backend(7)                   (CUPS)                    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

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          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 con-
               forms to the general form for network URIs.

          serial
               The device-uri refers to a serial device with config-
               urable 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

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          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 stop-
               ping 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.

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          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 envi-
          ronment 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 per-
          missions (0500 or 0700) that tell the scheduler to run them
          as the "root" user instead of an unprivileged user (typi-
          cally "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)

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     COPYRIGHT
          Copyright [co] 2007-2019 by Apple Inc.

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