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ef416fc2 1.\"
bfbd0197 2.\" Backend man page for CUPS.
ef416fc2 3.\"
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4.\" Copyright 2007-2014 by Apple Inc.
5.\" Copyright 1997-2006 by Easy Software Products.
ef416fc2 6.\"
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7.\" These coded instructions, statements, and computer programs are the
8.\" property of Apple Inc. and are protected by Federal copyright
9.\" law. Distribution and use rights are outlined in the file "LICENSE.txt"
10.\" which should have been included with this file. If this file is
11.\" file is missing or damaged, see the license at "http://www.cups.org/".
ef416fc2 12.\"
bfbd0197 13.TH backend 7 "CUPS" "4 April 2014" "Apple Inc."
ef416fc2 14.SH NAME
15backend \- cups backend transmission interfaces
ef416fc2 16.SH SYNOPSIS
17.B backend
18.br
19.B backend
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20.I job
21.I user
22.I title
23.I num-copies
24.I options
25[
ef416fc2 26.I filename
27]
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28.nf
29
30\fB#include <cups/cups.h>\fR
31
32\fBconst char *cupsBackendDeviceURI\fR(\fBchar **\fIargv\fR);
33
34\fBvoid cupsBackendReport\fR(\fBconst char *\fIdevice_scheme\fR,
35 \fBconst char *\fIdevice_uri\fR,
36 \fBconst char *\fIdevice_make_and_model\fR,
37 \fBconst char *\fIdevice_info\fR,
38 \fBconst char *\fIdevice_id\fR,
39 \fBconst char *\fIdevice_location\fR);
40
41\fBssize_t cupsBackChannelWrite\fR(\fBconst char *\fIbuffer\fR,
42 \fBsize_t \fIbytes\fR, \fBdouble \fItimeout\fR);
43
44\fBint cupsSideChannelRead\fR(\fBcups_sc_command_t *\fIcommand\fR,
45 \fBcups_sc_status_t *\fIstatus\fR, \fBchar *\fIdata\fR,
46 \fBint *\fIdatalen\fR, \fBdouble \fItimeout\fR);
47
48\fBint cupsSideChannelWrite\fR(\fBcups_sc_command_t \fIcommand\fR,
49 \fBcups_sc_status_t \fIstatus\fR, \fBconst char *\fIdata\fR,
50 \fBint \fIdatalen\fR, \fBdouble \fItimeout\fR);
51.fi
ef416fc2 52.SH DESCRIPTION
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53Backends are a special type of
54.BR filter (7)
55which is used to send print data to and discover different devices on the system.
ef416fc2 56.LP
bfbd0197 57Like 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.
ef416fc2 58.LP
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59The command name (\fIargv[0]\fR) is set to the device URI of the destination printer. Authentication information in
60.I argv[0]
61is removed, so backend developers are urged to use the
62.B DEVICE_URI
63environment variable whenever authentication information is required. The
64.BR cupsBackendDeviceURI ()
65function may be used to retrieve the correct device URI.
ef416fc2 66.LP
bfbd0197 67Back-channel data from the device should be relayed to the job filters using the \fIcupsBackChannelWrite\fR function.
f3c17241 68.LP
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69Backends are responsible for reading side-channel requests using the
70.BR cupsSideChannelRead ()
71function and responding with the
1515fed2 72.BR cupsSideChannelWrite ()
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73function. The
74.B CUPS_SC_FD
75constant defines the file descriptor that should be monitored for incoming requests.
76.SS DEVICE DISCOVERY
77When 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:
ef416fc2 78.nf
bfbd0197 79
bd7854cb 80 device-class scheme "Unknown" "device-info"
81 device-class device-uri "device-make-and-model" "device-info"
82 device-class device-uri "device-make-and-model" "device-info" "device-id"
749b1e90 83 device-class device-uri "device-make-and-model" "device-info" "device-id" "device-location"
ef416fc2 84.fi
ef416fc2 85.LP
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86The
87.BR cupsBackendReport ()
88function can be used to generate these lines and handle any necessary escaping of characters in the various strings.
89.LP
90The
91.I device-class
92field is one of the following values:
ef416fc2 93.TP 5
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94.B direct
95The device-uri refers to a specific direct-access device with no options, such as a parallel, USB, or SCSI device.
ef416fc2 96.TP 5
bfbd0197 97.B file
ef416fc2 98The device-uri refers to a file on disk.
ef416fc2 99.TP 5
bfbd0197 100.B network
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101The device-uri refers to a networked device and conforms to the general form for
102network URIs.
ef416fc2 103.TP 5
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104.B serial
105The 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.
ef416fc2 106.LP
fbfee7a8 107The
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108.I scheme
109field 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
110.I device-uri
111field specifies the full URI to use when communicating with the device.
ef416fc2 112.LP
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113The
114.I device-make-and-model
115field 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".
ef416fc2 116.LP
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117The
118.I device-info
119field 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".
ef416fc2 120.LP
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121The optional
122.I device-id
123field specifies the IEEE-1284 device ID string for the device, which is used to select a matching driver.
749b1e90 124.LP
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125The optional
126.I device-location
127field specifies the physical location of the device, which is often used to pre-populate the printer-location attribute when adding a printer.
128.SS PERMISSIONS
129Backends without world read and execute permissions are run as the root user. Otherwise, the backend is run using an unprivileged user account, typically "lp".
130.SH EXIT STATUS
131The following exit codes are defined for backends:
ef416fc2 132.TP 5
bfbd0197 133.B CUPS_BACKEND_OK
f3c17241 134The print file was successfully transmitted to the device or remote server.
ef416fc2 135.TP 5
bfbd0197 136.B CUPS_BACKEND_FAILED
ef416fc2 137.br
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138The 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
139.I printer-error-policy
f3c17241 140attribute.
ef416fc2 141.TP 5
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142.B CUPS_BACKEND_AUTH_REQUIRED
143The 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.
ef416fc2 144.TP 5
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145.B CUPS_BACKEND_HOLD
146The print file was not successfully transmitted because it cannot be printed at this time. The scheduler will respond to this by holding the job.
ef416fc2 147.TP 5
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148.B CUPS_BACKEND_STOP
149The print file was not successfully transmitted because it cannot be printed at this time. The scheduler will respond to this by stopping the queue.
22c9029b 150.TP 5
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151.B CUPS_BACKEND_CANCEL
152The 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.
22c9029b 153.TP 5
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154.B CUPS_BACKEND_RETRY
155The 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.
156.TP 5
157.B CUPS_BACKEND_RETRY_CURRENT
158The print file was not successfully transmitted because of a temporary issue. The scheduler will retry the job immediately without allowing intervening jobs.
ef416fc2 159.PP
160All other exit code values are reserved.
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161.SH ENVIRONMENT
162In addition to the environment variables listed in
163.BR cups (1)
164and
165.BR filter (7),
166CUPS backends can expect the following environment variable:
167.TP 5
168.B DEVICE_URI
169The device URI associated with the printer.
170.SH FILES
171.I /etc/cups/cups-files.conf
172.SH NOTES
d9b21b63 173CUPS backends are not generally designed to be run directly by the user. Aside from the device URI issue (
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174.I argv[0]
175and
176.B DEVICE_URI
8072030b 177environment 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.
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178.LP
179Unless you are a developer and know what you are doing, please do not run backends directly. Instead, use the
180.BR lp (1)
181or
182.BR lpr (1)
183programs to send print jobs or
184.BR lpinfo (8)
185to query for available printers using the backend. The one exception is the SNMP backend - see
186.BR cups-snmp (8)
187for more information.
ef416fc2 188.SH SEE ALSO
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189.IR cups (1),
190.IR cups-files.conf (5),
191.IR cups-snmp (8),
192.IR cupsd (8),
193.IR filter (7),
194.IR lp (1),
195.IR lpinfo (8),
196.IR lpr (1),
ef416fc2 197.br
bfbd0197 198CUPS Online Help (http://localhost:631/help)
ef416fc2 199.SH COPYRIGHT
bd5a2f28 200Copyright \[co] 2007-2017 by Apple Inc.