backend backend
job user title num-copies options [
filename
]
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. Starting with CUPS 1.1.22, any
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
CUPS API includes a cupsBackendDeviceURI function for
retrieving the correct device URI.
Back-channel data from the device should be relayed to the job
filters by writing to file descriptor 3. The CUPS API includes
the cupsBackChannelWrite function for this purpose.
DEVICE DISCOVERY
When run with no arguments, the backend should list the devices
and schemes it supports or is advertising to stdout. The output
consists of zero or more lines consisting of any of the following
forms:
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. "Acme 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. "Acme 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.
PERMISSIONS
Backends without world execute permissions are run as the root
user. Otherwise, the backend is run using the unprivileged user
account, typically "lp".
EXIT CODES
The following exit codes are defined for backends; C API
constants defined in the <cups/backend.h> header file are defined
in parenthesis:
0 (CUPS_BACKEND_OK)
The print file was successfully transmitted to the device or
remote server.
1 (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 error-policy attribute.
2 (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
authentication-required job-reasons keyword.
3 (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.
4 (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.
5 (CUPS_BACKEND_CANCEL)
The print file was not successfully transmitted because one or
more attributes are not supported. The scheduler will respond to
this by canceling the job.
