Documentation/filesystems/devpts.rst
Source file repositories/reference/linux-study-clean/Documentation/filesystems/devpts.rst
File Facts
- System
- Linux kernel
- Corpus path
Documentation/filesystems/devpts.rst- Extension
.rst- Size
- 1455 bytes
- Lines
- 37
- Domain
- Support Tooling And Documentation
- Bucket
- Documentation
- Inferred role
- Support Tooling And Documentation: documentation
- Status
- atlas-only
Why This File Exists
Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
- Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
Dependency Surface
- No C-style include directives detected by the generator.
Detected Declarations
- No top-level syscall, struct, function, initcall, or export declaration detected by the generator.
Annotated Snippet
.. 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.Annotation
- Atlas domain: Support Tooling And Documentation / Documentation.
- Implementation status: atlas-only.
Implementation Notes
- This generated page is the file-by-file coverage layer; curated subsystem chapters should link here when they synthesize a multi-file control flow.
- Core OS pages should be promoted from atlas-only to deep-reviewed when they explain data structures, invariants, locking, lifecycle, and C implementation snippets.
- Driver-family pages are intentionally pattern-oriented unless they are part of the selected PCIe/NVMe representative device path.