Synchronize multiple application - delphi

Question

I have multiple services processing some files. Each service must have exclusive access to the file while processing. I solved this problem a while ago by creating a global mutex that uses some temp files, something like this:
function AppLocked: boolean;
begin
result := FileExists(GetTempDir + '__MUTEX__' + LockExt);
end;
procedure AppLock;
var
F: TextFile;
begin
if FileExists(GetTempDir + '__MUTEX__' + LockExt) then
exit
else
try
AssignFile(F, GetTempDir + '__MUTEX__' + LockExt);
Rewrite(F);
Writeln(F, DateTimeToStr(Now));
CloseFile(F);
except
end;
end;
procedure AppUnLock;
begin
if FileExists(GetTempDir + '__MUTEX__' + LockExt) then
SysUtils.DeleteFile(GetTempDir + '__MUTEX__' + LockExt);
end;
This works pretty good, and I don't want to fix something that works, but I just wonder, is there a better solution?

Answer 1

An actual Mutex (as in win32 Mutex) is the preferred method.

Answer 2

Your solution has a problem, if the application terminates and you missed to unlock. This could happen on an abnormal termination. It would be better to create a file, that will automatically erase itself if the application terminates.
All the magic is done by FILE_FLAG_DELETE_ON_CLOSE
unit uAppLock;
interface
function AppLocked : Boolean;
function AppLock : Boolean;
procedure AppUnlock;
implementation
uses
Windows, SysUtils, Classes;
var
// unit global variable
LockFileHandle : THandle;
// function to build the filename
function GetLockFileName : string;
begin
// You have to point out, where to get these informations
Result := GetTempDir + '__MUTEX__' + LockExt;
end;
function AppLocked : Boolean;
begin
Result := FileExists( GetLockFileName );
end;
function AppLock : Boolean;
var
LFileName : string;
LLockFileStream : TStream;
LInfoStream : TStringStream;
begin
Result := False;
if AppLock
then
Exit;
LFileName := GetLockFileName;
// Retrieve the handle of the LockFile
LockFileHandle := CreateFile( PChar( LFileName ), GENERIC_READ or GENERIC_WRITE, FILE_SHARE_READ or FILE_SHARE_DELETE,
nil, CREATE_NEW, FILE_FLAG_DELETE_ON_CLOSE or FILE_ATTRIBUTE_TEMPORARY, 0 );
if LockFileHandle <> INVALID_HANDLE_VALUE
then
begin
Result := True;
LInfoStream := nil;
LLockFileStream := nil;
try
LInfoStream := TStringStream.Create;
LInfoStream.WriteString( DateTimeToStr( Now ) );
LInfoStream.Seek( 0, soFromBeginning );
LLockFileStream := THandleStream.Create( LockFileHandle );
LLockFileStream.CopyFrom( LInfoStream, LInfoStream.Size );
finally
LInfoStream.Free;
LLockFileStream.Free;
end;
end;
end;
procedure AppUnlock;
begin
// Just close the handle and the file will be deleted
CloseHandle( LockFileHandle );
end;
end.
BTW: GetTempDir looks to be a Directory, but you use it as a Path. So it would be better to rename it into GetTempPath instead :o)