LIRC(4) Linux Programmer's Manual LIRC(4)
lirc - lirc devices
The /dev/lirc* character devices provide a low-level bi-directional
interface to infra-red (IR) remotes. When receiving data, the driver
works in two different modes depending on the underlying hardware.
Some hardware (typically TV-cards) decodes the IR signal internally and
just provides decoded button presses as integer values. Drivers for
this kind of hardware work in LIRC_MODE_LIRCCODE mode. Such hardware
usually does not support sending IR signals. Furthermore, it usually
only works with a specific remote which is bundled with, for example, a
Other hardware provides a stream of pulse/space durations. Such driv-
ers work in LIRC_MODE_MODE2 mode. Sometimes, this kind of hardware
also supports sending IR data. Such hardware can be used with (almost)
any kind of remote.
The LIRC_GET_REC_MODE ioctl (see below) allows probing for the mode.
Reading input with the LIRC_MODE_MODE2 drivers
In the LIRC_MODE_MODE2 mode, the data returned by read(2) provides
32-bit values representing a space or a pulse duration, by convention
typed as lirc_t. The time of the duration (microseconds) is encoded in
the lower 24 bits. The upper 8 bit reflects the type of package:
Value reflects a space duration (microseconds).
Value reflects a pulse duration (microseconds).
Value reflects a frequency (Hz); see the LIRC_SET_MEASURE_CAR-
The package reflects a timeout; see the LIRC_SET_REC_TIME-
Reading input with the
In the LIRC_MODE_LIRCCODE mode, the data returned by read(2) reflects
decoded button presses. The length of each packet can be retrieved
using the LIRC_GET_LENGTH ioctl. Reads must be done in blocks matching
the bit count returned by the LIRC_GET_LENGTH ioctl, rounded up so it
matches full bytes.
When sending data, only the LIRC_MODE_PULSE mode is supported. The
data written to the character device using write(2) is a pulse/space
sequence of integer values. Pulses and spaces are only marked implic-
itly by their position. The data must start and end with a pulse, thus
it must always include an odd number of samples. The write(2) function
blocks until the data has been transmitted by the hardware. If more
data is provided than the hardware can send, the write(2) call fails
with the error EINVAL
The complete list of ioctl commands is maintained in the kernel docu-
mentation, see SEE ALSO. The ioctl commands presented here is a subset
of the kernel documentation.
The LIRC device's ioctl definition is bound by the ioctl function defi-
nition of struct file_operations, leaving us with an unsigned int for
the ioctl command and an unsigned long for the argument. For the pur-
poses of ioctl portability across 32-bit and 64-bit architectures,
these values are capped to their 32-bit sizes.
#include <lirc/include/media/lirc.h> /* But see BUGS */
int ioctl(int fd, int cmd, ...);
The following ioctls can be used to probe or change specific lirc hard-
ware settings. Many require a third argument, usually an int.
referred to below as val.
In general, each driver should have a default set of settings. The
driver implementation is expected to re-apply the default settings when
the device is closed by user space, so that every application opening
the device can rely on working with the default settings initially.
Always Supported Commands
/dev/lirc* devices always support the following commands:
Returns a bit mask of combined features bits; see FEATURES. Some
drivers have dynamic features which are not updated until after an
init() command. If a driver does not announce support of certain
features, calling of the corresponding ioctls is undefined.
Return the receive mode, which will be one of:
The driver returns a sequence of pulse/space durations.
The driver returns integer values, each of which represents
a decoded button press.
If a device returns an error code for LIRC_GET_REC_MODE, it is safe to
assume it is not a lirc device.
Some lirc devices support commands listed below. Unless otherwise
stated, these fail with the error ENOIOCTLCMD or with the error ENOSYS
if the operation isn't supported, or with the error EINVAL if the oper-
Set the receive mode. val is either LIRC_MODE_LIRCCODE or
Return the length of the returned codes for LIRC_MODE_LIRCCODE-
type drivers, otherwise fail with the error ENOIOCTLCMD.
Return the send mode. Currently, only LIRC_MODE_PULSE is sup-
Set the send mode. Currently serves no purpose since only
LIRC_MODE_PULSE is supported.
Get the modulation frequency (Hz).
Set the modulation frequency. The argument is the frequency
Get the modulation frequency used when decoding (Hz).
Set the carrier duty cycle. val is a number in the range
[0,100] which describes the pulse width as a percentage of the
total cycle. Currently, no special meaning is defined for 0 or
100, but the values are reserved for future use.
LIRC_GET_MIN_TIMEOUT (void), LIRC_GET_MAX_TIMEOUT (void)
Some devices have internal timers that can be used to detect
when there's no IR activity for a long time. This can help
lircd(8) in detecting that an IR signal is finished and can
speed up the decoding process. These operations return integer
values with the minimum/maximum timeout that can be set
(microseconds). Some devices have a fixed timeout. For such
drivers, LIRC_GET_MIN_TIMEOUT and LIRC_GET_MAX_TIMEOUT will
return the same value.
Set the integer value for IR inactivity timeout (microseconds).
To be accepted, the value must be within the limits defined by
LIRC_GET_MIN_TIMEOUT and LIRC_GET_MAX_TIMEOUT. A value of 0 (if
supported by the hardware) disables all hardware timeouts and
data should be reported as soon as possible. If the exact value
cannot be set, then the next possible value greater than the
given value should be set.
Enable (val is 1) or disable (val is 0) timeout packages in
LIRC_MODE_MODE2. By default, timeout reports should be turned
Set the receive carrier frequency (Hz).
After opening device, the first use of this operation sets the
lower bound of the carrier range, and the second use sets the
upper bound (Hz).
Enable (val is 1) or disable (val is 0) the measure mode. If
enabled, from the next key press on, the driver will send
LIRC_MODE2_FREQUENCY packets. By default this should be turned
Return the driver resolution (microseconds).
LIRC_GET_MIN_FILTER_PULSE (void), LIRC_GET_MAX_FILTER_PULSE (void)
Some devices are able to filter out spikes in the incoming sig-
nal using given filter rules. These ioctls return the hardware
capabilities that describe the bounds of the possible filters.
Filter settings depend on the IR protocols that are expected.
lircd(8) derives the settings from all protocols definitions
found in its lircd.conf(5) config file.
LIRC_GET_MIN_FILTER_SPACE (void), LIRC_GET_MAX_FILTER_SPACE (void)
Pulses/spaces shorter than this (microseconds) are filtered out
LIRC_SET_REC_FILTER_PULSE (int), LIRC_SET_REC_FILTER_SPACE (int)
Pulses/spaces shorter than this (microseconds) are filtered out
by hardware. If filters cannot be set independently for
pulse/space, the corresponding ioctls must return an error and
LIRC_SET_REC_FILTER should be used instead.
Enable the set of transmitters specified in val, which contains
a bit mask where each enabled transmitter is a 1. The first
transmitter is encoded by the least significant bit, and so on.
When an invalid bit mask is given, for example a bit is set even
though the device does not have so many transmitters, this oper-
ation returns the number of available transmitters and does
Some devices are equipped with a special wide band receiver
which is intended to be used to learn the output of an existing
remote. This ioctl can be used to enable (val equals 1) or dis-
able (val equals 0) this functionality. This might be useful
for devices that otherwise have narrow band receivers that pre-
vent them to be used with certain remotes. Wide band receivers
may also be more precise. On the other hand its disadvantage
usually is reduced range of reception.
Note: wide band receiver may be implicitly enabled if you enable
carrier reports. In that case, it will be disabled as soon as
you disable carrier reports. Trying to disable a wide band
receiver while carrier reports are active will do nothing.
LIRC_SETUP_START (void), LIRC_SETUP_END (void)
Setting of several driver parameters can be optimized by brack-
eting the actual ioctl calls LIRC_SETUP_START and LIRC_SET-
UP_END. When a driver receives a LIRC_SETUP_START ioctl, it can
choose to not commit further setting changes to the hardware
until a LIRC_SETUP_END is received. But this is open to the
driver implementation and every driver must also handle parame-
ter changes which are not encapsulated by LIRC_SETUP_START and
LIRC_SETUP_END. Drivers can also choose to ignore these ioctls.
This ioctl is called by lircd(8) whenever a successful decoding
of an incoming IR signal is possible. This can be used by sup-
porting hardware to give visual user feedback, for example by
flashing an LED.
The features returned by The LIRC_GET_FEATURES ioctl returns a bit mask
describing features of the driver. The following bits may be returned
in the mask:
The driver is capable of receiving using LIRC_MODE_RAW.
The driver is capable of receiving using LIRC_MODE_PULSE.
The driver is capable of receiving using LIRC_MODE_MODE2.
The driver is capable of receiving using LIRC_MODE_LIRCCODE.
The driver supports changing the modulation frequency using
The driver supports changing the duty cycle using
The driver supports changing the active transmitter(s) using
The driver supports setting the receive carrier frequency using
The driver supports LIRC_SET_REC_DUTY_CYCLE_RANGE.
The driver supports LIRC_SET_REC_CARRIER_RANGE.
The driver supports LIRC_GET_REC_RESOLUTION.
The driver supports LIRC_SET_REC_TIMEOUT.
The driver supports LIRC_SET_REC_FILTER.
The driver supports measuring of the modulation frequency using
The driver supports learning mode using LIRC_SET_WIDE-
The driver supports LIRC_NOTIFY_DECODE.
The driver supports sending using LIRC_MODE_RAW.
The driver supports sending using LIRC_MODE_PULSE.
The driver supports sending using LIRC_MODE_MODE2.
The driver supports sending. (This is uncommon, since LIRCCODE
drivers reflect hardware like TV-cards which usually dos not
Using these devices requires the kernel source header file lirc.h.
This file is not available before kernel release 4.6. Users of older
kernels could use the file bundled in <http://www.lirc.org>.
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
Linux 2016-07-17 LIRC(4)
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