doc/help/man-backend.html

   1 <!DOCTYPE HTML>
   2 <html>
   3 <!-- SECTION: Man Pages -->
   4 <head>
   5         <link rel="stylesheet" type="text/css" href="../cups-printable.css">
   6         <title>backend(7)</title>
   7 </head>
   8 <body>
   9 <h1 class="title">backend(7)</h1>
  10 <h2 class="title"><a name="NAME">Name</a></h2>
  11 backend - cups backend transmission interfaces
  12 <h2 class="title"><a name="SYNOPSIS">Synopsis</a></h2>
  13 <b>backend</b>
  14 <br>
  15 <b>backend</b>
  16 <i>job</i>
  17 <i>user</i>
  18 <i>title</i>
  19 <i>num-copies</i>
  20 <i>options</i>
  21 [
  22 <i>filename</i>
  23 ]
  24 <pre class="man">
  25
  26 <b>#include &lt;cups/cups.h></b>
  27
  28 <b>const char *cupsBackendDeviceURI</b>(<b>char **</b><i>argv</i>);
  29
  30 <b>void cupsBackendReport</b>(<b>const char *</b><i>device_scheme</i>,
  31                        <b>const char *</b><i>device_uri</i>,
  32                        <b>const char *</b><i>device_make_and_model</i>,
  33                        <b>const char *</b><i>device_info</i>,
  34                        <b>const char *</b><i>device_id</i>,
  35                        <b>const char *</b><i>device_location</i>);
  36
  37 <b>ssize_t cupsBackChannelWrite</b>(<b>const char *</b><i>buffer</i>,
  38                              <b>size_t </b><i>bytes</i>, <b>double </b><i>timeout</i>);
  39
  40 <b>int cupsSideChannelRead</b>(<b>cups_sc_command_t *</b><i>command</i>,
  41                         <b>cups_sc_status_t *</b><i>status</i>, <b>char *</b><i>data</i>,
  42                         <b>int *</b><i>datalen</i>, <b>double </b><i>timeout</i>);
  43
  44 <b>int cupsSideChannelWrite</b>(<b>cups_sc_command_t </b><i>command</i>,
  45                          <b>cups_sc_status_t </b><i>status</i>, <b>const char *</b><i>data</i>,
  46                          <b>int </b><i>datalen</i>, <b>double </b><i>timeout</i>);
  47 </pre>
  48 <h2 class="title"><a name="DESCRIPTION">Description</a></h2>
  49 Backends are a special type of
  50 <b>filter</b>(7)
  51 which is used to send print data to and discover different devices on the system.
  52 <p>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.
  53 <p>The command name (<i>argv[0]</i>) is set to the device URI of the destination printer.
  54 Authentication information in
  55 <i>argv[0]</i>
  56 is removed, so backend developers are urged to use the
  57 <b>DEVICE_URI</b>
  58 environment variable whenever authentication information is required. The
  59 <b>cupsBackendDeviceURI</b>()
  60 function may be used to retrieve the correct device URI.
  61 <p>Back-channel data from the device should be relayed to the job filters using the <i>cupsBackChannelWrite</i> function.
  62 <p>Backends are responsible for reading side-channel requests using the
  63 <b>cupsSideChannelRead</b>()
  64 function and responding with the
  65 <b>cupsSideChannelWrite</b>()
  66 function. The
  67 <b>CUPS_SC_FD</b>
  68 constant defines the file descriptor that should be monitored for incoming requests.
  69 <h3><a name="DEVICE_DISCOVERY">Device Discovery</a></h3>
  70 When run with no arguments, the backend should list the devices and schemes it supports or is advertising to the standard output.
  71 The output consists of zero or more lines consisting of any of the following forms:
  72 <pre class="man">
  73
  74     device-class scheme "Unknown" "device-info"
  75     device-class device-uri "device-make-and-model" "device-info"
  76     device-class device-uri "device-make-and-model" "device-info" "device-id"
  77     device-class device-uri "device-make-and-model" "device-info" "device-id" "device-location"
  78 </pre>
  79 <p>The
  80 <b>cupsBackendReport</b>()
  81 function can be used to generate these lines and handle any necessary escaping of characters in the various strings.
  82 <p>The
  83 <i>device-class</i>
  84 field is one of the following values:
  85 <dl class="man">
  86 <dt><b>direct</b>
  87 <dd style="margin-left: 5.0em">The device-uri refers to a specific direct-access device with no options, such as a parallel, USB, or SCSI device.
  88 <dt><b>file</b>
  89 <dd style="margin-left: 5.0em">The device-uri refers to a file on disk.
  90 <dt><b>network</b>
  91 <dd style="margin-left: 5.0em">The device-uri refers to a networked device and conforms to the general form for
  92 network URIs.
  93 <dt><b>serial</b>
  94 <dd style="margin-left: 5.0em">The device-uri refers to a serial device with configurable baud rate and other options.
  95 If the device-uri contains a baud value, it represents the maximum baud rate supported by the device.
  96 </dl>
  97 <p>The
  98 <i>scheme</i>
  99 field provides the URI scheme that is supported by the backend.
 100 Backends should use this form only when the backend supports any URI using that scheme.
 101 The
 102 <i>device-uri</i>
 103 field specifies the full URI to use when communicating with the device.
 104 <p>The
 105 <i>device-make-and-model</i>
 106 field specifies the make and model of the device, e.g. "Example Foojet 2000".
 107 If the make and model is not known, you must report "Unknown".
 108 <p>The
 109 <i>device-info</i>
 110 field specifies additional information about the device.
 111 Typically this includes the make and model along with the port number or network address, e.g. "Example Foojet 2000 USB #1".
 112 <p>The optional
 113 <i>device-id</i>
 114 field specifies the IEEE-1284 device ID string for the device, which is used to select a matching driver.
 115 <p>The optional
 116 <i>device-location</i>
 117 field specifies the physical location of the device, which is often used to pre-populate the printer-location attribute when adding a printer.
 118 <h3><a name="PERMISSIONS">Permissions</a></h3>
 119 Backends without world read and execute permissions are run as the root user.
 120 Otherwise, the backend is run using an unprivileged user account, typically "lp".
 121 <h2 class="title"><a name="EXIT_STATUS">Exit Status</a></h2>
 122 The following exit codes are defined for backends:
 123 <dl class="man">
 124 <dt><b>CUPS_BACKEND_OK</b>
 125 <dd style="margin-left: 5.0em">The print file was successfully transmitted to the device or remote server.
 126 <dt><b>CUPS_BACKEND_FAILED</b>
 127 <dd style="margin-left: 5.0em"><br>
 128 The print file was not successfully transmitted to the device or remote server.
 129 The scheduler will respond to this by canceling the job, retrying the job, or stopping the queue depending on the state of the
 130 <i>printer-error-policy</i>
 131 attribute.
 132 <dt><b>CUPS_BACKEND_AUTH_REQUIRED</b>
 133 <dd style="margin-left: 5.0em">The print file was not successfully transmitted because valid authentication information is required.
 134 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.
 135 <dt><b>CUPS_BACKEND_HOLD</b>
 136 <dd style="margin-left: 5.0em">The print file was not successfully transmitted because it cannot be printed at this time.
 137 The scheduler will respond to this by holding the job.
 138 <dt><b>CUPS_BACKEND_STOP</b>
 139 <dd style="margin-left: 5.0em">The print file was not successfully transmitted because it cannot be printed at this time.
 140 The scheduler will respond to this by stopping the queue.
 141 <dt><b>CUPS_BACKEND_CANCEL</b>
 142 <dd style="margin-left: 5.0em">The print file was not successfully transmitted because one or more attributes are not supported or the job was canceled at the printer.
 143 The scheduler will respond to this by canceling the job.
 144 <dt><b>CUPS_BACKEND_RETRY</b>
 145 <dd style="margin-left: 5.0em">The print file was not successfully transmitted because of a temporary issue.
 146 The scheduler will retry the job at a future time - other jobs may print before this one.
 147 <dt><b>CUPS_BACKEND_RETRY_CURRENT</b>
 148 <dd style="margin-left: 5.0em">The print file was not successfully transmitted because of a temporary issue.
 149 The scheduler will retry the job immediately without allowing intervening jobs.
 150 </dl>
 151 <p>All other exit code values are reserved.
 152 <h2 class="title"><a name="ENVIRONMENT">Environment</a></h2>
 153 In addition to the environment variables listed in
 154 <b>cups</b>(1)
 155 and
 156 <b>filter</b>(7),
 157 CUPS backends can expect the following environment variable:
 158 <dl class="man">
 159 <dt><b>DEVICE_URI</b>
 160 <dd style="margin-left: 5.0em">The device URI associated with the printer.
 161 </dl>
 162 <h2 class="title"><a name="FILES">Files</a></h2>
 163 <i>/etc/cups/cups-files.conf</i>
 164 <h2 class="title"><a name="NOTES">Notes</a></h2>
 165 CUPS backends are not generally designed to be run directly by the user.
 166 Aside from the device URI issue (
 167 <i>argv[0]</i>
 168 and
 169 <b>DEVICE_URI</b>
 170 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.
 171 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.
 172 <p>Unless you are a developer and know what you are doing, please do not run backends directly.
 173 Instead, use the
 174 <b>lp</b>(1)
 175 or
 176 <b>lpr</b>(1)
 177 programs to send print jobs or
 178 <b>lpinfo</b>(8)
 179 to query for available printers using the backend.
 180 The one exception is the SNMP backend - see
 181 <b>cups-snmp</b>(8)
 182 for more information.
 183 <h2 class="title"><a name="NOTES">Notes</a></h2>
 184 CUPS printer drivers and backends are deprecated and will no longer be supported in a future feature release of CUPS.
 185 Printers that do not support IPP can be supported using applications such as
 186 <b>ippeveprinter</b>(1).
 187 <h2 class="title"><a name="SEE_ALSO">See Also</a></h2>
 188 <i>cups</i>(1),
 189 <i>cups-files.conf</i>(5),
 190 <i>cups-snmp</i>(8),
 191 <i>cupsd</i>(8),
 192 <i>filter</i>(7),
 193 <i>lp</i>(1),
 194 <i>lpinfo</i>(8),
 195 <i>lpr</i>(1),
 196 <br>
 197 CUPS Online Help (<a href="http://localhost:631/help">http://localhost:631/help</a>)
 198 <h2 class="title"><a name="COPYRIGHT">Copyright</a></h2>
 199 Copyright &copy; 2007-2019 by Apple Inc.
 200
 201 </body>
 202 </html>