backend

backend(7)                        Apple Inc.                        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).

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 © 2007-2019 by Apple Inc.



26 April 2019                        CUPS                           backend(7)