man5/sysfs.5

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  25 .TH SYSFS 5 2018-04-30 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 sysfs \- a filesystem for exporting kernel objects
  28 .SH DESCRIPTION
  29 The
  30 .B sysfs
  31 filesystem is a pseudo-filesystem which provides an interface to
  32 kernel data structures.
  33 (More precisely, the files and directories in
  34 .B sysfs
  35 provide a view of the
  36 .IR kobject
  37 structures defined internally within the kernel.)
  38 The files under
  39 .B sysfs
  40 provide information about devices, kernel modules, filesystems,
  41 and other kernel components.
  42 .PP
  43 The
  44 .B sysfs
  45 filesystem is commonly mounted at
  46 .IR /sys .
  47 Typically, it is mounted automatically by the system,
  48 but it can also be mounted manually using a command such as:
  49 .PP
  50 .in +4n
  51 .EX
  52 mount \-t sysfs sysfs /sys
  53 .EE
  54 .in
  55 .PP
  56 Many of the files in the
  57 .B sysfs
  58 filesystem are read-only,
  59 but some files are writable, allowing kernel variables to be changed.
  60 To avoid redundancy,
  61 symbolic links are heavily used to connect entries across the filesystem tree.
  62 .\"
  63 .SS Files and directories
  64 The following list describes some of the files and directories under the
  65 .I /sys
  66 hierarchy.
  67 .TP
  68 .IR /sys/block
  69 This subdirectory contains one symbolic link for each block device
  70 that has been discovered on the system.
  71 The symbolic links point to corresponding directories under
  72 .IR /sys/devices .
  73 .TP
  74 .IR /sys/bus
  75 This directory contains one subdirectory for each of the bus types
  76 in the kernel.
  77 Inside each of these directories are two subdirectories:
  78 .RS
  79 .TP
  80 .IR devices
  81 This subdirectory contains symbolic links to entries in
  82 .IR /sys/devices
  83 that correspond to the devices discovered on this bus.
  84 .TP
  85 .IR drivers
  86 This subdirectory contains one subdirectory for each device driver
  87 that is loaded on this bus.
  88 .RE
  89 .TP
  90 .IR /sys/class
  91 This subdirectory contains a single layer of further subdirectories
  92 for each of the device classes that have been registered on the system
  93 (e.g., terminals, network devices, block devices, graphics devices,
  94 sound devices, and so on).
  95 Inside each of these subdirectories are symbolic links for each of the
  96 devices in this class.
  97 These symbolic links refer to entries in the
  98 .IR /sys/devices
  99 directory.
 100 .TP
 101 .IR /sys/class/net
 102 Each of the entries in this directory is a symbolic link
 103 representing one of the real or virtual networking devices
 104 that are visible in the network namespace of the process
 105 that is accessing the directory.
 106 Each of these symbolic links refers to entries in the
 107 .IR /sys/devices
 108 directory.
 109 .TP
 110 .IR /sys/dev
 111 This directory contains two subdirectories
 112 .IR block /
 113 and
 114 .IR char/ ,
 115 corresponding, respectively,
 116 to the block and character devices on the system.
 117 Inside each of these subdirectories are symbolic links with names of the form
 118 .IR major-ID : minor-ID ,
 119 where the ID values correspond to the major and minor ID of a specific device.
 120 Each symbolic link points to the
 121 .B sysfs
 122 directory for a device.
 123 The symbolic links inside
 124 .IR /sys/dev
 125 thus provide an easy way to look up the
 126 .B sysfs
 127 interface using the device IDs returned by a call to
 128 .BR stat (2)
 129 (or similar).
 130 .IP
 131 The following shell session shows an example from
 132 .IR /sys/dev :
 133 .IP
 134 .in +4n
 135 .EX
 136 $ \fBstat \-c "%t %T" /dev/null\fP
 137 1 3
 138 $ \fBreadlink /sys/dev/char/1\\:3\fP
 139 \&../../devices/virtual/mem/null
 140 $ \fBls \-Fd /sys/devices/virtual/mem/null\fP
 141 /sys/devices/virtual/mem/null/
 142 $ \fBls \-d1 /sys/devices/virtual/mem/null/*\fP
 143 /sys/devices/virtual/mem/null/dev
 144 /sys/devices/virtual/mem/null/power/
 145 /sys/devices/virtual/mem/null/subsystem@
 146 /sys/devices/virtual/mem/null/uevent
 147 .EE
 148 .in
 149 .TP
 150 .IR /sys/devices
 151 This is a directory that contains a filesystem representation of
 152 the kernel device tree,
 153 which is a hierarchy of
 154 .I device
 155 structures within the kernel.
 156 .TP
 157 .IR /sys/firmware
 158 This subdirectory contains interfaces for viewing and manipulating
 159 firmware-specific objects and attributes.
 160 .TP
 161 .IR /sys/fs
 162 This directory contains subdirectories for some filesystems.
 163 A filesystem will have a subdirectory here only if it chose
 164 to explicitly create the subdirectory.
 165 .TP
 166 .IR /sys/fs/cgroup
 167 This directory conventionally is used as a mount point for a
 168 .BR tmpfs (5)
 169 filesystem containing mount points for
 170 .BR cgroups (7)
 171 filesystems.
 172 .TP
 173 .IR /sys/fs/smackfs
 174 The directory contains configuration files for the SMACK LSM.
 175 See the kernel source file
 176 .IR Documentation/admin-guide/LSM/Smack.rst .
 177 .TP
 178 .IR /sys/hypervisor
 179 [To be documented]
 180 .TP
 181 .IR /sys/kernel
 182 This subdirectory contains various files and subdirectories that provide
 183 information about the running kernel.
 184 .TP
 185 .IR /sys/kernel/cgroup/
 186 For information about the files in this directory, see
 187 .BR cgroups (7).
 188 .TP
 189 .IR /sys/kernel/debug/tracing
 190 Mount point for the
 191 .I tracefs
 192 filesystem used by the kernel's
 193 .I ftrace
 194 facility.
 195 (For information on
 196 .IR ftrace ,
 197 see the kernel source file
 198 .IR Documentation/trace/ftrace.txt .)
 199 .TP
 200 .IR /sys/kernel/mm
 201 This subdirectory contains various files and subdirectories that provide
 202 information about the kernel's memory management subsystem.
 203 .TP
 204 .IR /sys/kernel/mm/hugepages
 205 This subdirectory contains one subdirectory for each of the
 206 huge page sizes that the system supports.
 207 The subdirectory name indicates the huge page size (e.g.,
 208 .IR hugepages-2048kB ).
 209 Within each of these subdirectories is a set of files
 210 that can be used to view and (in some cases) change settings
 211 associated with that huge page size.
 212 For further information, see the kernel source file
 213 .IR Documentation/admin-guide/mm/hugetlbpage.rst .
 214 .TP
 215 .IR /sys/module
 216 This subdirectory contains one subdirectory
 217 for each module that is loaded into the kernel.
 218 The name of each directory is the name of the module.
 219 In each of the subdirectories, there may be following files:
 220 .RS
 221 .TP
 222 .I coresize
 223 [to be documented]
 224 .TP
 225 .I initsize
 226 [to be documented]
 227 .TP
 228 .I initstate
 229 [to be documented]
 230 .TP
 231 .I refcnt
 232 [to be documented]
 233 .TP
 234 .I srcversion
 235 [to be documented]
 236 .TP
 237 .I taint
 238 [to be documented]
 239 .TP
 240 .I uevent
 241 [to be documented]
 242 .TP
 243 .I version
 244 [to be documented]
 245 .RE
 246 .IP
 247 In each of the subdirectories, there may be following subdirectories:
 248 .RS
 249 .TP
 250 .I drivers
 251 [To be documented]
 252 .TP
 253 .I holders
 254 [To be documented]
 255 .TP
 256 .I notes
 257 [To be documented]
 258 .TP
 259 .I parameters
 260 This directory contains one file for each module parameter,
 261 with each file containing the value of the corresponding parameter.
 262 Some of these files are writable, allowing the
 263 .TP
 264 .I sections
 265 This subdirectories contains files with information about module sections.
 266 This information is mainly used for debugging.
 267 .TP
 268 .I
 269 [To be documented]
 270 .RE
 271 .TP
 272 .IR /sys/power
 273 [To be documented]
 274 .SH VERSIONS
 275 The
 276 .B sysfs
 277 filesystem first appeared in Linux 2.6.0.
 278 .SH CONFORMING TO
 279 The
 280 .B sysfs
 281 filesystem is Linux-specific.
 282 .SH NOTES
 283 This manual page is incomplete, possibly inaccurate, and is the kind
 284 of thing that needs to be updated very often.
 285 .SH SEE ALSO
 286 .BR proc (5),
 287 .BR udev (7)
 288 .PP
 289 P. Mochel. (2005).
 290 .IR "The sysfs filesystem" .
 291 Proceedings of the 2005 Ottawa Linux Symposium.
 292 .\" https://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf
 293 .PP
 294 The kernel source file
 295 .I Documentation/filesystems/sysfs.txt
 296 and various other files in
 297 .IR Documentation/ABI
 298 and
 299 .IR Documentation/*/sysfs.txt