1backend(7)                       OpenPrinting                       backend(7)
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NAME

6       backend - cups backend transmission interfaces
7

SYNOPSIS

9       backend
10       backend job user title num-copies options [ filename ]
11
12       #include <cups/cups.h>
13
14       const char *cupsBackendDeviceURI(char **argv);
15
16       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
23       ssize_t cupsBackChannelWrite(const char *buffer,
24                                    size_t bytes, double timeout);
25
26       int cupsSideChannelRead(cups_sc_command_t *command,
27                               cups_sc_status_t *status, char *data,
28                               int *datalen, double timeout);
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30       int cupsSideChannelWrite(cups_sc_command_t command,
31                                cups_sc_status_t status, const char *data,
32                                int datalen, double timeout);
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DESCRIPTION

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
38       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
42       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.
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48       Back-channel data from the device should be relayed to the job  filters
49       using the cupsBackChannelWrite function.
50
51       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.
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56   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:
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62           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"
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67       The  cupsBackendReport()  function  can be used to generate these lines
68       and handle any necessary escaping of characters in the various strings.
69
70       The device-class field is one of the following values:
71
72       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
76       file The device-uri refers to a file on disk.
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78       network
79            The  device-uri  refers  to a networked device and conforms to the
80            general form for network URIs.
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82       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
87       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
92       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
96       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".
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100       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
103       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
107   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".
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EXIT STATUS

113       The following exit codes are defined for backends:
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115       CUPS_BACKEND_OK
116            The print file was successfully transmitted to the device  or  re‐
117            mote server.
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119       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.
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125       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.
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131       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.
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136       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
141       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.
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146       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.
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151       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
156       All other exit code values are reserved.
157

ENVIRONMENT

159       In  addition  to  the  environment variables listed in cups(1) and fil‐
160       ter(7), CUPS backends can expect the following environment variable:
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162       DEVICE_URI
163            The device URI associated with the printer.
164

FILES

166       /etc/cups/cups-files.conf
167

NOTES

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
179       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

NOTES

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

SEE ALSO

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.
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2012021-02-28                           CUPS                           backend(7)
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