5.1. Introduction

LIRC stands for Linux Infrared Remote Control. The LIRC device interface is a bi-directional interface for transporting raw IR and decoded scancodes data between userspace and kernelspace. Fundamentally, it is just a chardev (/dev/lircX, for X = 0, 1, 2, ...), with a number of standard struct file_operations defined on it. With respect to transporting raw IR and decoded scancodes to and fro, the essential fops are read, write and ioctl.

Example dmesg output upon a driver registering w/LIRC:

$ dmesg |grep lirc_dev
rc rc0: lirc_dev: driver mceusb registered at minor = 0, raw IR receiver, raw IR transmitter

What you should see for a chardev:

$ ls -l /dev/lirc*
crw-rw---- 1 root root 248, 0 Jul 2 22:20 /dev/lirc0

5.2. LIRC modes

LIRC supports some modes of receiving and sending IR codes, as shown on the following table.


This mode is for both sending and receiving IR.

For transmitting (aka sending), create a struct lirc_scancode with the desired scancode set in the scancode member, rc_proto set the IR protocol, and all other members set to 0. Write this struct to the lirc device.

For receiving, you read struct lirc_scancode from the lirc device, with scancode set to the received scancode and the IR protocol rc_proto. If the scancode maps to a valid key code, this is set in the keycode field, else it is set to KEY_RESERVED.

The flags can have LIRC_SCANCODE_FLAG_TOGGLE set if the toggle bit is set in protocols that support it (e.g. rc-5 and rc-6), or LIRC_SCANCODE_FLAG_REPEAT for when a repeat is received for protocols that support it (e.g. nec).

In the Sanyo and NEC protocol, if you hold a button on remote, rather than repeating the entire scancode, the remote sends a shorter message with no scancode, which just means button is held, a “repeat”. When this is received, the LIRC_SCANCODE_FLAG_REPEAT is set and the scancode and keycode is repeated.

With nec, there is no way to distinguish “button hold” from “repeatedly pressing the same button”. The rc-5 and rc-6 protocols have a toggle bit. When a button is released and pressed again, the toggle bit is inverted. If the toggle bit is set, the LIRC_SCANCODE_FLAG_TOGGLE is set.

The timestamp field is filled with the time nanoseconds (in CLOCK_MONOTONIC) when the scancode was decoded.


The driver returns a sequence of pulse and space codes to userspace, as a series of u32 values.

This mode is used only for IR receive.

The upper 8 bits determine the packet type, and the lower 24 bits the payload. Use LIRC_VALUE() macro to get the payload, and the macro LIRC_MODE2() will give you the type, which is one of:


Signifies the presence of IR in microseconds.


Signifies absence of IR in microseconds.


If measurement of the carrier frequency was enabled with ioctl LIRC_SET_MEASURE_CARRIER_MODE then this packet gives you the carrier frequency in Hertz.


If timeout reports are enabled with ioctl LIRC_SET_REC_TIMEOUT_REPORTS, when the timeout set with ioctl LIRC_GET_REC_TIMEOUT and LIRC_SET_REC_TIMEOUT expires due to no IR being detected, this packet will be sent, with the number of microseconds with no IR.


In pulse mode, a sequence of pulse/space integer values are written to the lirc device using LIRC write().

The values are alternating pulse and space lengths, in microseconds. The first and last entry must be a pulse, so there must be an odd number of entries.

This mode is used only for IR send.

5.3. Remote Controller protocol

An enum rc_proto in the LIRC Header File lists all the supported IR protocols:

enum rc_proto

the Remote Controller protocol


Protocol not known
Protocol known but proprietary
Philips RC5 protocol
Philips RC5x 20 bit protocol
StreamZap variant of RC5
JVC protocol
Sony 12 bit protocol
Sony 15 bit protocol
Sony 20 bit protocol
NEC protocol
Extended NEC protocol
NEC 32 bit protocol
Sanyo protocol
RC6-ish MCE keyboard
RC6-ish MCE mouse
Philips RC6-0-16 protocol
Philips RC6-6A-20 protocol
Philips RC6-6A-24 protocol
Philips RC6-6A-32 protocol
MCE (Philips RC6-6A-32 subtype) protocol
Sharp protocol
XMP protocol
CEC protocol
iMon Pad protocol