When your application calls open() or read() or stat() or whatever, it passes through libc and then makes a syscall to ask the kernel "hey, give me a file handle for /proc/foo", "hey, give me the first 500 bytes from this file handle", etc. Then in the kernel, that syscall triggers a function that figures out what filesystem you're talking to and passes the request to that filesystem driver. AFAIK, the procfs driver just reads (and writes) a bunch of data structures in kernel memory and passes the result through as a filesystem. So the actual underlying data structure does always exist and is a hard requirement for the system to work, but procfs is just a userspace view into that underlying data in the kernel. Incidentally sysfs would be the same - the stuff you see in /sys exists in the kernel regardless, but the kernel doesn't really care whether those data structures are exposed to userspace as a virtual filesystem.