backend - cups backend transmission interfaces
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);
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.
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.
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".
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.
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.
/etc/cups/cups-files.conf
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.
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).
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 © 2021-2022 by OpenPrinting.