLen = 0;
+	if (length >= Dll_BoxFilePathLen && 0 == Dll_NlsStrCmp(CopyPath, Dll_BoxFilePath, Dll_BoxFilePathLen))
+		prefixLen = Dll_BoxFilePathLen;
+	if (File_AltBoxPath && length >= File_AltBoxPathLen && 0 == Dll_NlsStrCmp(CopyPath, File_AltBoxPath, File_AltBoxPathLen))
+		prefixLen = File_AltBoxPathLen;
+	return prefixLen;
+}
+
+
 //---------------------------------------------------------------------------
 // File_GetName
 //---------------------------------------------------------------------------