man/backend.man

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   2 .\" Backend man page for CUPS.
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  13 .TH backend 7 "CUPS" "4 April 2014" "Apple Inc."
  14 .SH NAME
  15 backend \- cups backend transmission interfaces
  16 .SH SYNOPSIS
  17 .B backend
  18 .br
  19 .B backend
  20 .I job
  21 .I user
  22 .I title
  23 .I num-copies
  24 .I options
  25 [
  26 .I filename
  27 ]
  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
  52 .SH DESCRIPTION
  53 Backends are a special type of
  54 .BR filter (7)
  55 which is used to send print data to and discover different devices on the system.
  56 .LP
  57 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.
  58 .LP
  59 The command name (\fIargv[0]\fR) is set to the device URI of the destination printer. Authentication information in
  60 .I argv[0]
  61 is removed, so backend developers are urged to use the
  62 .B DEVICE_URI
  63 environment variable whenever authentication information is required. The
  64 .BR cupsBackendDeviceURI ()
  65 function may be used to retrieve the correct device URI.
  66 .LP
  67 Back-channel data from the device should be relayed to the job filters using the \fIcupsBackChannelWrite\fR function.
  68 .LP
  69 Backends are responsible for reading side-channel requests using the
  70 .BR cupsSideChannelRead ()
  71 function and responding with the
  72 .BR cupsSideChannelWrite()
  73 function. The
  74 .B CUPS_SC_FD
  75 constant defines the file descriptor that should be monitored for incoming requests.
  76 .SS DEVICE DISCOVERY
  77 When 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:
  78 .nf
  79
  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"
  83     device-class device-uri "device-make-and-model" "device-info" "device-id" "device-location"
  84 .fi
  85 .LP
  86 The
  87 .BR cupsBackendReport ()
  88 function can be used to generate these lines and handle any necessary escaping of characters in the various strings.
  89 .LP
  90 The
  91 .I device-class
  92 field is one of the following values:
  93 .TP 5
  94 .B direct
  95 The device-uri refers to a specific direct-access device with no options, such as a parallel, USB, or SCSI device.
  96 .TP 5
  97 .B file
  98 The device-uri refers to a file on disk.
  99 .TP 5
 100 .B network
 101 The device-uri refers to a networked device and conforms to the general form for
 102 network URIs.
 103 .TP 5
 104 .B serial
 105 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.
 106 .LP
 107 The
 108 .I scheme
 109 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
 110 .I device-uri
 111 field specifies the full URI to use when communicating with the device.
 112 .LP
 113 The
 114 .I device-make-and-model
 115 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".
 116 .LP
 117 The
 118 .I device-info
 119 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".
 120 .LP
 121 The optional
 122 .I device-id
 123 field specifies the IEEE-1284 device ID string for the device, which is used to select a matching driver.
 124 .LP
 125 The optional
 126 .I device-location
 127 field 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
 129 Backends 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
 131 The following exit codes are defined for backends:
 132 .TP 5
 133 .B CUPS_BACKEND_OK
 134 The print file was successfully transmitted to the device or remote server.
 135 .TP 5
 136 .B CUPS_BACKEND_FAILED
 137 .br
 138 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
 139 .I printer-error-policy
 140 attribute.
 141 .TP 5
 142 .B CUPS_BACKEND_AUTH_REQUIRED
 143 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" Job Description attribute.
 144 .TP 5
 145 .B CUPS_BACKEND_HOLD
 146 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.
 147 .TP 5
 148 .B CUPS_BACKEND_STOP
 149 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.
 150 .TP 5
 151 .B CUPS_BACKEND_CANCEL
 152 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.
 153 .TP 5
 154 .B CUPS_BACKEND_RETRY
 155 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.
 156 .TP 5
 157 .B CUPS_BACKEND_RETRY_CURRENT
 158 The print file was not successfully transmitted because of a temporary issue. The scheduler will retry the job immediately without allowing intervening jobs.
 159 .PP
 160 All other exit code values are reserved.
 161 .SH ENVIRONMENT
 162 In addition to the environment variables listed in
 163 .BR cups (1)
 164 and
 165 .BR filter (7),
 166 CUPS backends can expect the following environment variable:
 167 .TP 5
 168 .B DEVICE_URI
 169 The device URI associated with the printer.
 170 .SH FILES
 171 .I /etc/cups/cups-files.conf
 172 .SH NOTES
 173 CUPS backends are not generally design to be run directly by the user. Aside from the device URI issue (
 174 .I argv[0]
 175 and
 176 .B DEVICE_URI
 177 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 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.
 178 .LP
 179 Unless you are a developer and know what you are doing, please do not run backends directly. Instead, use the
 180 .BR lp (1)
 181 or
 182 .BR lpr (1)
 183 programs to send print jobs or
 184 .BR lpinfo (8)
 185 to query for available printers using the backend. The one exception is the SNMP backend - see
 186 .BR cups-snmp (8)
 187 for more information.
 188 .SH SEE ALSO
 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),
 197 .br
 198 CUPS Online Help (http://localhost:631/help)
 199 .SH COPYRIGHT
 200 Copyright \[co] 2007-2016 by Apple Inc.