backend(7) Apple Inc. backend(7)NAMEbackend - cups backend transmission interfaces
SYNOPSISbackend
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 cups‐
BackChannelWrite 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 snmp‐
backend(8) for more information.
DEVICE DISCOVERY
When run with no arguments, the backend should list the devices and
schemes it supports or is advertising 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 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 execute permissions are run as the root user.
Otherwise, the backend is run using an unprivileged user account, typi‐
cally "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-authenti‐
cation" 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 tem‐
porary 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 tem‐
porary issue. The scheduler will retry the job immediately with‐
out allowing intervening jobs.
All other exit code values are reserved.
SEE ALSOcups-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)
