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