Ubuntu Manpage: backend - cups backend transmission interfaces

Provided by: cups-daemon_2.2.7-1ubuntu2.10_amd64

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.

COPYRIGHT

       Copyright © 2007-2017 by Apple Inc.