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.