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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 ]

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.  Starting  with  CUPS
       1.1.22,  any authentication information in argv[0] is removed, so backend developers are urged to use
       the DEVICE_URI environment variable whenever authentication information is  required.  The  CUPS  API
       includes a cupsBackendDeviceURI function for retrieving the correct device URI.

       Back-channel  data from the device should be relayed to the job filters by writing to file descriptor
       3. The CUPS API includes the cupsBackChannelWrite function for this purpose.

WARNING
       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 a print job or lpinfo(8)  program  to  query  for  available
       printers  using  the  backend.  The  one  exception is the SNMP backend - see snmpbackend(8) for more
       information.

DEVICE DISCOVERY
       When run with no arguments, the backend should list the devices and schemes it supports or is  adver-tising advertising
       tising to stdout. The output 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 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 backend.  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 execute permissions are run as the root user. Otherwise, the  backend  is  run
       using an unprivileged user account, typically "lp".

EXIT CODES
       The  following  exit  codes are defined for backends; C API constants defined in the <cups/backend.h>
       header file are defined in parenthesis:

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

       1 (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 error-policy attribute.

       2 (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-authentication" "cups-held-forauthentication"
            authentication" keyword to the "job-reasons" attribute.

       3 (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.

       4 (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.

       5 (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.

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

       7 (CUPS_BACKEND_RETRY_CURRENT)
            The  print  file  was  not successfully transmitted because of a temporary issue.  The scheduler
            will retry the job immediately without allowing intervening jobs.

       All other exit code values are reserved.

SEE ALSO
       cups-snmp(8), cupsd(8), cupsd.conf(5), filter(7), lpinfo(8),
       http://localhost:631/help

COPYRIGHT
       Copyright 2007-2013 by Apple Inc.



23 April 2012                                       CUPS                                          backend(7)

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