How to create a file only if it doesn't exist?

Question

I wrote a UNIX daemon (targeting Debian, but it shouldn\'t matter) and I wanted to provide some way of creating a \".pid\" file, (a file which contains the process identifie

Answer 1

It would appear that there's no way to do it strictly using streams.

You can, instead, use open (as mentioned above by wildplasser) and if that succeeds, proceed to open the same file as a stream. Of course, if all you're writing to the file is a PID, it is unclear why you wouldn't just write it using C-style write().

O_EXCL only excludes other processes that are attempting to open the same file using O_EXCL. This, of course, means that you never have a perfect guarantee, but if the file name/location is somewhere nobody else is likely to be opening (other than folks you know are using O_EXCL) you should be OK.