Tribblix: manual page: backend.7

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() 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).

NAME

SYNOPSIS

DESCRIPTION

DEVICE DISCOVERY

PERMISSIONS

EXIT STATUS

ENVIRONMENT

FILES

NOTES

NOTES

SEE ALSO

COPYRIGHT