The LockFile API takes a file handle. I normally use TStream for file access, so I'm unsure how to get the appropriate handle, given an ANSIString filename only. My purpose is to lock a file (which may not exist originally) during a process, write some information to other users, and then unlock and delete it.
I would appreciate sample code or pointers to it to make this reliable.
It's pretty simple, actually. TFileStream has a Handle property that gives you the Windows handle to the file. And if you're using some other type of stream, there's no underlying file to work with.
You can use the LockFile function in conjunction with CreateFile and UnlockFile functions.
See this example
procedure TFrmMain.Button1Click(Sender: TObject);
var
aHandle : THandle;
aFileSize : Integer;
aFileName : String;
begin
aFileName :='C:\myfolder\myfile.ext';
aHandle := CreateFile(PChar(aFileName),GENERIC_READ, 0, nil, OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL,0); // get the handle of the file
try
aFileSize := GetFileSize(aHandle,nil); //get the file size for use in the lockfile function
Win32Check(LockFile(aHandle,0,0,aFileSize,0)); //lock the file
//your code
//
//
//
Win32Check(UnlockFile(aHandle,0,0,aFileSize,0));//unlock the file
finally
CloseHandle(aHandle);//Close the handle of the file.
end;
end;
Another option , if you want to lock the file using TFileStream you can opening the file using exclusive access (fmShareExclusive).
Var
MyStream :TFilestream;
begin
MyStream := TFilestream.Create( aFileName, fmOpenRead or fmShareExclusive );
end;
Note : in both examples the access is read-only, you must change the flags in order to write the files.
Another option is to create a file stream with exclusive read/write access:
fMask := fmOpenReadWrite or fmShareExclusive;
if not FileExists(Filename) then
fMask := fMask or fmCreate;
fstm := tFileStream.Create(Filename,fMask);