2026-01-21 18:59:54 +08:00
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.. SPDX-License-Identifier: GPL-2.0
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=====================
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The Devpts Filesystem
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=====================
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Each mount of the devpts filesystem is now distinct such that ptys
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2026-01-29 22:25:33 +08:00
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and their indices allocated in one mount are independent from ptys
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and their indices in all other mounts.
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2026-01-21 18:59:54 +08:00
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All mounts of the devpts filesystem now create a ``/dev/pts/ptmx`` node
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with permissions ``0000``.
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To retain backwards compatibility the a ptmx device node (aka any node
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created with ``mknod name c 5 2``) when opened will look for an instance
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of devpts under the name ``pts`` in the same directory as the ptmx device
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node.
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As an option instead of placing a ``/dev/ptmx`` device node at ``/dev/ptmx``
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it is possible to place a symlink to ``/dev/pts/ptmx`` at ``/dev/ptmx`` or
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to bind mount ``/dev/ptx/ptmx`` to ``/dev/ptmx``. If you opt for using
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the devpts filesystem in this manner devpts should be mounted with
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the ``ptmxmode=0666``, or ``chmod 0666 /dev/pts/ptmx`` should be called.
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Total count of pty pairs in all instances is limited by sysctls::
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kernel.pty.max = 4096 - global limit
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kernel.pty.reserve = 1024 - reserved for filesystems mounted from the initial mount namespace
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kernel.pty.nr - current count of ptys
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Per-instance limit could be set by adding mount option ``max=<count>``.
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This feature was added in kernel 3.4 together with
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``sysctl kernel.pty.reserve``.
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In kernels older than 3.4 sysctl ``kernel.pty.max`` works as per-instance limit.
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