Lock a file

[auba]

How would you avoid the reading of a file which currentely is written by another program? Let's say something like the iTunes database file, about 10MB plain text?
When you are working on win32, you can't simply create a tmp file, write it down, and then rename it. First, the file system of win32 seems to be too slow to realize, that a file is removed and you really fast get an "file exists" error when renaming the tmp file to the original name. Second, you'll get a small time span when the file does not exist as the old content is backuped and the new content is not yet renamed. Does that fact matter?

But I agree with you, both (wysota and euthmos). If I had mandatory locks, life would be much easier. But to say, that the Trolls are too lazy to implement them is the wrong way

[wysota]

How would you avoid the reading of a file which currentely is written by another program?

I would use proper IPC mechanisms on local filesystems. On remote filesystems there is nothing you can do, even on Windows they don't support locking.

[auba]

Well... with qt3.x we patched qfile_win.cpp to allow locking by sopen - and it even works on network drives. But I don't want to do that in the future...

[Malek]

What is the conclusion?
Can I lock the file until my thread finish writing on it? and how?

[squidge]

Sure, but the solution can be platform dependant depending on HOW you want to lock the file.

[Malek]

I work on Linux.
I need to lock the file until my thread finish writing on it. Then unlock it.

[squidge]

You still have not defined HOW you want to lock it.

For example, do you want to lock it to prevent your other threads from writing to the same file? Or writing and reading? Or prevent your other thread from even opening the file? Or prevent any application in the entire system from opening the file? Or perhaps allowing some apps to read the file but not write to it? etc...

[Malek]

do you want to lock it to prevent your other threads from writing to the same file?

Yes , this what I want to lock it for.
I am sorry if I was not clear.

[wysota]

On Unix systems there is no mandatory locking which means in a general case you cannot prevent someone else from writing to the same file while you have it open. It requires cooperation of all involved sides and use of advisory locking (e.g. using flock). Some filesystems may implement mandatory locking (through fcntl calls) but others will not and on some systems (like Linux) mandatory locks are unreliable (see the man page for fcntl).