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✏️ Editing: applications

How to write applications which use the hwmon drivers
=====================================================

You have several choices in accessing sensor devices using the hwmon drivers.
This document will briefly describe these methods, their advantages and
disadvantages, and provide examples.

From lowest-level to the highest-level, the access methods are:

1. sysfs access
   2. libsensors library
   3. sensors program

Details:

1. sysfs access
---------------
   Hwmon drivers create subdirectories in the sysfs filesystem (usually /sys)
   which can be accessed directly by applications.
   Naming and content standards for the entries in these subdirectories
   are documented in the file Documentation/hwmon/sysfs-interface in the 2.6
   kernel source tree. sysfs is the lowest-level access to sensor values.

Note that most drivers provide only raw sensor readings via /sys;
   many readings must be scaled or adjusted, and these adjustments
   must often be changed by the user. An application using /sys must
   generally provide adjustment facilities and the requirements
   of the adjustments can be quite complex. For this reason, this
   approach isn't recommended. Instead, consider the libsensors library,
   which offers a higher-level interface on top of sysfs (see below.)

Still, this method may work well for shell and perl scripts written to
   access a specific device.

For an example of a program using /sys accesses, see gkrellm (though
   recent versions use libsensors instead.) See also lib/sysfs.c.

2. libsensors library
---------------------
   The libsensors library provides standardized access to all chip drivers.
   It also provides a translation layer with settings in /etc/sensors.conf
   so that your application sees adjusted (scaled) values using settings
   provided by the user. Other facilities are sensor labelling, limit setting,
   and ignoring individual sensors. This is the recommended way to access
   the sensor chips.

The library API is documented in the libsensors(3) manual page. The
   configuration file format is documented in the sensors.conf(5) manual
   page.

Note that since version 3.2.0, libsensors falls under the LGPL, not the
   GPL as it used to. In more human language, that means it is now ALLOWED
   to link any application to the library, even to the shared version,
   regardless of the application's license.

For examples of programs using libsensors accesses, see prog/sensors and
   prog/sensord.

3. sensors program
------------------
   The 'sensors' program is a text-based application that uses libsensors.
   The output is fairly standardized; therefore this output could be
   used by other applications.
   One simple method is 'sensors|grep ALARM'.
   Note though that there is no guarantee that the output format won't change
   in the future, so be careful. We might improve the command-line interface
   of 'sensors' someday to make it more convenient for external programs to
   use it as a back-end.

MR SKB