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NAME | DESCRIPTION | IOCTL COMMANDS | FEATURES | BUGS | SEE ALSO | COLOPHON |
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 TV-card.
Other hardware provides a stream of pulse/space durations. Such
drivers 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:
LIRC_MODE2_SPACE.
Value reflects a space duration (microseconds).
LIRC_MODE2_PULSE.
Value reflects a pulse duration (microseconds).
LIRC_MODE2_FREQUENCY.
Value reflects a frequency (Hz); see the
LIRC_SET_MEASURE_CARRIER_MODE ioctl.
LIRC_MODE2_TIMEOUT.
The package reflects a timeout; see the
LIRC_SET_REC_TIMEOUT_REPORTS ioctl.
Reading input with the
LIRC_MODE_LIRCCODE drivers
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.
Sending data
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
implicitly 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
documentation, 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
definition of struct file_operations, leaving us with an unsigned int
for the ioctl command and an unsigned long for the argument. For the
purposes 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
hardware 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:
LIRC_GET_FEATURES (void)
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.
LIRC_GET_REC_MODE
Return the receive mode, which will be one of:
LIRC_MODE_MODE2 (void)
The driver returns a sequence of pulse/space durations.
LIRC_MODE_LIRCCODE
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.
Optional Commands
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 operation failed.
LIRC_SET_REC_MODE (int)
Set the receive mode. val is either LIRC_MODE_LIRCCODE or
LIRC_MODE_MODE2.
LIRC_GET_LENGTH (void)
Return the length of the returned codes for
LIRC_MODE_LIRCCODE-type drivers, otherwise fail with the error
ENOIOCTLCMD.
LIRC_GET_SEND_MODE (void)
Return the send mode. Currently, only LIRC_MODE_PULSE is
supported.
LIRC_SET_SEND_MODE (int)
Set the send mode. Currently serves no purpose since only
LIRC_MODE_PULSE is supported.
LIRC_GET_SEND_CARRIER (void)
Get the modulation frequency (Hz).
LIRC_SET_SEND_CARRIER (int)
Set the modulation frequency. The argument is the frequency
(Hz).
LIRC_GET_SEND_CARRIER (void)
Get the modulation frequency used when decoding (Hz).
SET_SEND_DUTY_CYCLE (int)
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.
LIRC_SET_REC_TIMEOUT (int)
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.
LIRC_SET_REC_TIMEOUT_REPORTS (int)
Enable (val is 1) or disable (val is 0) timeout packages in
LIRC_MODE_MODE2. By default, timeout reports should be turned
off.
LIRC_SET_REC_CARRIER (int)
Set the receive carrier frequency (Hz).
LIRC_SET_REC_CARRIER_RANGE (int)
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).
LIRC_SET_MEASURE_CARRIER_MODE (int)
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 off.
LIRC_GET_REC_RESOLUTION (void)
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
signal 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)
See LIRC_GET_MIN_FILTER_PULSE.
LIRC_SET_REC_FILTER (int)
Pulses/spaces shorter than this (microseconds) are filtered
out by hardware.
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.
LIRC_SET_TRANSMITTER_MASK
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 operation returns the number of available
transmitters and does nothing otherwise.
LIRC_SET_WIDEBAND_RECEIVER (int)
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 disable (val equals 0) this functionality. This might
be useful for devices that otherwise have narrow band
receivers that prevent 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
bracketing the actual ioctl calls LIRC_SETUP_START and
LIRC_SETUP_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 parameter changes which are not encapsulated by
LIRC_SETUP_START and LIRC_SETUP_END. Drivers can also choose
to ignore these ioctls.
LIRC_NOTIFY_DECODE (void)
This ioctl is called by lircd(8) whenever a successful
decoding of an incoming IR signal is possible. This can be
used by supporting 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:
LIRC_CAN_REC_RAW
The driver is capable of receiving using LIRC_MODE_RAW.
LIRC_CAN_REC_PULSE
The driver is capable of receiving using LIRC_MODE_PULSE.
LIRC_CAN_REC_MODE2
The driver is capable of receiving using LIRC_MODE_MODE2.
LIRC_CAN_REC_LIRCCODE
The driver is capable of receiving using LIRC_MODE_LIRCCODE.
LIRC_CAN_SET_SEND_CARRIER
The driver supports changing the modulation frequency using
LIRC_SET_SEND_CARRIER.
LIRC_CAN_SET_SEND_DUTY_CYCLE
The driver supports changing the duty cycle using
LIRC_SET_SEND_DUTY_CYCLE.
LIRC_CAN_SET_TRANSMITTER_MASK
The driver supports changing the active transmitter(s) using
LIRC_SET_TRANSMITTER_MASK.
LIRC_CAN_SET_REC_CARRIER
The driver supports setting the receive carrier frequency
using LIRC_SET_REC_CARRIER.
LIRC_CAN_SET_REC_DUTY_CYCLE_RANGE
The driver supports LIRC_SET_REC_DUTY_CYCLE_RANGE.
LIRC_CAN_SET_REC_CARRIER_RANGE
The driver supports LIRC_SET_REC_CARRIER_RANGE.
LIRC_CAN_GET_REC_RESOLUTION
The driver supports LIRC_GET_REC_RESOLUTION.
LIRC_CAN_SET_REC_TIMEOUT
The driver supports LIRC_SET_REC_TIMEOUT.
LIRC_CAN_SET_REC_FILTER
The driver supports LIRC_SET_REC_FILTER.
LIRC_CAN_MEASURE_CARRIER
The driver supports measuring of the modulation frequency
using LIRC_SET_MEASURE_CARRIER_MODE.
LIRC_CAN_USE_WIDEBAND_RECEIVER
The driver supports learning mode using
LIRC_SET_WIDEBAND_RECEIVER.
LIRC_CAN_NOTIFY_DECODE
The driver supports LIRC_NOTIFY_DECODE.
LIRC_CAN_SEND_RAW
The driver supports sending using LIRC_MODE_RAW.
LIRC_CAN_SEND_PULSE
The driver supports sending using LIRC_MODE_PULSE.
LIRC_CAN_SEND_MODE2
The driver supports sending using LIRC_MODE_MODE2.
LIRC_CAN_SEND_LIRCCODE
The driver supports sending. (This is uncommon, since
LIRCCODE drivers reflect hardware like TV-cards which usually
dos not support sending.)
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⟩.
lircd(8)
https://www.kernel.org/doc/htmldocs/media_api/lirc_dev.html
This page is part of release 4.12 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
https://www.kernel.org/doc/man-pages/.
Linux 2016-07-17 LIRC(4)