SYSFS(5) Linux Programmer's Manual SYSFS(5)
NAME
sysfs - a filesystem for exporting kernel objects
DESCRIPTION
The sysfs filesystem is a pseudo-filesystem which provides an interface
to kernel data structures. (More precisely, the files and directories
in sysfs provide a view of the kobject structures defined internally
within the kernel.) The files under sysfs provide information about
devices, kernel modules, filesystems, and other kernel components.
The sysfs filesystem is commonly mounted at /sys. Typically, it is
mounted automatically by the system, but it can also be mounted manu-
ally using a command such as:
mount -t sysfs sysfs /sys
Many of the files in the sysfs filesystem are read-only, but some files
are writable, allowing kernel variables to be changed. To avoid redun-
dancy, symbolic links are heavily used to connect entries across the
filesystem tree.
Files and directories
The following list describes some of the files and directories under
the /sys hierarchy.
/sys/block
This subdirectory contains one symbolic link for each block
device that has been discovered on the system. The symbolic
links point to corresponding directories under /sys/devices.
/sys/bus
This directory contains one subdirectory for each of the bus
types in the kernel. Inside each of these directories are two
subdirectories:
devices
This subdirectory contains symbolic links to entries in
/sys/devices that correspond to the devices discovered on
this bus.
drivers
This subdirectory contains one subdirectory for each
device driver that is loaded on this bus.
/sys/class
This subdirectory contains a single layer of further subdirecto-
ries for each of the device classes that have been registered on
the system (e.g., terminals, network devices, block devices,
graphics devices, sound devices, and so on). Inside each of
these subdirectories are symbolic links for each of the devices
in this class. These symbolic links refer to entries in the
/sys/devices directory.
/sys/class/net
Each of the entries in this directory is a symbolic link repre-
senting one of the real or virtual networking devices that are
visible in the network namespace of the process that is access-
ing the directory. Each of these symbolic links refers to
entries in the /sys/devices directory.
/sys/dev
This directory contains two subdirectories block/ and char/,
corresponding, respectively, to the block and character devices
on the system. Inside each of these subdirectories are symbolic
links with names of the form major-ID:minor-ID, where the ID
values correspond to the major and minor ID of a specific
device. Each symbolic link points to the sysfs directory for a
device. The symbolic links inside /sys/dev thus provide an easy
way to look up the sysfs interface using the device IDs returned
by a call to stat(2) (or similar).
The following shell session shows an example from /sys/dev:
$ stat -c "%t %T" /dev/null
1 3
$ readlink /sys/dev/char/1\:3
../../devices/virtual/mem/null
$ ls -Fd /sys/devices/virtual/mem/null
/sys/devices/virtual/mem/null/
$ ls -d1 /sys/devices/virtual/mem/null/*
/sys/devices/virtual/mem/null/dev
/sys/devices/virtual/mem/null/power/
/sys/devices/virtual/mem/null/subsystem@
/sys/devices/virtual/mem/null/uevent
/sys/devices
This is a directory that contains a filesystem representation of
the kernel device tree, which is a hierarchy of device struc-
tures within the kernel.
/sys/firmware
This subdirectory contains interfaces for viewing and manipulat-
ing firmware-specific objects and attributes.
/sys/fs
This directory contains subdirectories for some filesystems. A
filesystem will have a subdirectory here only if it chose to
explicitly create the subdirectory.
/sys/fs/cgroup
This directory conventionally is used as a mount point for a
tmpfs(5) filesystem containing mount points for cgroups(7)
filesystems.
/sys/hypervisor
[To be documented]
/sys/kernel
This subdirectory contains various files and subdirectories that
provide information about the running kernel.
/sys/kernel/cgroup/
For information about the files in this directory, see
cgroups(7).
/sys/kernel/debug/tracing
Mount point for the tracefs filesystem used by the kernel's
ftrace facility. (For information on ftrace, see the kernel
source file Documentation/trace/ftrace.txt.)
/sys/kernel/mm
This subdirectory contains various files and subdirectories that
provide information about the kernel's memory management subsys-
tem.
/sys/kernel/mm/hugepages
This subdirectory contains one subdirectory for each of the huge
page sizes that the system supports. The subdirectory name
indicates the huge page size (e.g., hugepages-2048kB). Within
each of these subdirectories is a set of files that can be used
to view and (in some cases) change settings associated with that
huge page size. For further information, see the kernel source
file Documentation/vm/hugetlbpage.txt.
/sys/module
This subdirectory contains one subdirectory for each module that
is loaded into the kernel. The name of each directory is the
name of the module. In each of the subdirectories, there may be
following files:
coresize
[to be documented]
initsize
[to be documented]
initstate
[to be documented]
refcnt [to be documented]
srcversion
[to be documented]
taint [to be documented]
uevent [to be documented]
version
[to be documented]
In each of the subdirectories, there may be following subdirec-
tories:
drivers
[To be documented]
holders
[To be documented]
notes [To be documented]
parameters
This directory contains one file for each module parame-
ter, with each file containing the value of the corre-
sponding parameter. Some of these files are writable,
allowing the
sections
This subdirectories contains files with information about
module sections. This information is mainly used for
debugging.
[To be documented]
/sys/power
[To be documented]
VERSIONS
The sysfs filesystem first appeared in Linux 2.6.0.
CONFORMING TO
The sysfs filesystem is Linux-specific.
NOTES
This manual page is incomplete, possibly inaccurate, and is the kind of
thing that needs to be updated very often.
SEE ALSO
proc(5), udev(7)
P. Mochel. (2005). The sysfs filesystem. Proceedings of the 2005
Ottawa Linux Symposium.
The kernel source file Documentation/filesystems/sysfs.txt and various
other files in Documentation/ABI and Documentation/*/sysfs.txt
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
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