Copy file even when destination exists (in Qt)

Question

In the QFile::copy documentation it says

If a file with the name newName already exists, copy() returns false (i.e., QFile will not overwrite it).

But I need to copy a file even if the destination exists. Any workaround available in Qt for that?

Deleting the file is the obvious solution but it invites a race condition...

Mccahill

QFile::remove? – Crifasi

Qt really likes to make your life hard sometimes.. they could simply add a third optional parameter to overwrite files but no.. – Rosena

Answer 1

if (QFile::exists("/home/user/dst.txt"))
{
    QFile::remove("/home/user/dst.txt");
}

QFile::copy("/home/user/src.txt", "/home/user/dst.txt");

Pandanus

Solution isn't atomic. – Mcdevitt

Answer 2

The obvious solution is of course to delete the file if it exists, before doing the copy.

Note however that doing so opens up the code to a classic race condition, since on a typical multitasking operating system a different process could re-create the file between your applications' delete and copy calls. That would cause the copy to still fail, so you need to be prepared (and perhaps re-try the delete, but that might introduce a need for count so you don't spend forever attempting, and on and on).

Astred

This answer is correct - but there's some irony here. The actual code for QFile::copy() itself has a race condition. – Recommendation

Answer 3

The simplest retrying I can think of is:

while !QFile::copy("/home/user/src.txt", "/home/user/dst.txt")
{
    QFile::remove("/home/user/dst.txt");
}

But this still isn't a real solution as some of the race conditions are things that don't block remove.

I'm currently hunting for a way to handle writing a web page as an output but without the auto refresh ever catching between the remove and the copy.

Balbur

Imagine you don't have file-permissions to do copy/remove. Now you have an infinite loop. – Salesperson

Answer 4

Just call remove() before calling copy()

Watchcase