Pathname-space mapping
You may have noticed that we've talked about files and directories appearing in their parent directories, rather than just saying that the parent directories contain these files. This is because in QNX OS, the pathname space is virtual, dictated not just by the filesystem that resides on media mounted at root, but rather by the paths and pathname aliases registered by the process manager.
In a typical disk-based QNX OS system, the directory / maps to the root of a filesystem on a physical hard drive partition. This filesystem on disk doesn't actually contain a /dev directory, which exists virtually, adopted via the process manager. In turn, the filename ser1 doesn't exist on a disk filesystem either; it has been adopted by the serial port driver.
This capability allows virtual directory unions or unioned filesystems to be created. This happens when multiple resource managers adopt files that lie in a common directory within the pathname space.
Which resource manager looks after which file or directory
depends on the length of the matched pathname and order in which the underlying filesystems are mounted,
as described in
Pathname Management
in the Process Manager chapter of the System Architecture guide.
For a more detailed example, see
Unioned filesystems
in the Resource Managers chapter of Getting Started with the QNX OS.