Lock a file

[euthymos]

Hi,

is there any way I can lock a file with Qt libraries?
For "lock" I mean that when my app opens the file for writing, no other process can write on it. This happens for many Windows applications working with files.
I've tried QtLockedFile, but it only works among processes using QtLockedFile to access the file.

I've tried to call the native Windows API LockFile but it does not work because I can't give it a valid HANDLE to the file (QFile::handle() returns rubbish).

How can I fix this?

Thank you
Giacomo

[wysota]

I've tried to call the native Windows API LockFile but it does not work because I can't give it a valid HANDLE to the file (QFile::handle() returns rubbish).

Did you cast this rubbish to a proper type?

[euthymos]

Did you cast this rubbish to a proper type?

Rubbish was not meant to be offensive, but ironic

However, I casted it to HANDLE, which is declared in windows.h

[wysota]

I didn't think it was offensive :-) Casting it to HANDLE should have worked. What exactly happens when you try to use the handle?

[euthymos]

I didn't think it was offensive :-) Casting it to HANDLE should have worked. What exactly happens when you try to use the handle?

It could have been offensive towards the developers...

However, if I cast it to HANDLE and pass it to LockFile(HANDLE, DWORD, DWORD, DWORD, DWORD) Windows API, the function returns false. GetLastError() returns 6, which is "invalid handle" error code.
I think this leaves no room to other possibilities: the handle is not valid.

[wysota]

Maybe you need to pass the result through some other function first?

[nish]

newbie reply...
Win32 CreateFile() to open file in locked mode;
QFile open again
QFile close
CloseFile (or whatever win api).

i really dont think that will work but just had this tickling in my mid..

[euthymos]

MrDeath: I'm a novice, too. I've been programming with Windows' APIs for a short time, and then in Java. Now I'm learning C++/Qt.

That solution might work fine. I think I'll try it when I get back to development machine.
However, I just can't see why QFile::handle() returns an invalid handle.

wysota: I don't know if I have to pass it to another function, the docs say nothing about that.

EDIT: there was a function to call! _get_osfhandle(int fd)
I've learnt that thanks to Qxt guys. See http://doc.libqxt.org/0.5.0/classQxtFileLock.html

[jonks]

1. Win32 CreateFile() to open file in locked mode;
2. QFile open again
3. QFile close
4. CloseFile (or whatever win api).

How can step 2 work? - it is trying to open a locked file.

[euthymos]

How can step 2 work? - it is trying to open a locked file.

Actually I don't know... but maybe, because it's the same application and the same thread accessing the file, it can be opened after the lock. Just guessing.

However, I solved with a great "native" solution inspired by Qxt's "QxtFileLock" class (http://doc.libqxt.org/0.5.0/classQxtFileLock.html). You should check this Qxt thing out, it's really good.

[nish]

Actually I don't know... but maybe, because it's the same application and the same thread accessing the file, it can be opened after the lock. Just guessing.

that was what i thought. Did it worked? i am on linux so cant test it.

However, I solved with a great "native" solution inspired by Qxt's "QxtFileLock" class (http://doc.libqxt.org/0.5.0/classQxtFileLock.html). You should check this Qxt thing out, it's really good.

this website is blocked at my office. can u please copy and paste the solution. thx..