UNIX for Dummies Questions & Answers

Hi,

I have 2 processes that will be accessing the same file. First process first reads from the file, closes it and then opens it again to write to it (this is done sequentially). The second process may delete the file on some condition. These processes are independent of each other now.

What I'd like to know is whether unix handles the delete synchronously i.e. if the file is being read and the first process is holding on to the file handler, will the second process fail deletion, or will the deletion succeed but the read will fail. Similarly when the first process is doing the write.

As of now we wouldn't like to introduce any special synchronization between these two processes if the behavior is known. Please let me know where I can find more details about this.

Thanks.

I'm not sure about what or why you're doing things...?

This is how unlink (remove() in C or rm in shell ) works: it deletes the file from the directory, but the file's data and any currently open file descriptors can see that data.
When the last file descriptor to the file close, then the data itself goes away.

This means process #1 can keep on reading the file. When it is done with the file -- and if it was deleted by process #2, then the file's data will no longer be accessible.

If process #2 opens another file with the same file name it will not affect process #1's data.

So, why are you doing this? The only reason I know of to delete a file that is open is to hide it from all other processes. It's a good security measure.

You can achieve synchronization using a lock file - another file that tells process #1 and process #2 when they can do something with the data - if you need it.

Thanks for your input. So this means when process #1 is doing read while process #2 performs the delete on the file - both will succeed without any failure. Will this also be true when process #1 is writing to the file?

Yes. The reason we want to delete the file is to hide from any other process but don't want to fail a process that maybe currently having an open file descriptor. So long as we are guarenteed there will not be a failure, we'd like to not do any explicit synchronization.

Thanks again.