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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 bidirectional
interface to infra-red (IR) remotes. Most of these devices can
receive, and some can send. When receiving or sending data, the
driver works in two different modes depending on the underlying
hardware.
Some hardware (typically TV-cards) decodes the IR signal internally
and provides decoded button presses as scancode values. Drivers for
this kind of hardware work in LIRC_MODE_SCANCODE mode. Such hardware
usually does not support sending IR signals. Furthermore, such
hardware can only decode a limited set of IR protocols, usually only
the protocol of the 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. This type of hardware can also be
used in LIRC_MODE_SCANCODE mode, in which case the kernel IR decoders
will decode the IR. These decoders can be written in extended BPF
(see bpf(2)) and attached to the lirc device.
The LIRC_GET_FEATURES ioctl (see below) allows probing for whether
receiving and sending is supported, and in which modes, amongst other
features.
Reading input with the LIRC_MODE_MODE2 mode
In the LIRC_MODE_MODE2 mode, the data returned by read(2) provides
32-bit values representing a space or a pulse duration. The time of
the duration (microseconds) is encoded in the lower 24 bits. The
upper 8 bits indicate 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
Value reflects a space duration (microseconds). The package
reflects a timeout; see the LIRC_SET_REC_TIMEOUT_REPORTS ioctl.
Reading input with the LIRC_MODE_SCANCODE mode
In the LIRC_MODE_SCANCODE mode, the data returned by read(2) reflects
decoded button presses, in the struct lirc_scancode. The scancode is
stored in the scancode field, and the IR protocol is stored in
rc_proto. This field has one the values of the enum rc_proto.
Writing output with the LIRC_MODE_PULSE mode
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.
Writing output with the LIRC_MODE_SCANCODE mode
The data written to the character devices must be a single struct
lirc_scancode. The scancode and rc_proto fields must filled in, all
other fields must be 0. The kernel IR encoders will convert the
scancode to pulses and spaces. The protocol or scancode is invalid,
or the lirc device cannot transmit.
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 <linux/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.
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.
If a device returns an error code for LIRC_GET_FEATURES, it is safe
to assume it is not a lirc device.
Optional Commands
Some lirc devices support the commands listed below. Unless
otherwise stated, these fail with the error ENOTTY if the operation
isn't supported, or with the error EINVAL if the operation failed, or
invalid arguments were provided. If a driver does not announce
support of certain features, invoking the corresponding ioctls will
fail with the error ENOTTY.
LIRC_GET_REC_MODE (void)
If the lirc device has no receiver, this operation fails with
the error ENOTTY. Otherwise, it returns the receive mode,
which will be one of:
LIRC_MODE_MODE2
The driver returns a sequence of pulse/space durations.
LIRC_MODE_SCANCODE
The driver returns struct lirc_scancode values, each of
which represents a decoded button press.
LIRC_SET_REC_MODE (int)
Set the receive mode. val is either LIRC_MODE_SCANCODE or
LIRC_MODE_MODE2. If the lirc device has no receiver, this
operation fails with the error ENOTTY.
LIRC_GET_SEND_MODE (void)
Return the send mode. LIRC_MODE_PULSE or LIRC_MODE_SCANCODE
is supported. If the lirc device cannot send, this operation
fails with the error ENOTTY.
LIRC_SET_SEND_MODE (int)
Set the send mode. val is either LIRC_MODE_SCANCODE or
LIRC_MODE_PULSE. If the lirc device cannot send, this
operation fails with the error ENOTTY.
LIRC_SET_SEND_CARRIER (int)
Set the modulation frequency. The argument is the frequency
(Hz).
LIRC_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 has been 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 fail with the error ENOTTY.
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_GET_REC_TIMEOUT (void)
Return the current inactivity timeout (microseconds).
Available since Linux 4.18.
LIRC_SET_REC_TIMEOUT_REPORTS (int)
Enable (val is 1) or disable (val is 0) timeout packages in
LIRC_MODE_MODE2. The behavior of this operation has varied
across kernel versions:
* Since Linux 4.16: each time the lirc device is opened,
timeout reports are by default enabled for the resulting
file descriptor. The LIRC_SET_REC_TIMEOUT operation can be
used to disable (and, if desired, to later re-enable) the
timeout on the file descriptor.
* In Linux 4.15 and earlier: timeout reports are disabled by
default, and enabling them (via LIRC_SET_REC_TIMEOUT) on
any file descriptor associated with the lirc device has the
effect of enabling timeouts for all file descriptors
referring to that device (until timeouts are disabled
again).
LIRC_SET_REC_CARRIER (int)
Set the upper bound of the receive carrier frequency (Hz).
See LIRC_SET_REC_CARRIER_RANGE.
LIRC_SET_REC_CARRIER_RANGE (int)
Sets the lower bound of the receive carrier frequency (Hz).
For this to take affect, first set the lower bound using the
LIRC_SET_REC_CARRIER_RANGE ioctl, and then the upper bound
using the LIRC_SET_REC_CARRIER ioctl.
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_SET_TRANSMITTER_MASK (int)
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, their 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.
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_MODE2
The driver is capable of receiving using LIRC_MODE_MODE2.
LIRC_CAN_REC_SCANCODE
The driver is capable of receiving using LIRC_MODE_SCANCODE.
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. Any lirc device since the drivers
were merged in kernel release 2.6.36 must have
LIRC_CAN_SET_REC_CARRIER_RANGE set if LIRC_CAN_SET_REC_CARRIER
feature is set.
LIRC_CAN_SET_REC_CARRIER_RANGE
The driver supports LIRC_SET_REC_CARRIER_RANGE. The lower
bound of the carrier must first be set using the
LIRC_SET_REC_CARRIER_RANGE ioctl, before using the
LIRC_SET_REC_CARRIER ioctl to set the upper bound.
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_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_SEND_PULSE
The driver supports sending using LIRC_MODE_PULSE or
LIRC_MODE_SCANCODE
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⟩.
ir-ctl(1), lircd(8), bpf(2)
https://www.kernel.org/doc/html/latest/media/uapi/rc/lirc-dev.html
This page is part of release 5.08 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 2019-03-06 LIRC(4)
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