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man:sysfs

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/vir-
                tual/mem/null/dev       /sys/devices/virtual/mem/null/power/
                /sys/devices/virtual/mem/null/subsystem@   /sys/devices/vir-
                tual/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/fs/smackfs
            The  directory  contains  configuration files for the SMACK LSM.
            See    the    kernel    source     file     Documentation/admin-
            guide/LSM/Smack.rst.
     /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 4.16 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/.

Linux 2018-04-30 SYSFS(5)

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