backend(7) OpenPrinting 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 whenever
authentication information is required. The cupsBackendDeviceURI() func-
tion 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 de-
vice, 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 at-
tribute 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 au-
thentication 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 tempo-
rary 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 tempo-
rary 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
/etc/cups/cups-files.conf
NOTES
CUPS backends are not generally designed 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 en-
vironment variables and file descriptors, and typically run in a user
session that (on macOS) 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.
NOTES
CUPS printer drivers and backends are deprecated and will no longer be
supported in a future feature release of CUPS. Printers that do not
support IPP can be supported using applications such as
ippeveprinter(1).
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 © 2020-2024 by OpenPrinting.
2021-02-28 CUPS backend(7)
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