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backend(7)			  Apple Inc.			    backend(7)

NAME
       backend - cups backend transmission interfaces

SYNOPSIS
       backend
       backend job user title num-copies options [ filename ]

       #include <cups/cups.h>

       const char *cupsBackendDeviceURI(char **argv);

       void cupsBackendReport(const char *device_scheme,
			      const char *device_uri,
			      const char *device_make_and_model,
			      const char *device_info,
			      const char *device_id,
			      const char *device_location);

       ssize_t cupsBackChannelWrite(const char *buffer,
				    size_t bytes, double timeout);

       int cupsSideChannelRead(cups_sc_command_t *command,
			       cups_sc_status_t *status, char *data,
			       int *datalen, double timeout);

       int cupsSideChannelWrite(cups_sc_command_t command,
				cups_sc_status_t status, const char *data,
				int datalen, double timeout);

DESCRIPTION
       Backends	 are  a	 special type of filter(7) which is used to send print
       data to and discover different devices on the system.

       Like filters, backends must be capable of reading from  a  filename  on
       the command-line or from the standard input, copying the standard input
       to a temporary file as required by the physical interface.

       The command name (argv[0]) is set to the device URI of the  destination
       printer.	 Authentication	 information in argv[0] is removed, so backend
       developers are urged to use the DEVICE_URI environment  variable	 when‐
       ever authentication information is required. The cupsBackendDeviceURI()
       function may be used to retrieve the correct device URI.

       Back-channel data from the device should be relayed to the job  filters
       using the cupsBackChannelWrite function.

       Backends	 are  responsible  for reading side-channel requests using the
       cupsSideChannelRead() function and responding with the cupsSideChannel‐
       Write()	function.  The CUPS_SC_FD constant defines the file descriptor
       that should be monitored for incoming requests.

   DEVICE DISCOVERY
       When run with no arguments, the backend should  list  the  devices  and
       schemes	it supports or is advertising to the standard output. The out‐
       put consists of zero or more lines consisting of any of	the  following
       forms:

	   device-class scheme "Unknown" "device-info"
	   device-class device-uri "device-make-and-model" "device-info"
	   device-class device-uri "device-make-and-model" "device-info" "device-id"
	   device-class device-uri "device-make-and-model" "device-info" "device-id" "device-location"

       The  cupsBackendReport()	 function  can be used to generate these lines
       and handle any necessary escaping of characters in the various strings.

       The device-class field is one of the following values:

       direct
	    The device-uri refers to a specific direct-access device  with  no
	    options, such as a parallel, USB, or SCSI device.

       file The device-uri refers to a file on disk.

       network
	    The	 device-uri  refers  to a networked device and conforms to the
	    general form for network URIs.

       serial
	    The device-uri refers to a serial device  with  configurable  baud
	    rate  and  other options. If the device-uri contains a baud value,
	    it represents the maximum baud rate supported by the device.

       The scheme field provides the URI scheme that is supported by the back‐
       end.  Backends  should use this form only when the backend supports any
       URI using that scheme. The device-uri field specifies the full  URI  to
       use when communicating with the device.

       The  device-make-and-model  field  specifies  the make and model of the
       device, e.g. "Example Foojet 2000". If the make and model is not known,
       you must report "Unknown".

       The  device-info	 field	specifies  additional  information  about  the
       device. Typically this includes the make and model along with the  port
       number or network address, e.g. "Example Foojet 2000 USB #1".

       The  optional  device-id field specifies the IEEE-1284 device ID string
       for the device, which is used to select a matching driver.

       The optional device-location field specifies the physical  location  of
       the  device,  which  is often used to pre-populate the printer-location
       attribute when adding a printer.

   PERMISSIONS
       Backends without world read and execute permissions are run as the root
       user. Otherwise, the backend is run using an unprivileged user account,
       typically "lp".

EXIT STATUS
       The following exit codes are defined for backends:

       CUPS_BACKEND_OK
	    The print file was	successfully  transmitted  to  the  device  or
	    remote server.

       CUPS_BACKEND_FAILED
	    The	 print	file was not successfully transmitted to the device or
	    remote server. The scheduler will respond to this by canceling the
	    job,  retrying  the	 job,  or  stopping the queue depending on the
	    state of the printer-error-policy attribute.

       CUPS_BACKEND_AUTH_REQUIRED
	    The print file was	not  successfully  transmitted	because	 valid
	    authentication information is required. The scheduler will respond
	    to this by holding the job and adding the 'cups-held-for-authenti‐
	    cation' keyword to the "job-reasons" Job Description attribute.

       CUPS_BACKEND_HOLD
	    The	 print file was not successfully transmitted because it cannot
	    be printed at this time. The scheduler will	 respond  to  this  by
	    holding the job.

       CUPS_BACKEND_STOP
	    The	 print file was not successfully transmitted because it cannot
	    be printed at this time. The scheduler will	 respond  to  this  by
	    stopping the queue.

       CUPS_BACKEND_CANCEL
	    The	 print	file  was  not successfully transmitted because one or
	    more attributes are not supported or the job was canceled  at  the
	    printer. The scheduler will respond to this by canceling the job.

       CUPS_BACKEND_RETRY
	    The	 print file was not successfully transmitted because of a tem‐
	    porary issue. The scheduler will retry the job at a future time  -
	    other jobs may print before this one.

       CUPS_BACKEND_RETRY_CURRENT
	    The	 print file was not successfully transmitted because of a tem‐
	    porary issue. The scheduler will retry the job immediately without
	    allowing intervening jobs.

       All other exit code values are reserved.

ENVIRONMENT
       In  addition  to	 the  environment variables listed in cups(1) and fil‐
       ter(7), CUPS backends can expect the following environment variable:

       DEVICE_URI
	    The device URI associated with the printer.

FILES
       /usr/local/etc/cups/cups-files.conf

NOTES
       CUPS backends are not generally design to be run directly by the	 user.
       Aside  from  the	 device URI issue ( argv[0] and DEVICE_URI environment
       variable contain the device URI), CUPS backends	also  expect  specific
       environment variables and file descriptors, and typically run in a user
       session that (on OS X) has additional restrictions that affect  how  it
       runs.  Backends can also be installed with restricted permissions (0500
       or 0700) that tell the scheduler to run them as the "root" user instead
       of an unprivileged user (typically "lp") on the system.

       Unless  you  are a developer and know what you are doing, please do not
       run backends directly. Instead, use the lp(1)  or  lpr(1)  programs  to
       send  print jobs or lpinfo(8) to query for available printers using the
       backend. The one exception is the SNMP backend - see  cups-snmp(8)  for
       more information.

SEE ALSO
       cups(1),	 cups-files.conf(5), cups-snmp(8), cupsd(8), filter(7), lp(1),
       lpinfo(8), lpr(1),
       CUPS Online Help (http://localhost:631/help)

COPYRIGHT
       Copyright © 2007-2014 by Apple Inc.

4 April 2014			     CUPS			    backend(7)
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