1.. SPDX-License-Identifier: GPL-2.0 2 3===================== 4The Devpts Filesystem 5===================== 6 7Each mount of the devpts filesystem is now distinct such that ptys 8and their indices allocated in one mount are independent from ptys 9and their indices in all other mounts. 10 11All mounts of the devpts filesystem now create a ``/dev/pts/ptmx`` node 12with permissions ``0000``. 13 14To retain backwards compatibility the a ptmx device node (aka any node 15created with ``mknod name c 5 2``) when opened will look for an instance 16of devpts under the name ``pts`` in the same directory as the ptmx device 17node. 18 19As an option instead of placing a ``/dev/ptmx`` device node at ``/dev/ptmx`` 20it is possible to place a symlink to ``/dev/pts/ptmx`` at ``/dev/ptmx`` or 21to bind mount ``/dev/ptx/ptmx`` to ``/dev/ptmx``. If you opt for using 22the devpts filesystem in this manner devpts should be mounted with 23the ``ptmxmode=0666``, or ``chmod 0666 /dev/pts/ptmx`` should be called. 24 25Total count of pty pairs in all instances is limited by sysctls:: 26 27 kernel.pty.max = 4096 - global limit 28 kernel.pty.reserve = 1024 - reserved for filesystems mounted from the initial mount namespace 29 kernel.pty.nr - current count of ptys 30 31Per-instance limit could be set by adding mount option ``max=<count>``. 32 33This feature was added in kernel 3.4 together with 34``sysctl kernel.pty.reserve``. 35 36In kernels older than 3.4 sysctl ``kernel.pty.max`` works as per-instance limit. 37