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man pages section 7: Device and Network Interfaces

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Updated: July 2017



prnio - generic printer interface


#include <sys/prnio.h>


The prnio generic printer interface defines ioctl commands and data structures for printer device drivers.

prnio defines and provides facilities for five basic phases of the printing process:

  • Identification — Retrieve device information/attributes

  • Setup — Set device attributes

  • Transfer — Transfer data to or from the device

  • Cleanup — Transfer phase conclusion

  • Abort — Transfer phase interruption

During the Identification phase, the application retrieves a set of device capabilities and additional information using the PRNIOC_GET_IFCAP , PRNIOC_GET_STATUS, PRNIOC_GET_TIMEOUTS, PRNIOC_GET_IFINFO and PRNIOC_GET_1284_DEVID commands.

During the Setup phase the application sets some interface attributes and probably resets the printer as described in the PRNIOC_SET_IFCAP, PRNIOC_SET_TIMEOUTS and PRNIOC_RESET sections.

During the Transfer phase, data is transferred in a forward (host to peripheral) or reverse direction (peripheral to host). Transfer is accomplished using write(2) and read(2) system calls. For prnio compliant printer drivers, forward transfer support is mandatory, while reverse transfer support is optional. Applications can also use PRNIOC_GET_STATUS and PRNIOC_GET_1284_STATUS commands during the transfer to monitor the device state.

The Cleanup phase is accomplished by closing the device using close(2). Device drivers supporting prnio may set non-zero error code as appropriate. Applications should explicitly close(2) a device before exiting and check errno value.

The Abort phase is accomplished by interrupting the write(2) and read(2) system calls. The application can perform some additional cleanup during the Abort phase as described in PRNIOC_GET_IFCAP section.



Application can retrieve printer interface capabilities using this command. The ioctl(2) argument is a pointer to uint_t, a bit field representing a set of properties and services provided by a printer driver. Set bit means supported capability. The following values are defined:

  • PRN_BIDI - When this bit is set, the interface operates in a bidirectional mode, instead of forward-only mode.
  • PRN_HOTPLUG - If this bit is set, the interface allows device hot-plugging.
  • PRN_1284_DEVID - If this bit is set, the device is capable of returning 1284 device ID (see PRNIOC_GET_1284_DEVID.)
  • PRN_1284_STATUS - If this bit is set, the device driver can return device status lines (see PRNIOC_GET_1284_STATUS ). Some devices support this ioctl in unidirectional mode only.
  • PRN_TIMEOUTS - If this bit is set the peripheral may stall during the transfer phase and the driver can timeout and return from the write(2) and read(2) returning the number of bytes that have been transferred. If PRN_TIMEOUTS is set, the driver supports this functionality and the timeout values can be retrieved and modified via the PRNIOC_GET_TIMEOUTS and PRNIOC_SET_TIMEOUTS ioctls. Otherwise, applications can implement their own timeouts and abort phase.
  • PRN_STREAMS - This bit impacts the application abort phase behaviour. If the device claimed PRN_STREAMS capability, the application must issue an I_FLUSH ioctl(2) before close(2) to dismiss the untransferred data. Only STREAMS drivers can support this capability.

This ioctl can be used to change interface capabilities. The argument is a pointer to uint_t bit field that is described in detail in the PRNIOC_GET_IFCAP section. Capabilities should be set one at a time; otherwise the command will return EINVAL. The following capabilities can be changed by this ioctl:

  • PRN_BIDI - When this capability is set, the interface operates in a bidirectional mode, instead of forward-only mode. Devices that support only one mode will not return error; applications should use PRNIOC_GET_IFCAP to check if the mode was successfully changed. Because some capabilities may be altered as a side effect of changing other capabilities, this command should be followed by PRNIOC_GET_IFCAP.

This command can be used to retrieve printer interface info string, which is an arbitrary format string usually describing the bus type. The argument is a pointer to struct prn_interface_info as described below.

struct prn_interface_info {
  uint_t	  if_len;   /* length of buffer */
  uint_t	  if_rlen;  /* actual info length */
  char   *if_data;  /* buffer address */

The application allocates a buffer and sets if_data and if_len values to its address and length, respectively. The driver returns the string to this buffer and sets if_len to its length. If if_len is less that if_rlen , the driver must return the first if_len bytes of the string. The application may then repeat the command with a bigger buffer.

Although prnio does not limit the contents of the interface info string, some values are recommended and defined in <sys/prnio.h > by the following macros:

  • PRN_PARALLEL - Centronics or IEEE 1284 compatible devices
  • PRN_SERIAL - EIA-232/EIA-485 serial ports
  • PRN_USB - Universal Serial Bus printers
  • PRN_1394 - IEEE 1394 peripherals
  • Printer interface info string is for information only: no implications should be made from its value.

Some applications may want to reset the printer state during Setup and/or Cleanup phase using PRNIOC_RESET command. Reset semantics are device-specific, and in general, applications using this command should be aware of the printer type.

Each prnio compliant driver is required to accept this request, although performed actions are completely driver-dependent. More information on the PRNIOC_RESET implementation for the particular driver is available in the corresponding man page and printer manual.


This command can be used to retrieve printer device ID as defined by IEEE 1284-1994.The ioctl(2) argument is a pointer to struct prn_1284_device_id as described below.

struct prn_1284_device_id {
   uint_t	  id_len;   /* length of buffer */
   uint_t	  id_rlen;  /* actual ID length */
   char      *id_data;  /* buffer address */

For convenience, the two-byte length field is not considered part of device ID string and is not returned in the user buffer. Instead, id_rlen value shall be set to (length - 2) by the driver, where length is the ID length field value. If buffer length is less than id_rlen, the driver returns the first id_len bytes of the ID.

The printer driver must return the most up-to-date value of the device ID.


This command can be used by applications to retrieve current device status. The argument is a pointer to uint_t, where the status word is returned. Status is a combination of the following bits:

  • PRN_ONLINE - For devices that support PRN_HOTPLUG capability, this bit is set when the device is online, otherwise the device is offline. Devices without PRN_HOTPLUG support should always have this bit set.
  • PRN_READY - This bit indicates if the device is ready to receive/send data. Applications may use this bit for an outbound flow control

Devices that support PRN_1284_STATUS capability accept this ioctl to retrieve the device status lines defined in IEEE 1284 for use in Compatibility mode. The following bits may be set by the driver:

  • PRN_1284_NOFAULT - Device is not in error state
  • PRN_1284_SELECT - Device is selected
  • PRN_1284_PE - Paper error
  • PRN_1284_BUSY - Device is busy

This command retrieves current transfer timeout values for the driver. The argument is a pointer to struct prn_timeouts as described below.

struct prn_timeouts {
  uint_t	tmo_forward;  /* forward transfer timeout */
  uint_t	tmo_reverse;  /* reverse transfer timeout */

tmo_forward and tmo_reverse define forward and reverse transfer timeouts in seconds. This command is only valid for drivers that support PRN_TIMEOUTS capability.


This command sets current transfer timeout values for the driver. The argument is a pointer to struct prn_timeouts. See PRNIOC_GET_TIMEOUTS for description of this structure. This command is only valid for drivers that support PRN_TIMEOUTS capability.


See attributes(5) for descriptions of the following attributes:

Interface Stability

See Also

close(2), ioctl(2), read(2), write(2), attributes(5), ecpp(7D), usbprn(7D)

IEEE Std 1284-1994