Ubuntu Manpage: lirc - lirc devices

NAME

       lirc - lirc devices

DESCRIPTION

       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.

IOCTL COMMANDS

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.

FEATURES

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

BUGS

       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⟩.

COLOPHON