CODEWITHSUNDEEP
windowsfile-iomutex
vsz
vsz

Reputation: 4909

LockFileEx returns success, but seems to have no effect

I'm trying to lock a file, because it is sitting on a network drive, and multiple instances of a program from multiple computers need to edit it. To prevent damage, I intend to set it up so that only one of the instances has rights to it at a time.

I implemented a lock, which would theoretically lock the first 100 bytes of the file from any access. I'm using Qt with its own file handling, but it has a method of returning a generic file handle.

QFile file(path);
HANDLE handle = (HANDLE)_get_osfhandle(file.handle());
OVERLAPPED ov1;
memset(&ov1, 0, sizeof(ov1));
ov1.Offset = 0;
ov1.OffsetHigh = 0;
if (handle == INVALID_HANDLE_VALUE)
{
    // error
    return;
}

LockFileEx(handle, LOCKFILE_FAIL_IMMEDIATELY | LOCKFILE_EXCLUSIVE_LOCK, 0, 100, 0, &ov1);
qDebug() << file.readLine();

LockFileEx() returns 1, so it seems to have been successful. However, if I run the program in multiple instances, all of them can read and print the first line of the file. More than this, I can freely edit the file with any text editor. Being a network file is not an issue, as it behaves similarly with a local file.

Upvotes: 1

Views: 190

Answers (1)

vsz
vsz

Reputation: 4909

The problem was that, while the program does not terminate, the QFile variable was local, so after finishing the function, the destructor of the QFile was called, so it released the file. The OS then seemed to have released the lock.

If my QFile survives the scope, everything works just fine. A minor issue is, that while I expected the file to be locked against reading, external programs do have a read-only access to it. It's not a problem, as my program can check whether it can create a lock, and detect failure to do so. This means that the intended mutex functionality works.

Upvotes: 1

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