LED(4)

🌈 LED(4) — the led manual, bestie edition ✨

okay so this is the Kernel Interfaces Manual page for led, and honestly? it's lowkey iconic. it's all about making LEDs and lamps and lil annunciator thingies blink 💡✨

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✨ NAME ✨

led — the API for manipulating LED's, lamps and other annunciators 🎀

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💅 SYNOPSIS 💅

#include <dev/led/led.h>

typedef void led_t(void *priv, int onoff);

struct cdev *
led_create_state(led_t *func, void *priv, char const *name, int state);

struct cdev *
led_create(led_t *func, void *priv, char const *name);

void
led_destroy(struct cdev *);

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🦋 DESCRIPTION 🦋

The led driver is giving generic support energy for handling LEDs, lamps and other annunciators. it's literally the main character of blinky things 👑

So here's the vibe: the hardware driver must supply a function that turns the annunciator on and off, plus the device name of the annunciator relative to /dev/led/. The priv argument gets passed right back to that on/off function, and the hardware driver can use it however it sees fit — no cap, total freedom 🫶

You control the lamp by opening and writing ASCII strings to the /dev/led/bla device.

In the following, we're gonna use this special notation to show the resulting output of the annunciator:

*       The annunciator is on for 1/10th second.
_       The annunciator is off for 1/10th second.

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🌸 Setting state directly 🌸

State can be set directly, and since the change happens immediately, you can flash the annunciator with very short periods and sync it with program events. so slay for that ✨

⚠️ but heads up bestie — there's a non-trivial overhead, so this may not be usable for benchmarking or measuring short intervals. like, it's giving "cute but not a stopwatch" energy 💀

0       Turn the annunciator off immediately.
1       Turn the annunciator on immediately.

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🎀 Flashing 🎀

Flashing can be set with a given period. The pattern continues endlessly — fr fr, it does not stop 🔥

f       _*
f1      _*
f2      __**
f3      ___***
...
f9      _________*********

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👑 The three high-level commands 👑

Three high-level commands are available, and yes they ate:

d%d     Numbers.  Each digit is blinked out at 1/10th second, zero
        as ten pulses.  Between digits a one second pause and
        after the last digit a two second pause after which the
        sequence is repeated.

s%s     String.  This gives full control over the annunciator.
        Letters ‘A’ ... ‘J’ turn the annunciator on for from
        1/10th to one full second.  Letters ‘a’ ... ‘j’ turn the
        annunciator off for 1/10th to one full second.  Letters
        ‘u’ and ‘U’ turn the annunciator off and on respectively
        when the next UTC second starts.  Unless terminated with a
        ‘.’, the sequence is immediately repeated.

m%s     Morse.

              ‘.’     becomes ‘_*’
              ‘-’     becomes ‘_***’
              ‘ ’     becomes ‘__’
              ‘\n’    becomes ‘____’

The sequence is repeated after a one second pause ⭐

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🍓 FILES 🍓

• /dev/led/*

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💜 EXAMPLES 💜

A d12 flashes the lamp, like so:

*__________*_*______________________________

A sAaAbBa flashes:

*_*__**_

And this one is my personal fave 🧚:

/usr/bin/morse -l "Soekris rocks" > /dev/led/error

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🌈 SEE ALSO 🌈

• morse(6)

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🎀 HISTORY 🎀

The led driver first appeared in FreeBSD 5.2. a true OG 💅

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🌸 AUTHORS 🌸

This software was written by Poul-Henning Kamp <phk@FreeBSD.org>.

This manual page was written by Sergey A. Osokin <osa@FreeBSD.org> and Poul-Henning Kamp <phk@FreeBSD.org>.

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okay and that's the whole page bestie, no notes 🙈 this cutie came from Debian, dated April 24, 2007 — go blink something pretty 💖✨🎀