.\"
.\" "$Id: backend.man 7810 2008-07-29 01:11:15Z mike $"
.\"
.\" Backend man page for CUPS.
.\"
.\" Copyright 2007-2012 by Apple Inc.
.\" Copyright 1997-2006 by Easy Software Products.
.\"
.\" These coded instructions, statements, and computer programs are the
.\" property of Apple Inc. and are protected by Federal copyright
.\" law. Distribution and use rights are outlined in the file "LICENSE.txt"
.\" which should have been included with this file. If this file is
.\" file is missing or damaged, see the license at "http://www.cups.org/".
.\"
.TH backend 7 "CUPS" "23 April 2012" "Apple Inc."
.SH NAME
backend \- cups backend transmission interfaces
.SH SYNOPSIS
.B backend
.br
.B backend
job user title num-copies options [
.I filename
]
.SH DESCRIPTION
Backends are a special type of \fIfilter(7)\fR which is used to send print data
to and discover different devices on the system.
.LP
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.
.LP
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
\fIcupsBackendDeviceURI\fR function for retrieving the correct device URI.
.LP
Back-channel data from the device should be relayed to the job
filters by writing to file descriptor 3. The CUPS API includes
the \fIcupsBackChannelWrite\fR function for this purpose.
.SH WARNING
CUPS backends are not generally design 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 OS X) 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.
.LP
Unless you are a developer and know what you are doing, please do not run
backends directly. Instead, use the \fIlp(1)\fR or \fIlpr(1)\fR programs to send
a print job or \fIlpinfo(8)\fR program to query for available printers using the
backend. The one exception is the SNMP backend - see \fIsnmpbackend(8)\fR for
more information.
.SH 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:
.nf
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"
.fi
.LP
The \fIdevice-class\fR field is one of the following values:
.TP 5
direct
.br
The device-uri refers to a specific direct-access device with no options, such
as a parallel, USB, or SCSI device.
.TP 5
file
.br
The device-uri refers to a file on disk.
.TP 5
network
.br
The device-uri refers to a networked device and conforms to the general form for
network URIs.
.TP 5
serial
.br
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.
.LP
The \fIscheme\fR 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 \fIdevice-uri\fR field specifies the full URI to use when
communicating with the device.
.LP
The \fIdevice-make-and-model\fR 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".
.LP
The \fIdevice-info\fR 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".
.LP
The optional \fIdevice-id\fR field specifies the IEEE-1284 device ID string for
the device, which is used to select a matching driver.
.LP
The optional \fIdevice-location\fR field specifies the physical location of
the device, which is often used to pre-populate the printer-location attribute
when adding a printer.
.SH PERMISSIONS
Backends without world execute permissions are run as the root user. Otherwise,
the backend is run using an unprivileged user account, typically "lp".
.SH 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:
.TP 5
0 (CUPS_BACKEND_OK)
.br
The print file was successfully transmitted to the device or remote server.
.TP 5
1 (CUPS_BACKEND_FAILED)
.br
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.
.TP 5
2 (CUPS_BACKEND_AUTH_REQUIRED)
.br
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"
attribute.
.TP 5
3 (CUPS_BACKEND_HOLD)
.br
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.
.TP 5
4 (CUPS_BACKEND_STOP)
.br
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.
.TP 5
5 (CUPS_BACKEND_CANCEL)
.br
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.
.TP 5
6 (CUPS_BACKEND_RETRY)
.br
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.
.TP 5
7 (CUPS_BACKEND_RETRY_CURRENT)
.br
The print file was not successfully transmitted because of a temporary issue.
The scheduler will retry the job immediately without allowing intervening jobs.
.PP
All other exit code values are reserved.
.SH SEE ALSO
\fIcups-snmp(8)\fR, \fIcupsd(8)\fR, \fIcupsd.conf(5)\fR, \fIfilter(7)\fR,
\fIlpinfo(8)\fR,
.br
http://localhost:631/help
.SH COPYRIGHT
Copyright 2007-2012 by Apple Inc.
.\"
.\" End of "$Id: backend.man 7810 2008-07-29 01:11:15Z mike $".
.\"