Ubuntu Manpage: sysfs - a filesystem for exporting kernel objects

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 manually 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 redundancy, 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 subdirectories 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 representing one of the
real or virtual networking devices that are visible in the network namespace of the
process that is accessing 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 structures within the kernel.

/sys/firmware
This subdirectory contains interfaces for viewing and manipulating 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 subsystem.

/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 subdirectories:

drivers
[To be documented]

holders
[To be documented]

notes [To be documented]

parameters
This directory contains one file for each module parameter, with each file
containing the value of the corresponding 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.

COLOPHON