Delphi - finding the process that is accessing a file from my program

Question

I have a Delphi app that regularly writes to a local disk file. Occasionally it is unable to access the file - a sharing violation results when it tries to open it. A retry after a short delay is all that is needed, but when it occurs, I would like to report the process that prevented the access.

Is it feasible when a sharing violation occurs for my program to enumerate all the file handles in use, inspect the filename, and if it matches the name of my data file, retrieves the process name associated with that handle?

Some example code would be nice.

Answer 1

You have basically two ways

The Easy Way

if you are using Windows Vista or newer try the IFileIsInUse interface

The Hard Way

if you need a method compatible with Windows XP,Vista,7 and so on. then you use the NtQuerySystemInformation, NtQueryInformationFile and NtQueryObject functions.

These are the steps to proceed

1. Call the NTQuerySystemInformation passing the undocumented SystemHandleInformation ($10) value to get the list of handles
2. then process the list of handles (only for ObjectType = 28) which are files.
3. call OpenProcess with PROCESS_DUP_HANDLE
4. then call DuplicateHandle for get a real handle to the file.
5. get the name of the filename asociated to the handle using the NtQueryInformationFile and NtQueryObject functions.

Note 1 : the tricky part of this method is resolve the filename based in a handle. the function NtQueryInformationFile hangs in some scenarios (system handles and others) a workaround to prevent the entire application from hanging is call the function from a separate thread.

Note 2 : exist another functions like GetFileInformationByHandleEx and GetFinalPathNameByHandle to resolve the filename of a handle. but both exist since Windows viste an d in such case is better use IFileIsInUse.

Check this sample application tested in Delphi 2007, XE2 and Windows XP and 7. from here you can take some ideas to resolve your issue.

Note : The function GetProcessIdUsingFile Only compares the name of the files (not the path).

{$APPTYPE CONSOLE}


uses
  Windows,
  SysUtils;

const
  SystemHandleInformation = $10;
  STATUS_SUCCESS          = $00000000;
  FileNameInformation     = 9;
  ObjectNameInformation   = 1;

type
 SYSTEM_HANDLE=packed record
   uIdProcess:ULONG;
   ObjectType:UCHAR;
   Flags     :UCHAR;
   Handle    :Word;
   pObject   :Pointer;
   GrantedAccess:ACCESS_MASK;
 end;

 SYSTEM_HANDLE_ARRAY = Array[0..0] of SYSTEM_HANDLE;

 SYSTEM_HANDLE_INFORMATION=packed record
 uCount:ULONG;
 Handles:SYSTEM_HANDLE_ARRAY;
 end;
 PSYSTEM_HANDLE_INFORMATION=^SYSTEM_HANDLE_INFORMATION;

  NT_STATUS = Cardinal;

  PFILE_NAME_INFORMATION = ^FILE_NAME_INFORMATION;
  FILE_NAME_INFORMATION = packed record
    FileNameLength: ULONG;
    FileName: array [0..MAX_PATH - 1] of WideChar;
  end;

  PUNICODE_STRING = ^TUNICODE_STRING;
  TUNICODE_STRING = packed record
    Length : WORD;
    MaximumLength : WORD;
    Buffer : array [0..MAX_PATH - 1] of WideChar;
  end;

  POBJECT_NAME_INFORMATION = ^TOBJECT_NAME_INFORMATION;
  TOBJECT_NAME_INFORMATION = packed record
    Name : TUNICODE_STRING;
  end;

  PIO_STATUS_BLOCK = ^IO_STATUS_BLOCK;
  IO_STATUS_BLOCK = packed record
    Status: NT_STATUS;
    Information: DWORD;
  end;

  PGetFileNameThreadParam = ^TGetFileNameThreadParam;
  TGetFileNameThreadParam = packed record
    hFile    : THandle;
    Result   : NT_STATUS;
    FileName : array [0..MAX_PATH - 1] of AnsiChar;
  end;

  function NtQueryInformationFile(FileHandle: THandle;
    IoStatusBlock: PIO_STATUS_BLOCK; FileInformation: Pointer;
    Length: DWORD; FileInformationClass: DWORD): NT_STATUS;
    stdcall; external 'ntdll.dll';

  function NtQueryObject(ObjectHandle: THandle;
    ObjectInformationClass: DWORD; ObjectInformation: Pointer;
    ObjectInformationLength: ULONG;
    ReturnLength: PDWORD): NT_STATUS; stdcall; external 'ntdll.dll';

  function NtQuerySystemInformation(SystemInformationClass: DWORD; SystemInformation: Pointer; SystemInformationLength: ULONG; ReturnLength: PULONG): NT_STATUS; stdcall; external 'ntdll.dll' name 'NtQuerySystemInformation';


function GetFileNameHandleThr(Data: Pointer): DWORD; stdcall;
var
  dwReturn: DWORD;
  FileNameInfo: FILE_NAME_INFORMATION;
  ObjectNameInfo: TOBJECT_NAME_INFORMATION;
  IoStatusBlock: IO_STATUS_BLOCK;
  pThreadParam: TGetFileNameThreadParam;
begin
  ZeroMemory(@FileNameInfo, SizeOf(FILE_NAME_INFORMATION));
  pThreadParam := PGetFileNameThreadParam(Data)^;
  Result := NtQueryInformationFile(pThreadParam.hFile, @IoStatusBlock,  @FileNameInfo, MAX_PATH * 2, FileNameInformation);
  if Result = STATUS_SUCCESS then
  begin
    Result := NtQueryObject(pThreadParam.hFile, ObjectNameInformation,  @ObjectNameInfo, MAX_PATH * 2, @dwReturn);
    if Result = STATUS_SUCCESS then
    begin
      pThreadParam.Result := Result;
      WideCharToMultiByte(CP_ACP, 0, @ObjectNameInfo.Name.Buffer[ObjectNameInfo.Name.MaximumLength - ObjectNameInfo.Name.Length], ObjectNameInfo.Name.Length, @pThreadParam.FileName[0], MAX_PATH, nil, nil);
    end
    else
    begin
      pThreadParam.Result := STATUS_SUCCESS;
      Result := STATUS_SUCCESS;
      WideCharToMultiByte(CP_ACP, 0, @FileNameInfo.FileName[0], IoStatusBlock.Information, @pThreadParam.FileName[0], MAX_PATH, nil, nil);
    end;
  end;
  PGetFileNameThreadParam(Data)^ := pThreadParam;
  ExitThread(Result);
end;

function GetFileNameHandle(hFile: THandle): String;
var
  lpExitCode: DWORD;
  pThreadParam: TGetFileNameThreadParam;
  hThread: THandle;
begin
  Result := '';
  ZeroMemory(@pThreadParam, SizeOf(TGetFileNameThreadParam));
  pThreadParam.hFile := hFile;
  hThread := CreateThread(nil, 0, @GetFileNameHandleThr, @pThreadParam, 0, PDWORD(nil)^);
  if hThread <> 0 then
  try
    case WaitForSingleObject(hThread, 100) of
      WAIT_OBJECT_0:
      begin
        GetExitCodeThread(hThread, lpExitCode);
        if lpExitCode = STATUS_SUCCESS then
          Result := pThreadParam.FileName;
      end;
      WAIT_TIMEOUT:
        TerminateThread(hThread, 0);
    end;
  finally
    CloseHandle(hThread);
  end;
end;

//get the pid of the process which had open the specified file
function GetProcessIdUsingFile(const TargetFileName:string): DWORD;
var
 hProcess    : THandle;
 hFile       : THandle;
 ReturnLength: DWORD;
 SystemInformationLength : DWORD;
 Index       : Integer;
 pHandleInfo : PSYSTEM_HANDLE_INFORMATION;
 hQuery      : THandle;
 FileName    : string;
begin
  Result:=0;
  pHandleInfo      := nil;
  ReturnLength     := 1024;
  pHandleInfo      := AllocMem(ReturnLength);
  hQuery           := NTQuerySystemInformation(DWORD(SystemHandleInformation), pHandleInfo, 1024, @ReturnLength);
  if ReturnLength<>0 then
  begin
    FreeMem(pHandleInfo);
    SystemInformationLength := ReturnLength;
    pHandleInfo             := AllocMem(ReturnLength+1024);
    hQuery                  := NTQuerySystemInformation(DWORD(SystemHandleInformation), pHandleInfo, SystemInformationLength, @ReturnLength);//Get the list of handles
  end
  else
   RaiseLastOSError;

  try
    if(hQuery = STATUS_SUCCESS) then
    begin
      for Index:=0 to pHandleInfo^.uCount-1 do
      if pHandleInfo.Handles[Index].ObjectType=28 then
      begin
        hProcess := OpenProcess(PROCESS_DUP_HANDLE, FALSE, pHandleInfo.Handles[Index].uIdProcess);
        if(hProcess <> INVALID_HANDLE_VALUE) then
        begin
          try
           if not DuplicateHandle(hProcess, pHandleInfo.Handles[Index].Handle,GetCurrentProcess(), @hFile,  0 ,FALSE, DUPLICATE_SAME_ACCESS) then
            hFile := INVALID_HANDLE_VALUE;
          finally
           CloseHandle(hProcess);
          end;

          if (hFile<>INVALID_HANDLE_VALUE) then
          begin
            try
              FileName:=GetFileNameHandle(hFile);
            finally
              CloseHandle(hFile);
            end;
          end
          else
          FileName:='';

          //Writeln(FileName);
           if CompareText(ExtractFileName(FileName), TargetFileName)=0 then
            Result:=pHandleInfo.Handles[Index].uIdProcess;
        end;
      end;
    end;
  finally
   if pHandleInfo<>nil then
     FreeMem(pHandleInfo);
  end;
end;

function SetDebugPrivilege: Boolean;
var
  TokenHandle: THandle;
  TokenPrivileges : TTokenPrivileges;
begin
  Result := false;
  if OpenProcessToken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES or TOKEN_QUERY, TokenHandle) then
  begin
    if LookupPrivilegeValue(nil, PChar('SeDebugPrivilege'), TokenPrivileges.Privileges[0].Luid) then
    begin
      TokenPrivileges.PrivilegeCount := 1;
      TokenPrivileges.Privileges[0].Attributes := SE_PRIVILEGE_ENABLED;
      Result := AdjustTokenPrivileges(TokenHandle, False,
        TokenPrivileges, 0, PTokenPrivileges(nil)^, PDWord(nil)^);
    end;
  end;
end;

begin
  try
   SetDebugPrivilege;
   Writeln('Processing');
   Writeln(GetProcessIdUsingFile('MyFile.txt'));
   Writeln('Done');
  except
    on E:Exception do
      Writeln(E.Classname, ': ', E.Message);
  end;
  Readln;
end.