procfs began life (on Solaris?) as a virtual filesystem dedicated to exporting process information. Over time, it has grown substantially on any number of operating systems. It is pretty much a required interface on Linux, and strongly recommended on Solaris and FreeBSD.
Each entity associated with a non-zero PID (this includes most kernel threads) has a corresponding toplevel procfs directory named by its PID (e.g. when using systemd as init and mounting procfs at `/proc`, systemd's primary process is described by `/proc/1` (the process only appears in procfs mounts within the same PID namespace). One of the entries is `proc/PID/task`, a directory which contains the threads making up the process, using the TID as name:
[schwarzgerat](1) $ grep ^Threads: /proc/`pidof rtorrent`/status Threads: 3 [schwarzgerat](0) $ ls /proc/`pidof rtorrent`/task 4282 4283 4309 [schwarzgerat](0) $
procfs since Linux 3.3 accepts a mount option `hidepid`, taking one of three values:
- 0: everyone may access all `proc/PID` directories
- 1: users can only access their own `proc/PID` directories
- 2: users can only *see* their own `proc/PID` directories
Linux 3.3 also introduced the `gid` parameter, which specifies a group ID. Members of this group are exempted from `hidepid` restrictions.
- The sysfs page