| .. SPDX-License-Identifier: GPL-2.0 | |
| ===================== | |
| The Devpts Filesystem | |
| ===================== | |
| Each mount of the devpts filesystem is now distinct such that ptys | |
| and their indices allocated in one mount are independent from ptys | |
| and their indices in all other mounts. | |
| All mounts of the devpts filesystem now create a ``/dev/pts/ptmx`` node | |
| with permissions ``0000``. | |
| To retain backwards compatibility the a ptmx device node (aka any node | |
| created with ``mknod name c 5 2``) when opened will look for an instance | |
| of devpts under the name ``pts`` in the same directory as the ptmx device | |
| node. | |
| As an option instead of placing a ``/dev/ptmx`` device node at ``/dev/ptmx`` | |
| it is possible to place a symlink to ``/dev/pts/ptmx`` at ``/dev/ptmx`` or | |
| to bind mount ``/dev/ptx/ptmx`` to ``/dev/ptmx``. If you opt for using | |
| the devpts filesystem in this manner devpts should be mounted with | |
| the ``ptmxmode=0666``, or ``chmod 0666 /dev/pts/ptmx`` should be called. | |
| Total count of pty pairs in all instances is limited by sysctls:: | |
| kernel.pty.max = 4096 - global limit | |
| kernel.pty.reserve = 1024 - reserved for filesystems mounted from the initial mount namespace | |
| kernel.pty.nr - current count of ptys | |
| Per-instance limit could be set by adding mount option ``max=<count>``. | |
| This feature was added in kernel 3.4 together with | |
| ``sysctl kernel.pty.reserve``. | |
| In kernels older than 3.4 sysctl ``kernel.pty.max`` works as per-instance limit. |