1backend(7) OpenPrinting backend(7)
2
6 backend - cups backend transmission interfaces
7
9 backend
10 backend job user title num-copies options [ filename ]
11 #include <cups/cups.h>
13 const char *cupsBackendDeviceURI(char **argv);
15 void cupsBackendReport(const char *device_scheme,
17 const char *device_uri,
18 const char *device_make_and_model,
19 const char *device_info,
20 const char *device_id,
21 const char *device_location);
22 ssize_t cupsBackChannelWrite(const char *buffer,
24 size_t bytes, double timeout);
25 int cupsSideChannelRead(cups_sc_command_t *command,
27 cups_sc_status_t *status, char *data,
28 int *datalen, double timeout);
29 int cupsSideChannelWrite(cups_sc_command_t command,
31 cups_sc_status_t status, const char *data,
32 int datalen, double timeout);
33
35 Backends are a special type of filter(7) which is used to send print
36 data to and discover different devices on the system.
37 Like filters, backends must be capable of reading from a filename on
39 the command-line or from the standard input, copying the standard input
40 to a temporary file as required by the physical interface.
41 The command name (argv[0]) is set to the device URI of the destination
43 printer. Authentication information in argv[0] is removed, so backend
44 developers are urged to use the DEVICE_URI environment variable when‐
45 ever authentication information is required. The cupsBackendDeviceURI()
46 function may be used to retrieve the correct device URI.
47 Back-channel data from the device should be relayed to the job filters
49 using the cupsBackChannelWrite function.
50 Backends are responsible for reading side-channel requests using the
52 cupsSideChannelRead() function and responding with the cupsSideChannel‐
53 Write() function. The CUPS_SC_FD constant defines the file descriptor
54 that should be monitored for incoming requests.
55 DEVICE DISCOVERY
57 When run with no arguments, the backend should list the devices and
58 schemes it supports or is advertising to the standard output. The out‐
59 put consists of zero or more lines consisting of any of the following
60 forms:
61 device-class scheme "Unknown" "device-info"
63 device-class device-uri "device-make-and-model" "device-info"
64 device-class device-uri "device-make-and-model" "device-info" "device-id"
65 device-class device-uri "device-make-and-model" "device-info" "device-id" "device-location"
66 The cupsBackendReport() function can be used to generate these lines
68 and handle any necessary escaping of characters in the various strings.
69 The device-class field is one of the following values:
71 direct
73 The device-uri refers to a specific direct-access device with no
74 options, such as a parallel, USB, or SCSI device.
75 file The device-uri refers to a file on disk.
77 network
79 The device-uri refers to a networked device and conforms to the
80 general form for network URIs.
81 serial
83 The device-uri refers to a serial device with configurable baud
84 rate and other options. If the device-uri contains a baud value,
85 it represents the maximum baud rate supported by the device.
86 The scheme field provides the URI scheme that is supported by the back‐
88 end. Backends should use this form only when the backend supports any
89 URI using that scheme. The device-uri field specifies the full URI to
90 use when communicating with the device.
91 The device-make-and-model field specifies the make and model of the de‐
93 vice, e.g. "Example Foojet 2000". If the make and model is not known,
94 you must report "Unknown".
95 The device-info field specifies additional information about the de‐
97 vice. Typically this includes the make and model along with the port
98 number or network address, e.g. "Example Foojet 2000 USB #1".
99 The optional device-id field specifies the IEEE-1284 device ID string
101 for the device, which is used to select a matching driver.
102 The optional device-location field specifies the physical location of
104 the device, which is often used to pre-populate the printer-location
105 attribute when adding a printer.
106 PERMISSIONS
108 Backends without world read and execute permissions are run as the root
109 user. Otherwise, the backend is run using an unprivileged user ac‐
110 count, typically "lp".
111
113 The following exit codes are defined for backends:
114 CUPS_BACKEND_OK
116 The print file was successfully transmitted to the device or re‐
117 mote server.
118 CUPS_BACKEND_FAILED
120 The print file was not successfully transmitted to the device or
121 remote server. The scheduler will respond to this by canceling
122 the job, retrying the job, or stopping the queue depending on the
123 state of the printer-error-policy attribute.
124 CUPS_BACKEND_AUTH_REQUIRED
126 The print file was not successfully transmitted because valid au‐
127 thentication information is required. The scheduler will respond
128 to this by holding the job and adding the 'cups-held-for-authenti‐
129 cation' keyword to the "job-reasons" Job Description attribute.
130 CUPS_BACKEND_HOLD
132 The print file was not successfully transmitted because it cannot
133 be printed at this time. The scheduler will respond to this by
134 holding the job.
135 CUPS_BACKEND_STOP
137 The print file was not successfully transmitted because it cannot
138 be printed at this time. The scheduler will respond to this by
139 stopping the queue.
140 CUPS_BACKEND_CANCEL
142 The print file was not successfully transmitted because one or
143 more attributes are not supported or the job was canceled at the
144 printer. The scheduler will respond to this by canceling the job.
145 CUPS_BACKEND_RETRY
147 The print file was not successfully transmitted because of a tem‐
148 porary issue. The scheduler will retry the job at a future time -
149 other jobs may print before this one.
150 CUPS_BACKEND_RETRY_CURRENT
152 The print file was not successfully transmitted because of a tem‐
153 porary issue. The scheduler will retry the job immediately with‐
154 out allowing intervening jobs.
155 All other exit code values are reserved.
157
159 In addition to the environment variables listed in cups(1) and fil‐
160 ter(7), CUPS backends can expect the following environment variable:
161 DEVICE_URI
163 The device URI associated with the printer.
164
166 /etc/cups/cups-files.conf
167
169 CUPS backends are not generally designed to be run directly by the
170 user. Aside from the device URI issue ( argv[0] and DEVICE_URI envi‐
171 ronment variable contain the device URI), CUPS backends also expect
172 specific environment variables and file descriptors, and typically run
173 in a user session that (on macOS) has additional restrictions that af‐
174 fect how it runs. Backends can also be installed with restricted per‐
175 missions (0500 or 0700) that tell the scheduler to run them as the
176 "root" user instead of an unprivileged user (typically "lp") on the
177 system.
178 Unless you are a developer and know what you are doing, please do not
180 run backends directly. Instead, use the lp(1) or lpr(1) programs to
181 send print jobs or lpinfo(8) to query for available printers using the
182 backend. The one exception is the SNMP backend - see cups-snmp(8) for
183 more information.
184
186 CUPS printer drivers and backends are deprecated and will no longer be
187 supported in a future feature release of CUPS. Printers that do not
188 support IPP can be supported using applications such as
189 ippeveprinter(1).
190
192 cups(1), cups-files.conf(5), cups-snmp(8), cupsd(8), filter(7), lp(1),
193 lpinfo(8), lpr(1),
194 CUPS Online Help (http://localhost:631/help)
195
197 Copyright © 2021-2022 by OpenPrinting.
1982021-02-28 CUPS backend(7)