lirc(4) — Linux manual page


lirc(4)                 Kernel Interfaces Manual                 lirc(4)

NAME         top

       lirc - lirc devices

DESCRIPTION         top

       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.  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.
       Sometimes, this kind of hardware also supports sending IR data.

       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.  Pulse (also known as flash) indicates a duration of
       infrared light being detected, and space (also known as gap)
       indicates a duration with no infrared.  If the duration of space
       exceeds the inactivity timeout, a special timeout package is
       delivered, which marks the end of a message.  The upper 8 bits
       indicate the type of package:

           Value reflects a space duration (microseconds).

           Value reflects a pulse duration (microseconds).

           Value reflects a frequency (Hz); see the

           Value reflects a space duration (microseconds).  The package
           reflects a timeout; see the LIRC_SET_REC_TIMEOUT_REPORTS

           The IR receiver encountered an overflow, and as a result data
           is missing (since Linux 5.18).

   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

   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         top

       #include <linux/lirc.h>    /* But see BUGS */

       int ioctl(int fd, int cmd, int *val);

       The following ioctl(2) operations are provided by the lirc
       character device to probe or change specific lirc hardware

   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:

                     The driver returns a sequence of pulse/space

                     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.

              Set the modulation frequency.  The argument is the
              frequency (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.

              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.

              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.

              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 5.17: timeout packages are always enabled
                 and this ioctl is a no-op.

              •  Since Linux 4.16: timeout packages are enabled by
                 default.  Each time the lirc device is opened, 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 packages 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).

              Set the upper bound of the receive carrier frequency (Hz).

              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.

              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.

              Return the driver resolution (microseconds).

              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.

              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         top

       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_MODE2.

              The driver is capable of receiving using

              The driver supports changing the modulation frequency
              using LIRC_SET_SEND_CARRIER.

              The driver supports changing the duty cycle using

              The driver supports changing the active transmitter(s)
              using LIRC_SET_TRANSMITTER_MASK.

              The driver supports setting the receive carrier frequency
              using LIRC_SET_REC_CARRIER.  Any lirc device since the
              drivers were merged in Linux 2.6.36 must have
              LIRC_CAN_SET_REC_CARRIER_RANGE set if
              LIRC_CAN_SET_REC_CARRIER feature is set.

              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.

              The driver supports LIRC_GET_REC_RESOLUTION.

              The driver supports LIRC_SET_REC_TIMEOUT.

              The driver supports measuring of the modulation frequency
              using LIRC_SET_MEASURE_CARRIER_MODE.

              The driver supports learning mode using

              The driver supports sending using LIRC_MODE_PULSE or

BUGS         top

       Using these devices requires the kernel source header file
       lirc.h.  This file is not available before Linux 4.6.  Users of
       older kernels could use the file bundled in 

SEE ALSO         top

       ir-ctl(1), lircd(8), bpf(2) man-pages (unreleased)   2024-05-02                        lirc(4)