Provided by: cups-daemon_2.4.11-0ubuntu1_amd64 
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() 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 cupsSideChannelWrite() 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 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 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 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 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-authentication' 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 temporary 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 temporary 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 filter(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 environment 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
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