libgpiod/README

# SPDX-License-Identifier: CC-BY-SA-4.0
# SPDX-FileCopyrightText: 2017-2023 Bartosz Golaszewski <brgl@bgdev.pl>

libgpiod
========

  libgpiod - C library and tools for interacting with the linux GPIO
             character device (gpiod stands for GPIO device)

Since linux 4.8 the GPIO sysfs interface is deprecated. User space should use
the character device instead. Version 2 of libgpiod requires GPIO character
device uAPI v2 which was first released in linux 5.10. This library
encapsulates the ioctl calls and data structures behind a straightforward API.

RATIONALE
---------

The new character device interface guarantees all allocated resources are
freed after closing the device file descriptor and adds several new features
that are not present in the obsolete sysfs interface (like event polling,
setting/reading multiple values at once or open-source and open-drain GPIOs).

Unfortunately interacting with the linux device file can no longer be done
using only standard command-line tools. This is the reason for creating a
library encapsulating the cumbersome, ioctl-based kernel-userspace interaction
in a set of convenient functions and opaque data structures.

Additionally this project contains a set of command-line tools that should
allow an easy conversion of user scripts to using the character device.

BUILDING
--------

This is a pretty standard autotools project. The core C library does not have
any external dependencies other than the standard C library with GNU extensions.

The build system requires autotools, autoconf-archive, libtool and pkg-config
to be installed on the host system for the basic build. Development files for
additional libraries may be required depending on selected options. The
configure script will report any missing additional required dependencies.

The command-line tools optionally depend on libedit for the interactive feature.

To build the project (including command-line utilities) run:

    ./autogen.sh --enable-tools=yes --prefix=<install path>
    make
    make install

The autogen script will execute ./configure and pass all the command-line
arguments to it.

If building from release tarballs, the configure script is already provided and
there's no need to invoke autogen.sh.

For all configure features, see: ./configure --help.

TOOLS
-----

There are currently six command-line tools available:

* gpiodetect - list all gpiochips present on the system, their names, labels
               and number of GPIO lines

* gpioinfo   - list lines, their gpiochip, offset, name, and direction, and
               if in use then the consumer name and any other configured
               attributes, such as active state, bias, drive, edge detection
               and debounce period

* gpioget    - read values of specified GPIO lines

* gpioset    - set values of specified GPIO lines, holding the lines until the
               process is killed or otherwise exits

* gpiomon    - wait for edge events on GPIO lines, specify which edges to watch
               for, how many events to process before exiting, or if the events
               should be reported to the console

* gpionotify - wait for changed to the info for GPIO lines, specify which
               changes to watch for, how many events to process before exiting,
               or if the events should be reported to the console

Examples:

    (using a Raspberry Pi 4B)

    # Detect the available gpiochips.
    $ gpiodetect
    gpiochip0 [pinctrl-bcm2711] (58 lines)
    gpiochip1 [raspberrypi-exp-gpio] (8 lines)

    # Read the info for all the lines on a gpiochip.
    $ gpioinfo -c 1
    gpiochip1 - 8 lines:
     line   0: "BT_ON"          output
     line   1: "WL_ON"          output
     line   2: "PWR_LED_OFF"    output active-low consumer="led1"
     line   3: "GLOBAL_RESET"   output
     line   4: "VDD_SD_IO_SEL"  output consumer="vdd-sd-io"
     line   5: "CAM_GPIO"       output consumer="cam1_regulator"
     line   6: "SD_PWR_ON"      output consumer="sd_vcc_reg"
     line   7: "SD_OC_N"        input

    # Read the info for particular lines.
    $ ./gpioinfo PWR_LED_OFF STATUS_LED_G_CLK GLOBAL_RESET
    gpiochip0 42 "STATUS_LED_G_CLK" output consumer="led0"
    gpiochip1 2 "PWR_LED_OFF"    output active-low consumer="led1"
    gpiochip1 3 "GLOBAL_RESET"   output

    # Read the value of a single GPIO line by name.
    $ gpioget RXD1
    "RXD1"=active

    # Read the value of a single GPIO line by chip and offset.
    $ gpioget -c 0 15
    "15"=active

    # Read the value of a single GPIO line as a numeric value.
    $ gpioget --numeric RXD1
    1

    # Read two values at the same time. Set the active state of the lines
    # to low and without quoted names.
    $ gpioget --active-low --unquoted GPIO23 GPIO24
    GPIO23=active GPIO24=active

    # Set the value of a line and hold the line until killed.
    $ gpioset GPIO23=1

    # Set values of two lines, then daemonize and hold the lines.
    $ gpioset --daemonize GPIO23=1 GPIO24=0

    # Set the value of a single line, hold it for 20ms, then exit.
    $ gpioset --hold-period 20ms -t0 GPIO23=1

    # Blink an LED on GPIO22 at 1Hz
    $ gpioset -t500ms GPIO22=1

    # Blink an LED on GPIO22 at 1Hz with a 20% duty cycle
    $ gpioset -t200ms,800ms GPIO22=1

    # Set some lines interactively (requires --enable-gpioset-interactive)
    $ gpioset --interactive --unquoted GPIO23=inactive GPIO24=active
    gpioset> get
    GPIO23=inactive GPIO24=active
    gpioset> toggle
    gpioset> get
    GPIO23=active GPIO24=inactive
    gpioset> set GPIO24=1
    gpioset> get
    GPIO23=active GPIO24=active
    gpioset> toggle
    gpioset> get
    GPIO23=inactive GPIO24=inactive
    gpioset> toggle GPIO23
    gpioset> get
    GPIO23=active GPIO24=inactive
    gpioset> exit

    # Wait for three rising edge events on a single GPIO line, then exit.
    $ gpiomon --num-events=3 --edges=rising GPIO22
    10002.907638045	rising	"GPIO22"
    10037.132562259	rising	"GPIO22"
    10047.179790748	rising	"GPIO22"

    # Wait for three edge events on a single GPIO line, with time in local time
    # and with unquoted line name, then exit.
    $ gpiomon --num-events=3 --edges=both --localtime --unquoted GPIO22
    2022-11-15T10:36:59.109615508	rising	GPIO22
    2022-11-15T10:36:59.129681898	falling	GPIO22
    2022-11-15T10:36:59.698971886	rising	GPIO22

    # Wait for falling edge events with a custom output format.
    $ gpiomon --format="%e %c %o %l %S" --edges=falling -c gpiochip0 22
    2 gpiochip0 22 GPIO22 10946.693481859
    2 gpiochip0 22 GPIO22 10947.025347604
    2 gpiochip0 22 GPIO22 10947.283716669
    2 gpiochip0 22 GPIO22 10947.570109430
    ...

    # Block until an edge event occurs. Don't print anything.
    $ gpiomon --num-events=1 --quiet GPIO22

    # Monitor multiple lines, exit after the first edge event.
    $ gpiomon --quiet --num-events=1 GPIO5 GPIO6 GPIO12 GPIO17

    # Monitor a line for changes to info.
    $ gpionotify GPIO23
    11571.816473718	requested	"GPIO23"
    11571.816535124	released	"GPIO23"
    11572.722894029	requested	"GPIO23"
    11572.722932843	released	"GPIO23"
    11573.222998598	requested	"GPIO23"
    ...

    # Monitor a line for requests, reporting UTC time and unquoted line name.
     $ gpionotify --utc --unquoted GPIO23
    2022-11-15T03:05:23.807090687Z	requested	GPIO23
    2022-11-15T03:05:23.807151390Z	released	GPIO23
    2022-11-15T03:05:24.784984280Z	requested	GPIO23
    2022-11-15T03:05:24.785023873Z	released	GPIO23
    ...

    # Monitor multiple lines, exit after the first is requested.
    $ gpionotify --quiet --num-events=1 --event=requested GPIO5 GPIO6 GPIO12 GPIO17

    # Block until a line is released.
    $ gpionotify --quiet --num-events=1 --event=released GPIO6

BINDINGS
--------

High-level, object-oriented bindings for C++, GLib, python3 and Rust are
provided. They can be enabled by passing --enable-bindings-cxx,
--enable-bindings-glib, --enable-bindings-python and --enable-bindings-rust
arguments respectively to configure.

C++ bindings require C++11 support and autoconf-archive collection if building
from git.

GLib bindings requires GLib (as well as GObject, GIO and GIO-Unix) v2.54.

Python bindings require python3 support and libpython development files. Please
refer to bindings/python/README.md for more information.

Rust bindings require cargo support. When building the Rust bindings along the
C library using make, they will be automatically configured to build against the
build results of the C library. Please refer to bindings/rust/libgpiod/README.md
for more information.

DBUS
----

A commonly requested feature for the GPIO character device was state persistence
after releasing the lines (as a kernel feature) or providing a central authority
(in user-space) that would be in charge of keeping the lines requested and in a
certain state (similarily to how the sysfs ABI works). DBus API has been
provided to address this requirement. We define an interface covering the
majority of the GPIO chardev's functionality and implement it from both the
server and client sides in the form of the gpio-manager daemon and the gpiocli
command-line utility for talking to the manager.

DBus support can be built by passing --enable-dbus to configure. The daemon
is bundled with a systemd unit file and an example configuration file for the
io.gpiod1 interface that allows all users to access basic information about the
GPIOs in the system but only root to request lines or change their values.

With the manager running the user can run gpiocli to control GPIOs by asking
gpio-manager to act on their behalf:

    # Detect chips in the system.
    $ gpiocli detect
    gpiochip0 [INT34C6:00] (463 lines)

    # Request a set of lines. Note that gpiocli exits immediately but the
    # state of the lines is retained because it's the gpio-manager that
    # requested them.
    $ gpiocli request --output foo=active
    request0

    # Previous invocation printed out the name of the request by which the
    # caller can refer to it later. All active requests can also be inspected
    # at any time.
    $ gpiocli requests
    request0 (gpiochip1) Offsets: [5]

    # We can print the information about the requested line using the
    # information above.
    $ gpiocli info --chip=gpiochip1 5
    gpiochip1   5:	"foo"		[used,consumer="gpiocli request",managed="request0",output,push-pull]

    # We can now change the value of the line.
    $ gpiocli set foo=inactive

    # And read it.
    $ gpiocli get foo
    "foo"=inactive

    # We can even reconfigure it to input and enable edge-detection.
    $ gpiocli reconfigure --input --both-edges request0

    # And wait for edge events.
    $ gpiocli monitor cos
    21763952894920 rising  "foo"

    # And finally release the request.
    $ gpiocli release request0

For more information please refer to the output of gpiocli --help as well as
gpiocli <command> --help which prints detailed info on every available command.

Of course - this being DBus - users can talk to gpio-manager using any DBus
library available and are not limited to the provided client.

TESTING
-------

A comprehensive testing framework is included with the library and can be
used to test both the core library code as well as the kernel-to-user-space
interface.

The minimum kernel version required to run the tests can be checked in the
tests/gpiod-test.c source file (it's subject to change if new features are
added to the kernel). The tests work together with the gpio-sim kernel module
which must either be built-in or available for loading using kmod. A helper
library - libgpiosim - is included to enable straightforward interaction with
the module.

To build the testing executable add the '--enable-tests' option when running
the configure script. If enabled, the tests will be installed next to
gpio-tools.

As opposed to standard autotools projects, libgpiod doesn't execute any tests
when invoking 'make check'. Instead the user must run them manually with
superuser privileges.

The testing framework uses the GLib unit testing library so development package
for GLib must be installed.

The gpio-tools programs can be tested separately using the gpio-tools-test.bash
script. It requires shunit2[1] to run and assumes that the tested executables are
in the same directory as the script.

C++, Rust and Python bindings also include their own test-suites. All three
reuse the libgpiosim library to avoid code duplication when interacting with
gpio-sim.

Python test-suite uses the standard unittest package. C++ tests use an external
testing framework - Catch2 - which must be installed in the system. Rust
bindings use the standard tests module layout and the #[test] attribute.

DOCUMENTATION
-------------

All API symbols exposed by the core C API and C++ bindings are documented with
doxygen markup blocks. Doxygen documentation can be generated by executing
'make doc' given that the doxygen executable is available in the system.

Python bindings contain help strings that can be accessed with the help
builtin.

Rust bindings use rustdoc.

Man pages for command-line programs are generated automatically if gpio-tools
were selected and help2man is available in the system.

CONTRIBUTING
------------

Contributions are welcome - please send questions, patches and bug reports
to the linux-gpio mailing list[2] by e-mailing to linux-gpio@vger.kernel.org
(add the [libgpiod] prefix to the e-mail subject line).
Note that the mailing list quietly drops HTML formatted e-mail, so be sure
to send plain text[3].

Code submissions should stick to the linux kernel coding style[4] and
follow the kernel patch submission process[5] as applied to the libgpiod
source tree. All shell scripts must pass `shellcheck` tests[9]. All files
must have a license and copyright SPDX headers and the repo is expected to
pass the `reuse lint` check[10].

The mailing list archive[6] contains all the historical mails to the list,
and is the place to check to ensure your e-mail has been received.
Search for "libgpiod" to filter the list down to relevant messages.
Those also provide examples of the expected formatting.
Allow some time for your e-mail to propagate to the list before retrying,
particularly if there are no e-mails in the list more recent than yours.

There is a libgpiod github page[7] available for reporting bugs and general
discussions and although PRs can be submitted and discussed, upstreambound
patches need to go through the mailing list nevertheless while release
tarballs should be fetched from kernel.org[8].

For more information, refer to CONTRIBUTING.md in this repository.

[1]  https://github.com/kward/shunit2
[2]  http://vger.kernel.org/vger-lists.html#linux-gpio
[3]  https://docs.kernel.org/process/email-clients.html
[4]  https://docs.kernel.org/process/coding-style.html
[5]  https://docs.kernel.org/process/submitting-patches.html
[6]  https://lore.kernel.org/linux-gpio/
[7]  https://github.com/brgl/libgpiod
[8]  https://mirrors.edge.kernel.org/pub/software/libs/libgpiod/
[9]  https://www.shellcheck.net/
[10] https://reuse.software/