backend - Man Page

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.

    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 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 © 2020-2024 by OpenPrinting.

cupsd(8), cupsd-helper(8), cups-snmp(8), filter(7).