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System Utilities sysfs Library - libsysfs
=========================================
Version: 0.4.0
December 16, 2003
Contents
--------
1. Introduction
2. Requirements
3. Definitions
4. Overview
5. Data Structures
5.1 Directory and Attribute Data Structures
5.1.1 Attribute Structure
5.1.2 Link Structure
5.1.3 Directory Structure
5.2 Bus Data Structure
5.3 Class Data Structures
5.4 Root Device Data Structure
5.5 Device Data Structure
5.6 Driver Data Structure
6. Functions
6.1 Utility Functions
6.2 Filesystem Functions
6.2.1 Attribute Functions
6.2.2 Directory Link Functions
6.2.3 Directory Functions
6.3 Bus Functions
6.4 Class Functions
6.5 Device Functions
6.6 Driver Functions
7. Navigating a dlist
8. Usage
9. Conclusion
1. Introduction
---------------
Libsysfs' purpose is to provide a consistent and stable interface for
querying system device information exposed through the sysfs filesystem.
The library implements functions for querying filesystem information,
such as reading directories and files. It also contains routines for
working with buses, classes, and the device tree.
2. Requirements
---------------
The library must satisfy the following requirements:
- It must provide a stable programming interfaces that applications can
be built upon.
- It must provide functions to retrieve device Vital Product Data (VPD)
information for Error Log Analysis (ELA) support. ELA will provide
device driver and device bus address information.
- It must provide access to all system devices and information exposed
by sysfs.
- It must provide a function to find sysfs' current mount point.
- It must provide a function for udev to retrieve a device's major and
minor numbers.
3. Definitions
--------------
- sysfs: Sysfs is a virtual filesystem in 2.5+ Linux kernels that
presents a hierarchical representation of all system physical and
virtual devices. It presents system devices by bus, by class, and
by topology. Callbacks to device drivers are exposed as files in
device directories. Sysfs, for all purposes, is our tree of system
devices. For more information, please see:
http://www.kernel.org/pub/linux/kernel/people/mochel/doc/
- udev: Udev is Greg Kroah-Hartman's User Space implementation of devfs.
Udev creates /dev nodes for devices upon Hotplug events. The Hotplug
event provides udev with a sysfs directory location of the device. Udev
must use that directory to grab device's major and minor number, which it
will use to create the /dev node. For more information, please see:
http://www.kernel.org/pub/linux/utils/kernel/hotplug/
4. Overview
-----------
Libsysfs grew from a common need. There are several applications under
development that need access to sysfs and system devices. Udev, on a
hotplug event, must take a sysfs device path and create a /dev node. Our
diagnostic client needs to list all system devices. Finally, our Error
Log Analysis piece is required to retrieve VPD information for a
failing device. We divided to create a single library interface rather
than having these separate applications create their own accesses to
sysfs involving reading directories and files.
Libsysfs will also provide stability for applications to be built upon. Sysfs
currently doesn't enforce any standards for callback or file names. File
names change depending on bus or class. Sysfs is also changing, it is
currently being developed. Libsysfs will provide a stable interface to
applications while allowing sysfs to change underneath it.
Like sysfs, the library will provide devices to applications by bus, by
class, and by topology. The library will function similar to directories
and files that lie underneath it. To query a device on a PCI bus, one would
"open" the bus to scan or read devices and "close" the bus when
completed. Besides supplying functions to retrieve devices, the library
will also provide some utility functions like getting sysfs mount point.
5. Data Structures
------------------
Libsysfs will classify system devices following sysfs' example, dividing
them by bus, class, and devices. The library presents this information
generically. It doesn't, for example, differentiate between PCI and USB
buses. Device attributes are presented with values as they are exposed
by sysfs, the values are not formatted.
The library will provide standard definitions for working with sysfs
and devices, here's some examples:
#define SYSFS_FSTYPE_NAME "sysfs"
#define SYSFS_PROC_MNTS "/proc/mounts"
#define SYSFS_BUS_DIR "/bus"
#define SYSFS_BUS_NAME "bus"
#define SYSFS_CLASS_DIR "/class"
#define SYSFS_CLASS_NAME "class"
#define SYSFS_BLOCK_DIR "/block"
#define SYSFS_BLOCK_NAME "block"
#define SYSFS_DEVICES_DIR "/devices"
#define SYSFS_DEVICES_NAME "devices"
#define SYSFS_DRIVERS_DIR "/drivers"
#define SYSFS_DRIVERS_NAME "drivers"
#define SYSFS_NAME_ATTRIBUTE "name"
The library uses some definitions to mark maximum size of a sysfs name or
path length:
#define SYSFS_PATH_MAX 255
#define SYSFS_NAME_LEN 50
#define SYSFS_BUS_ID_SIZE 20
NOTE:
As of release 0.4.0 of libsysfs, a number of changes have been made
so that the dlists and "directory" references in all libsysfs's
structures are not populated until such time that it is absolutely
necessary. Hence, these elements may not contain valid data at all
times (as was the case before).
5.1 Directory and Attribute Data Structures
-------------------------------------------
The library implements structures to represent sysfs directories, links,
and files.
5.1.1 Attribute Structure
-------------------------
A file in sysfs represents a device or driver attribute. Attributes can be
read only, write only, or read and write. File data can be ASCII and
binary. The library has the following structure to represent files:
struct sysfs_attribute {
unsigned char *value;
unsigned short len; /* value length */
unsigned short method; /* show and store */
unsigned char name[SYSFS_NAME_LEN];
unsigned char path[SYSFS_PATH_MAX];
};
Path represents the file/attribute's full path. Value is used when reading
from or writing to an attribute. "len" is the length of data in "value".
Method is a bitmask for defining if the attribute supports show(read)
and/or store(write).
5.1.2 Link Structure
--------------------
Symbolic links are used in sysfs to link bus or class views with
particular devices.
struct sysfs_link {
unsigned char name[SYSFS_NAME_LEN];
unsigned char path[SYSFS_PATH_MAX];
unsigned char target[SYSFS_PATH_MAX];
};
Link's name is stored in "name' and it's target stored in "target". Absolute
path to the link is stored in "path".
5.1.3 Directory Structure
-------------------------
The directory structure represents a sysfs directory:
struct sysfs_directory {
struct dlist *subdirs;
struct dlist *links;
struct dlist *attributes;
unsigned char name[SYSFS_NAME_LEN];
unsigned char path[SYSFS_PATH_MAX];
};
The sysfs_directory structure includes the list of subdirectories, links and
attributes. The "name" and absolute "path" are also stored in the structure.
The sysfs_directory structure is intended for use internal to the library.
Applications needing access to attributes and links from the directory
will need to make appropriate calls (described below) to get the same.
5.2 Bus Data Structure
----------------------
All buses look similar in sysfs including lists of devices and drivers,
therefore we use the following structure to represent all sysfs buses:
struct sysfs_bus {
unsigned char name[SYSFS_NAME_LEN];
unsigned char path[SYSFS_PATH_MAX];
/* internal use only */
struct dlist *drivers;
struct dlist *devices;
struct sysfs_directory *directory;
};
The sysfs_bus structure contains the bus "name", while the "path" to bus
directory is also stored. It also contains lists of devices on the bus
and drivers that are registered on it. The bus' directory is represented
by the sysfs_directory structure and it contains references to all the
subdirectories, links, and attributes. The sysfs_directory structure
is for internal use only. The following functions may be used by
applications to retrieve data from the sysfs_directory structure:
struct dlist *sysfs_get_bus_attributes(struct sysfs_bus *bus)
struct sysfs_attribute *sysfs_get_bus_attribute(struct sysfs_bus *bus,
unsigned char *attrname)
5.3 Class Data Structures
-------------------------
The library uses two data structures to represent classes in sysfs. Sysfs
classes contains a class directory like "net" or "scsi_host" and then
class devices like "eth0", "lo", or "eth1" for the "net" class.
struct sysfs_class {
unsigned char name[SYSFS_NAME_LEN];
unsigned char path[SYSFS_PATH_MAX];
/* for internal use only */
struct dlist *devices;
struct sysfs_directory *directory;
};
The sysfs_class represents device classes in sysfs like "net". It contains
the class "name", "path" to the class, a list of class devices and the
directory representation (for internal use only).
struct sysfs_class_device {
unsigned char name[SYSFS_NAME_LEN];
unsigned char classname[SYSFS_NAME_LEN];
unsigned char path[SYSFS_PATH_MAX];
/* for internal use only */
struct sysfs_class_device *parent;
struct sysfs_device *sysdevice; /* NULL if virtual */
struct sysfs_driver *driver; /* NULL if not implemented */
struct sysfs_directory *directory;
};
A class device isn't the same as a sysfs_device, it's specific to the class in
which it belongs. The class device structure contains the name of the class
the class device belongs to, its sysfs_device reference and that device's
driver reference (if any). It also contains the name of the class device
- like "eth0", its parent point (if present) and its sysfs directory
information including links and attributes (for internal use only).
The following function may be used by applications to retrieve data
from the sysfs_directory structure:
struct dlist *sysfs_get_classdev_attributes(struct sysfs_class_device *cdev);
5.4 Root Device Data Structure
------------------------------
Device hierarchies in sysfs are represented under the /sys/devices directory
structure. Sysfs devices typically spawn off from base devices which are
represented by a sysfs_root_device.
struct sysfs_root_device {
unsigned char name[SYSFS_NAME_LEN];
unsigned char path[SYSFS_PATH_MAX];
/* for internal use only */
struct dlist *devices;
struct sysfs_directory *directory;
};
The sysfs_root_device structure contains a list of "devices" that spawn off it.
The name of the root device as represented under /sys/devices is read into
"name" and the absolute path into "path" and its sysfs_directory information
intended to be used internal to the library.
5.5 Device Data Structure
-------------------------
The sysfs_device structure represents a system device that's exposed
in sysfs under the /sys/devices directory structure.
struct sysfs_device {
unsigned char name[SYSFS_NAME_LEN];
unsigned char bus_id[SYSFS_NAME_LEN];
unsigned char bus[SYSFS_NAME_LEN];
unsigned char driver_name[SYSFS_NAME_LEN];
unsigned char path[SYSFS_PATH_MAX];
/* for internal use only */
struct sysfs_device *parent;
struct dlist *children;
struct sysfs_directory *directory;
};
The sysfs_device structure contains a "parent" pointer, a list of child
devices, if any, device's directory, its bus id - which is the name of
device's directory, the bus name on which this device is registered and
its driver name. The device structure also contains the absolute path
to the device and a directory structure, which contains a list of the
device's attributes (for internal use only). The following functions
may be used to obtain information from sysfs_directory structure:
struct sysfs_attribute *sysfs_get_device_attribute(struct sysfs_device *dev,
const unsigned char *name)
struct dlist *sysfs_get_device_attributes(struct sysfs_device *device)
5.6 Driver Data Structure
-------------------------
The sysfs_driver structure represents a device driver.
struct sysfs_driver {
unsigned char name[SYSFS_NAME_LEN];
unsigned char path[SYSFS_PATH_MAX];
/* for internal use only */
struct dlist *devices;
struct sysfs_directory *directory;
};
The sysfs_driver structure contains a list of devices that use this driver,
the name of the driver, its path, and its directory information, which
includes the driver's attributes (for internal use only). The following
function may be used to retrieve driver attribute information from the
sysfs_directory structure:
struct dlist *sysfs_get_driver_attributes(struct sysfs_driver *driver)
6. Functions
------------
Libsysfs will provide functions to access system devices by bus, by class,
and by device. Functions will act like accessing directories and files,
using "open" and "close". Open returns a structure and close is used
to clean that structure up.
6.1 Utility Functions
---------------------
The library will provide a few utility functions for working with sysfs.
-------------------------------------------------------------------------------
Name: sysfs_get_mnt_path
Description: Function finds the mount path for filesystem type "sysfs".
Arguments: unsigned char *mnt_path Mount path buffer
size_t len Size of mount path buffer
Returns: Zero with success.
-1 with error. Errno will be set with error:
- EINVAL for invalid argument, if buffer is NULL.
Prototype: sysfs_get_mnt_path(unsigned char *mnt_path, size_t len);
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_name_from_path
Description: Function returns the last directory or file name from the
included path.
Arguments: const unsigned char *path Path to parse name from
unsigned char *name Buffer to put parsed name into
size_t *len Size of name buffer
Returns: 0 with success.
-1 on Error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: int sysfs_get_name_from_path(const unsigned char *path,
unsigned char *name, size_t *len)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_link
Description: Sysfs readlink function, reads the link at supplied path
and returns its target path.
Arguments: const unsigned char *path Link's path
unsigned char *target Buffer to place link's target path
size_t len Size of target buffer
Returns: 0 with success
-1 with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: int sysfs_get_link(const unsigned char *path,
unsigned char *target, size_t len)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_subsystem_list
Description: Function returns the list of entries for the given subsystem. If
the argument is "bus", this function will return a list of buses
("pci", "scsi", etc) supported on the system.
sysfs_close_list() has to be called to free the list obtained
from this call.
Arguments: unsigned char *name Subsystem to open, like "bus"..
Returns: dlist of entries for the subsystem on success
NULL with error indicating the "name" subsystem is invalid.
Prototype: struct dlist *sysfs_open_subsystem_list(unsigned char *name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_bus_devices_list
Description: Function returns the list of devices on the given bus.
sysfs_close_list() has to be called to free the list obtained
from this call.
Arguments: unsigned char *name Bus name to open "pci"/"scsi"/"usb"..
Returns: dlist of device names for the given bus on success
NULL with error indicating the bus is not supported.
Prototype: struct dlist *sysfs_open_bus_devices_list(unsigned char *name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_close_list
Description: Closes a given dlist. This can be used as a generic list close
routine.
Arguments: struct dlist *list List to be closed
Prototype: void sysfs_close_list(struct dlist *list)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_path_is_dir
Description: Utility function to verify if a given path is to a directory.
Arguments: unsigned char *path Path to verify
Returns: 0 on success, 1 on error
- EINVAL for invalid arguments
Prototype: int sysfs_path_is_dir(unsigned char *path)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_path_is_file
Description: Utility function to verify if a given path is to a file.
Arguments: unsigned char *path Path to verify
Returns: 0 on success, 1 on error
- EINVAL for invalid arguments
Prototype: int sysfs_path_is_file(unsigned char *path)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_path_is_link
Description: Utility function to verify if a given path is to a link.
Arguments: unsigned char *path Path to verify
Returns: 0 on success, 1 on error
- EINVAL for invalid arguments
Prototype: int sysfs_path_is_link(unsigned char *path)
-------------------------------------------------------------------------------
6.2 Filesystem Functions
------------------------
Libsysfs provides a set of functions to open, read, and close directories
and attributes/files in sysfs. These functions mirror their filesystem
function counterparts.
6.2.1 Attribute Functions
-------------------------
Along with the usual open, read, and close functions, libsysfs provides
a couple other functions for accessing attribute values.
-------------------------------------------------------------------------------
Name: sysfs_open_attribute
Description: Opens up a file in sysfs and creates a sysfs_attribute
structure. File isn't read with this function.
Arguments: const unsigned char *path File/Attribute's path
Returns: struct sysfs_attribute * with success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_attribute *sysfs_open_attribute
(const unsigned char *path)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_close_attribute
Description: Cleans up and closes sysfs_attribute structure.
Arguments: struct sysfs_attribute *sysattr Attribute to close
Prototype: void sysfs_close_attribute(struct sysfs_attribute *sysattr)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_read_dir_attributes
Description: Reads the given sysfs_directory to create a list of attributes.
Arguments: struct sysfs_directory *sysdir sysfs_directory whose
attributes to read
Returns: struct dlist * of attributes on success
NULL with error. Errno will be set on error, returning EINVAL
for invalid arguments
Prototype: struct dlist *sysfs_read_dir_attributes
(struct sysfs_directory *sysdir)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_refresh_attributes
Description: Given a list of attributes, this function refreshes the values
of attributes in the list.
Arguments: struct dlist *attrlist list of attributes to refresh
Returns: 0 with success.
1 with error. Errno will be set on error, returning EINVAL
for invalid arguments
Prototype: int sysfs_refresh_attributes(struct dlist *attrlist)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_read_attribute
Description: Reads the supplied attribute. Since the maximum transfer
from a sysfs attribute is a pagesize, function reads in
up to a page from the file and stores it in the "value"
field in the attribute.
Arguments: struct sysfs_attribute *sysattr Attribute to read
Returns: 0 with success.
-1 with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: int sysfs_read_attribute(struct sysfs_attribute *sysattr)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_write_attribute
Description: Writes to the supplied attribute. Function validates if attribute
is writable, and writes the new value to the attribute. Value to
write as well as its length is user supplied. In case the length
written is not equal to the length requested to be written, the
original value is restored and an error is returned.
Arguments: struct sysfs_attribute *sysattr Attribute to write to
const unsigned char *new_value New value for the attribute
size_t len Length of "new_value"
Returns: 0 with success.
-1 with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: int sysfs_write_attribute(struct sysfs_attribute *sysattr,
const unsigned char *new_value, size_t len)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_read_attribute_value
Description: Given a path to a specific attribute, function reads and
returns its value to the supplied value buffer.
Arguments: const unsigned char *attrpath Attribute path to read
unsigned char *value Buffer to place attribute's value
size_t vsize Size of buffer
Returns: 0 with success.
-1 with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: int sysfs_read_attribute_value(const unsigned char *attrpath,
unsigned char *value, size_t vsize)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_value_from_attributes
Description: Function takes a single or linked list of sysfs attribute
structures and returns the value of the specified attribute
name.
Arguments: struct sysfs_attribute *attr
Attribute list to search through
const unsigned char *name Name of attribute to return value
Returns: unsigned char * attribute value with success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: unsigned char *sysfs_get_value_from_attributes
(struct sysfs_attribute *attr, const unsigned char * name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_directory_attribute
Description: Function walks the list of attributes for the given sysfs
directory and returns the sysfs_attribute structure for
the specified attribute name.
Arguments: struct sysfs_directory *dir Directory in which to search
unsigned char *attrname Attribute name to look for
Returns: struct sysfs_attribute on success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_attribute *sysfs_get_directory_attribute
(struct sysfs_directory *dir, unsigned char *attrname)
-------------------------------------------------------------------------------
6.2.2 Link Functions
--------------------
Sysfs contains many symbolic links, like bus links to bus devices. Libsysfs
treats links differently than directories due to processing differences. A
link in the /sys/bus/"busname"/devices/ directory indicates a device in the
/sys/devices directory. Through links we give the functionality to know
what is and what isn't a link and the ability to query the links target.
-------------------------------------------------------------------------------
Name: sysfs_open_link
Description: Opens a directory link.
Arguments: const unsigned char *linkpath Path to link
Returns: struct sysfs_link * with success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_link *sysfs_open_link
(const unsigned char *linkpath)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_close_link
Description: Closes a directory link structure.
Arguments: struct sysfs_link *ln Link to close
Prototype: void sysfs_close_link(struct sysfs_link *ln)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_read_dir_links
Description: Reads the given sysfs_directory to create a list of links.
Arguments: struct sysfs_directory *sysdir sysfs_directory whose
links to read
Returns: struct dlist * of links with success
NULL with error. Errno will be set on error, returning EINVAL
for invalid arguments
Prototype: struct dlist *sysfs_read_dir_links
(struct sysfs_directory *sysdir)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_directory_link
Description: Function walks the list of links for the given sysfs directory
and returns the sysfs_link structure for the specified link
name.
Arguments: struct sysfs_directory *dir Directory in which to search
unsigned char *linkname Link name to look for
Returns: struct sysfs_link * with success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_link *sysfs_get_directory_link
(struct sysfs_directory *dir, unsigned char *linkname)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_subdirectory_link
Description: Function walks the list of links for the given sysfs directory
and its subdirectories returns the sysfs_link structure for
the specified link name.
Arguments: struct sysfs_directory *dir Directory in which to search
unsigned char *linkname Link name to look for
Returns: struct sysfs_link * with success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_link *sysfs_get_subdirectory_link
(struct sysfs_directory *dir, unsigned char *linkname)
-------------------------------------------------------------------------------
6.2.3 Directory Functions
-------------------------
Sysfs directories can represent every directory under sysfs. The structures
keep track of subdirectories, links, and files. Like opendir, readdir, and
closedir, libsysfs provides open, read, and close functions for working with
sysfs directories. Open creates the sysfs_directory structure. Read reads in
its contents - like subdirectories, links, and files. Close cleans it all
up.
-------------------------------------------------------------------------------
Name: sysfs_open_directory
Description: Opens a sysfs directory at a specific path
Arguments: const unsigned char *path Directory path to open
Returns: struct sysfs_directory * with success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_directory *sysfs_open_directory
(const unsigned char *path)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_close_directory
Description: Closes specific directory, its subdirectories, links, and
files.
Arguments: struct sysfs_directory *sysdir Directory to close
Prototype: void sysfs_close_directory(struct sysfs_directory *sysdir)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_read_dir_subdirs
Description: Reads the given sysfs_directory to create a list of subdirs.
Arguments: struct sysfs_directory *sysdir sysfs_directory whose
subdirs have to be read
Returns: struct dlist * of links with success
NULL with error. Errno will be set on error, returning EINVAL
for invalid arguments
Prototype: struct dlist *sysfs_read_dir_subdirs
(struct sysfs_directory *sysdir)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_read_directory
Description: Read the supplied directory. Reading fills in the directory's
contents like subdirectories, links, and attributes.
Arguments: struct sysfs_directory *sysdir Directory to read
Returns: 0 with success.
-1 with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: int sysfs_read_directory(struct sysfs_directory *sysdir)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_read_all_subdirs
Description: Reads all subdirs under a given supplied directory.
Arguments: struct sysfs_directory *sysdir Directory to read
Returns: 0 with success.
-1 with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: int sysfs_read_all_subdirs(struct sysfs_directory *sysdir)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_subdirectory
Description: Function walks the directory tree for the given directory and
returns a sysfs_directory structure for the specified directory
name.
Arguments: struct sysfs_directory *dir Directory in which to search
unsigned char *subname Name of directory to look for
Returns: struct sysfs_directory with success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
-------------------------------------------------------------------------------
6.3 Bus Functions
-----------------
The library provides a functions for viewing buses represented in sysfs.
The sysfs_open_bus opens a bus in the /sys/bus directory, such as "pci",
"usb", or "scsi". The open command returns a sysfs_bus structure that
contains a list of the bus' devices. The sysfs_close_bus function is
used to clean up the bus structure. Given a device or a driver,
functions are provided to determine what bus they are on.
-------------------------------------------------------------------------------
Name: sysfs_open_bus
Description: Function opens up one of the buses represented in sysfs in
the /sys/bus directory. It returns a sysfs_bus structure
that includes a list of bus devices and drivers.
Arguments: const unsigned char *name Bus name to open, like "pci"...
Returns: struct sysfs_bus * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_bus *sysfs_open_bus(const unsigned char *name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_close_bus
Description: Function closes up the sysfs_bus structure including its
devices, drivers, and directory.
Arguments: sysfs_bus *bus Bus structure to close
Prototype: void sysfs_close_bus(struct sysfs_bus *bus)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_bus_devices
Description: Function returns a list of devices that are registered with
this bus.
Arguments: struct sysfs_bus *bus Bus whose devices list to return
Returns: struct dlist * of sysfs_devices on success
NULL with error. Errno will be sent with error, returning
- EINVAL for invalid arguments
Prototype: struct dlist *sysfs_get_bus_devices(struct sysfs_bus *bus)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_bus_drivers
Description: Function returns a list of drivers that are registered with
this bus.
Arguments: struct sysfs_bus *bus Bus whose drivers list to return
Returns: struct dlist * of sysfs_drivers on success
NULL with error. Errno will be sent with error, returning
- EINVAL for invalid arguments
Prototype: struct dlist *sysfs_get_bus_drivers(struct sysfs_bus *bus)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_bus_device
Description: Function takes a sysfs_bus structure(obtained on a successful
return from a sysfs_open_bus() call) and looks for the given
device on this bus. On success, it returns a sysfs_device
structure corresponding to the device.
Arguments: struct sysfs_bus *bus Bus structure on which to search
unsigned char *id Device to look for
Returns: struct sysfs_device * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_device *sysfs_get_bus_device
(struct sysfs_bus *bus, unsigned char *id)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_bus_driver
Description: Function takes a sysfs_bus structure (obtained on a successful
return from a sysfs_open_bus() call) and looks for the given
driver on this bus. On success, it returns a sysfs_driver
structure corresponding to the driver.
Arguments: struct sysfs_bus *bus Bus structure on which to search
unsigned char *drvname Driver to look for
Returns: struct sysfs_driver * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_device *sysfs_get_bus_driver
(struct sysfs_bus *bus, unsigned char *drvname)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_bus_attributes
Description: Function takes a sysfs_bus structure and returns a list of
attributes for the bus.
Arguments: struct sysfs_bus *bus Bus for which attributes are required
Returns: struct dlist * of attributes with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct dlist *sysfs_get_bus_attributes(struct sysfs_bus *bus)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_bus_attribute
Description: Function takes a sysfs_bus structure and looks for the required
attribute on the bus. On success, it returns a sysfs_attribute
structure corresponding to the given attribute.
Arguments: struct sysfs_bus *bus Bus structure on which to search
unsigned char *attrname Attribute to look for
Returns: struct sysfs_attribute * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_attribute *sysfs_get_bus_attribute
(struct sysfs_bus *bus, unsigned char *attrname)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_bus_device
Description: Given the name of the bus on which to look for, this function
locates a given device on the bus and returns a sysfs_device
structure corresponding to the requested device.
NOTE:
1. The sysfs_device structure obtained upon successful return
from this function has to be closed by calling
sysfs_close_device().
Arguments: unsigned char *busname Bus on which to search
unsigned char *dev_id Name of the device to look for
Returns: struct sysfs_device * on success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_device *sysfs_open_bus_device
(unsigned char *busname, unsigned char *dev_id)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_find_driver_bus
Description: Given the name of a driver, this function finds the name of the
bus the driver is on
Arguments: const unsigned char *driver Name of the driver to look for
unsigned char *busname Buffer to return the bus name
size_t bsize Size of the "busname" buffer
Returns: 0 with success.
-1 with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: int sysfs_find_driver_bus(const unsigned char *driver,
unsigned char *busname, size_t bsize)
-------------------------------------------------------------------------------
6.4 Class Functions
-------------------
Libsysfs provides functions to open sysfs classes and their class devices.
These functions too operate with open and close, close must be called to
clean up the class structures. Given a class device name, functions are
provided to determine what class they belong to. Once a class device
name and the class it belongs to is known, a function to open the
class device is provided. This method can be used when details of
a single class device is required.
-------------------------------------------------------------------------------
Name: sysfs_open_class
Description: Function opens up one of the classes represented in sysfs in
the /sys/class directory. It returns a sysfs_class structure
that includes a list of class devices.
Arguments: const unsigned char *name Class name to open, like "net"..
Returns: struct sysfs_class * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_class *sysfs_open_class(const unsigned char *name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_close_class
Description: Function closes up the sysfs_class structure including its
class devices.
Arguments: sysfs_class *class Class structure to close
Prototype: void sysfs_close_class(struct sysfs_class *class);
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_class_device_path
Description: Function opens up one of the class devices represented in
sysfs in sysfs/class/"class"/ directory. It returns a
sysfs_class_device structure.
Arguments: const unsigned char *path Path to class device
Returns: struct sysfs_class_device * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_class_device *sysfs_open_class_device_path
(const unsigned char *path)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_close_class_device
Description: Function closes up the sysfs_class_device structure.
Arguments: sysfs_class_device *dev Class device structure to close
Prototype: void sysfs_close_class_device(struct sysfs_class_device *dev)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_class_device
Description: Function takes a sysfs_class structure(obtained on a successful
return from a sysfs_open_class() call) and looks for the given
device in this class. On success, it returns a sysfs_class_device
structure corresponding to the class device.
Arguments: struct sysfs_class *class Class on which to search
unsigned_char *name Class device "name" to look for
Returns: struct sysfs_class_device * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_class_device *sysfs_get_class_device
(struct sysfs_class *class, unsigned char *name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_class_devices
Description: Function returns a list of class devices for the given class.
Arguments: struct sysfs_class *cls Class whose class device list
is required
Returns: struct dlist * of sysfs_class_devices on success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct dlist *sysfs_get_class_devices(struct sysfs_class *cls)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_class_device
Description: Given the name of the class on which to look for, this function
locates a given class device and returns a sysfs_class_device
structure corresponding to the requested class device.
NOTE:
1. The sysfs_class_device structure obtained upon successful
return from this function has to be closed by calling
sysfs_close_class_device().
2. Class this device belongs to must be known prior to calling
this function.
Arguments: const unsigned char *classname Class on which to search
unsigned char *name Class device "name" to open
Returns: struct sysfs_class_device * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_class_device *sysfs_open_class_device
(const unsigned char *classname, unsigned char *class)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_classdev_device
Description: Function returns the sysfs_device reference (if present) for the
given class device.
Arguments: struct sysfs_class_device *clsdev Class device whose
sysfs_device reference
is required
Returns: struct sysfs_device * on success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_device *sysfs_get_classdev_device
(struct sysfs_class_device *clsdev)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_classdev_driver
Description: Function returns the sysfs_driver reference (if present) for the
given class device.
Arguments: struct sysfs_class_device *clsdev Class device whose
sysfs_driver reference
is required
Returns: struct sysfs_driver * on success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_driver *sysfs_get_classdev_driver
(struct sysfs_class_device *clsdev)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_classdev_parent
Description: Function returns the sysfs_class_device reference for the parent
(if present) of the given class device.
Arguments: struct sysfs_class_device *clsdev Class device whose
parent reference
is required
Returns: struct sysfs_class_device * on success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_class_device *sysfs_get_classdev_parent
(struct sysfs_class_device *clsdev)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_classdev_attributes
Description: Function takes a sysfs_class_device structure and returns a list
of attributes for the class device.
Arguments: struct sysfs_class_device *cdev Class device for which
attributes are required
Returns: struct dlist * of attributes with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct dlist *sysfs_get_classdev_attributes
(struct sysfs_class_device *cdev)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_classdev_attr
Description: Searches supplied class device's attributes by name and returns
the attribute.
Arguments: struct sysfs_class_device *clsdev Device to search
const unsigned char *name Attribute name to find
Returns: struct sysfs_attribute * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_attribute *sysfs_get_classdev_attr
(struct sysfs_class_device *clsdev, const unsigned char *name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_classdev_attr
Description: Function takes as arguments, a the name of the class, the class
device name and the name of the required attribute.
NOTE:
1. The struct sysfs_attribute * obtained upon successful
return from this function has to be closed by making
a call to sysfs_close_attribute()
Arguments: unsigned char *classname Class name on which to search
unsigned char *dev Name of the class device
unsigned char *attrib Attribute to open
Returns: struct sysfs_attribute * with success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_attribute *sysfs_write_classdev_attr
(const unsigned char *classname, const unsigned char *dev,
const unsigned char *attrib)
-------------------------------------------------------------------------------
6.5 Device Functions
--------------------
Devices represent everything in sysfs under /sys/devices, which is a
hierarchical view of system devices. Besides the expected open and
close functions, libsysfs provides open and close functions for
root devices. These functions recursively open or close a device
and all of its children.
-------------------------------------------------------------------------------
Name: sysfs_open_device_path
Description: Opens up a device at a specific path. It opens the device's
directory, reads the directory, and returns a sysfs_device
structure.
Arguments: const unsigned char *path Path to device
Returns: struct sysfs_device * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_device *sysfs_open_device_path
(const unsigned char *path)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_close_device
Description: Function closes up the sysfs_device structure.
Arguments: sysfs_device *dev Device structure to close
Prototype: void sysfs_close_device(struct sysfs_device *dev)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_root_device
Description: Function opens up one of the root devices represented in sysfs
in the /sys/devices directory. It returns a sysfs_root_device
structure that includes a list of devices in the tree.
Arguments: const unsigned char *name Name of the root device to open
Returns: struct sysfs_root_device * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_device *sysfs_open_root_device
(const unsigned char *name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_close_root_device
Description: Function closes up the sysfs_root_device structure including the
devices in the root device tree.
Arguments: sysfs_device *root Root device structure to close
Prototype: void sysfs_close_root_device(struct sysfs_root_device *root)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_device_parent
Description: Function returns the sysfs_device reference for the parent
(if present) of the given sysfs_device.
Arguments: struct sysfs_device *dev sysfs_device whose parent
reference is required
Returns: struct sysfs_device * on success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_device *sysfs_get_device_parent
(struct sysfs_device *dev)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_root_devices
Description: Function returns a list of devices under the given root device.
Arguments: struct sysfs_root_device *root sysfs_root_device whose devices
list is required
Returns: struct dlist * of sysfs_devices on success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct dlist *sysfs_get_root_devices
(struct sysfs_root_device *root)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_device_attr
Description: Searches supplied device's attributes by name and returns
the attribute.
Arguments: struct sysfs_device *dev Device to search
const unsigned char *name Attribute name to find
Returns: struct sysfs_attribute * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_attribute *sysfs_get_device_attr
(struct sysfs_device *dev, const unsigned char *name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_device_attributes
Description: Function takes a sysfs_device structure and returns a list
of attributes for the device.
Arguments: struct sysfs_device *device Device for which
attributes are required
Returns: struct dlist * of attributes with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct dlist *sysfs_get_device_attributes
(struct sysfs_device *device)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_device
Description: Given the name of the bus on which to look for, this function
locates a given device and returns a sysfs_device structure
corresponding to the requested device.
Arguments: const unsigned char *bus_id Device to look for
const unsigned char *bus Bus on which to search
Returns: struct sysfs_device * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_device *sysfs_open_device
(const unsigned char *bus_id, const unsigned char *bus)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_device_attr
Description: Function takes as arguments, the bus on which to search for a
device, and an attribute of the device to open.
NOTE:
1. The struct sysfs_attribute * obtained upon successful
return from this function has to be closed by making
a call to sysfs_close_attribute()
Arguments: unsigned char *bus Bus on which to search
unsigned char *bus_id Device to look for
unsigned char *attrib Name of the attribute to open
Returns: struct sysfs_attribute * with success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_attribute *sysfs_open_device_attr
(const unsigned char *bus, const unsigned char *bus_id,
const unsigned char *attrib)
-------------------------------------------------------------------------------
6.6 Driver Functions
--------------------
Drivers are represented in sysfs under the /sys/bus/xxx/drivers (xxx being
the bus type, such as "pci", "usb, and so on). Functions are provided to
open and close drivers.
-------------------------------------------------------------------------------
Name: sysfs_open_driver_path
Description: Opens driver at specific path.
Arguments: const unsigned char *path Path to driver
Returns: struct sysfs_driver * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_driver *sysfs_open_driver_path
(const unsigned char *path)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_close_driver
Description: Closes and cleans up sysfs_driver structure.
Arguments: sysfs_driver *driver Driver structure to close
Prototype: void sysfs_close_driver(struct sysfs_driver *driver)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_driver_devices
Description: Function returns a list of devices that use this driver.
Arguments: struct sysfs_driver *driver Driver whose devices list is
required
Returns: struct dlist * of sysfs_devices on success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct dlist *sysfs_get_driver_devices
(struct sysfs_driver *driver)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_driver_device
Description: Function returns a sysfs_device reference for the device with
"name" that uses this driver
Arguments: struct sysfs_driver *driver Driver on which to search
const unsigned char *name Name of the device to look for
Returns: struct sysfs_device * corresponding to "name" on success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct dlist *sysfs_get_driver_device
(struct sysfs_driver *driver, const unsigned char *name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_driver_attr
Description: Searches supplied driver's attributes by name and returns
the attribute.
Arguments: struct sysfs_driver *drv Driver to search
const unsigned char *name Attribute name to find
Returns: struct sysfs_attribute * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_attribute *sysfs_get_driver_attr
(struct sysfs_driver *drv, const unsigned char *name)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_driver_attributes
Description: Function takes a sysfs_driver structure and returns a list
of attributes for the driver.
Arguments: struct sysfs_driver *driver Driver for which
attributes are required
Returns: struct dlist * of attributes with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct dlist *sysfs_get_driver_attributes
(struct sysfs_driver *driver)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_driver
Description: Given the name of the bus on which to look for, this function
locates a given driver and returns a sysfs_driver structure
corresponding to the requested device.
NOTE:
1. The sysfs_driver structure obtained upon successful return
from this function has to be closed by calling
sysfs_close_driver_by_name().
2. Bus on which to look for this driver should be known prior
to calling this function. Use sysfs_find_driver_bus()
to determine this.
Arguments: const unsigned char *drv_name Driver to look for
const unsigned char *bus Bus on which to search
size_t bsize Size of "bus"
Returns: struct sysfs_driver * with success
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_driver *sysfs_open_driver
(const unsigned char *drv_name,
const unsigned char *bus, size_t bsize)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_get_driver_links
Description: Function returns a list of links for a given driver
Arguments: struct sysfs_driver *driver Driver to get links from
Returns: struct dlist * of links on success
NULL with error
Prototype: struct dlist *sysfs_get_driver_links
(struct sysfs_driver *driver)
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Name: sysfs_open_driver_attr
Description: Function takes as arguments, the bus the driver is registered
on, the driver name and the name of the attribute to open.
NOTE:
1. The struct sysfs_attribute * obtained upon successful
return from this function has to be closed by making
a call to sysfs_close_attribute()
Arguments: unsigned char *bus Bus on which driver is present
unsigned char *drv Driver to look for
unsigned char *attrib Name of the attribute to open
Returns: struct sysfs_attribute * with success.
NULL with error. Errno will be set with error, returning
- EINVAL for invalid arguments
Prototype: struct sysfs_attribute *sysfs_open_driver_attr
(const unsigned char *bus, const unsigned char *drv,
const unsigned char *attrib)
-------------------------------------------------------------------------------
7. Navigating a dlist
---------------------
Libsysfs uses (yet another) list implementation thanks to Eric J Bohm.
Some library functions return a dlist of devices/drivers/attributes, etc.
To navigate the list returned the macro "dlist_for_each_data" is to be used.
------------------------------------------------------------------------------
Function/Macro name: dlist_for_each_data
Description: Walk the given list, returning a known data type/
structure in each iteration.
Arguments: struct dlist *list List pointer
data_iterator Data type/structure variable
contained in the list
datatype Data type/structure contained
in the list
Returns: On each iteration, "data_iterator" will contain a list
element of "datatype"
Usage example: The function sysfs_get_classdev_attributes() returns a
dlist of attributes. To navigate the list:
struct sysfs_attribute *attr = NULL;
struct dlist *attrlist = NULL;
.
.
.
attrlist = sysfs_get_classdev_attributes
(struct sysfs_class_device *cdev)
if (attrlist != NULL) {
dlist_for_each_data(attrlist, attr,
struct sysfs_attribute) {
.
.
.
}
}
-------------------------------------------------------------------------------
8. Usage
--------
Accessing devices through libsysfs is supposed to mirror accessing devices
in the filesystem it represents. Here's a typical order of operation:
- get sysfs mount point
- "open" sysfs category, ie. bus, class, or device
- work with category
- "close" sysfs category
9. Conclusion
-------------
Libsysfs is meant to provide a stable application programming interface to
sysfs. Applications can depend upon the library to access system devices
and functions exposed through sysfs.
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