We make a device that can appear as a USB serial port on a variety of POSIX-compliant systems. I'm supporting an API that allows callers to retrieve a list of all the currently available (i.e. not in use, have correct privileges, etc.) instances of our device available to them. The caller can also get a list where the in-use devices are also indicated.
The simple way to do this of course is just attempt to open/close each candidate serial port and take note of which ones are open-able. I would prefer not to do this as it is possible that multiple applications using this API (which would be a library / dylib in their app) could create race conditions where they both try to query the same serial port simultaneously which would cause one app to erroneously think the device was in use.
So what I need is a way of knowing the port is open-able without actually opening it. I have seen others use lock-file schemes where there is a special file created to indicate the port is in use - the assumption being that if the file is not present the port is available. My problem is that there may be other users of the port other than my library that do not abide by such as scheme, so I cannot rely on it.
Is there perhaps some low-level POSIX functionality that lets me query a file's status in this regard without actually attempting to open it?
