[Brian_K_White]

You don't use exists and then open a file. If you actually want to open a file, you open it or fail. You don't check if it's ok and then do it, you do it and then check if it failed.

exists followed by open is a pointless exists because anything can happen in between.

Conversely, if you are neither the producer nor consumer of the file at this time, you don't want to actually open it just as a way to stat it, because actually opening it updates it's access time and interferes with the actual consumer(s). Even read-only non-exclusive.

[gus_massa]

You can use it as an improvised lock.

[Brian_K_White]

How do you impliment this lock? You use an atomic operation. Which atomic operation? You try to open the file in exclusive mode.

[gus_massa]

Create the file to lock and delete to unlock. I'm not sure if there are corner cases.

[Brian_K_White]

I misread the comment first, I missed the word "it".

I thought you were saying you could use a seperate lock to guard the time between exist() and open(), which is possible but silly.

Now I'm not sure what you meant. Is "it" exists() or open()? You can use open() as a lock for other things. It doesn't need any other locks itself. You can't use exists() as a lock.

Or, I guess you are saying one process can open the file and others can merely check exists()? Yes depending on what the purpose was, what you intend to do with the results of exists(), because it isn't true any more the instant after exists() returns, only while exists() is actually running.

That makes it sound useless and therefor what is the problem that VB6 didn't have it? That might very well be why it was omitted.

But you do need some form of stat(), a way to get info about files without affecting them in any way. A process needs to be able to watch a files access time to know if it's been accessed yet, or when last, without that very observation changing the access time or making the file appear open()ed to other processes that may care about that. Even being opened read-only andbnon-exclusive is still opened. Another process may need to know that it has eclusive access or else it can't do it's thing, so it will fail it's own open() exclusive.

Even if the other process retries and open succeeds a few cycles later, changing the access time breaks the process that's waiting for the file to be accessed by it's actual consumer so it can be automatically deleted for example, or simply logged the fact that the customer has downloaded the edi file, the remote client/browser has loaded the temp file, someone somewhere has accessed super secret file, etc.

(setting aside that these days at least on *ix you frequently disable atime updates in the filesystem altogether and just do not build anything on top of that any more for performance and flash write life reasons. But I have processes that use it only in tmpfs in ram even while it's off for the real filesystem.)

[gus_massa]

I used something like that 20 years ago, so I forgot the details. Perhaps I just tried to create the file without checking if it exist(?????). The code is long gone.