doc/help/man-backend.html

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   8     <h1 id="backend-7">backend(7)</h1>
   9     <h2 id="backend-7.name">Name</h2>
  10 <p>backend - cups backend transmission interfaces
  11 </p>
  12     <h2 id="backend-7.synopsis">Synopsis</h2>
  13 <p><strong>backend</strong>
  14 <br>
  15 <strong>backend</strong>
  16 <em>job</em>
  17 <em>user</em>
  18 <em>title</em>
  19 <em>num-copies</em>
  20 <em>options</em>
  21 [
  22 <em>filename</em>
  23 ]
  24 </p>
  25     <pre>
  26 <strong>#include &lt;cups/cups.h></strong>
  27
  28 <strong>const char *cupsBackendDeviceURI</strong>(<strong>char **</strong><em>argv</em>);
  29
  30 <strong>void cupsBackendReport</strong>(<strong>const char *</strong><em>device_scheme</em>,
  31                        <strong>const char *</strong><em>device_uri</em>,
  32                        <strong>const char *</strong><em>device_make_and_model</em>,
  33                        <strong>const char *</strong><em>device_info</em>,
  34                        <strong>const char *</strong><em>device_id</em>,
  35                        <strong>const char *</strong><em>device_location</em>);
  36
  37 <strong>ssize_t cupsBackChannelWrite</strong>(<strong>const char *</strong><em>buffer</em>,
  38                              <strong>size_t </strong><em>bytes</em>, <strong>double </strong><em>timeout</em>);
  39
  40 <strong>int cupsSideChannelRead</strong>(<strong>cups_sc_command_t *</strong><em>command</em>,
  41                         <strong>cups_sc_status_t *</strong><em>status</em>, <strong>char *</strong><em>data</em>,
  42                         <strong>int *</strong><em>datalen</em>, <strong>double </strong><em>timeout</em>);
  43
  44 <strong>int cupsSideChannelWrite</strong>(<strong>cups_sc_command_t </strong><em>command</em>,
  45                          <strong>cups_sc_status_t </strong><em>status</em>, <strong>const char *</strong><em>data</em>,
  46                          <strong>int </strong><em>datalen</em>, <strong>double </strong><em>timeout</em>);
  47 </pre>
  48     <h2 id="backend-7.description">Description</h2>
  49 <p>Backends are a special type of
  50 <a href="filter.html"><strong>filter</strong>(7)</a>
  51
  52 which is used to send print data to and discover different devices on the system.
  53 </p>
  54     <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.
  55 </p>
  56     <p>The command name (<em>argv[0]</em>) is set to the device URI of the destination printer.
  57 Authentication information in
  58 <em>argv[0]</em>
  59 is removed, so backend developers are urged to use the
  60 <strong>DEVICE_URI</strong>
  61 environment variable whenever authentication information is required. The
  62 <strong>cupsBackendDeviceURI</strong>()
  63
  64 function may be used to retrieve the correct device URI.
  65 </p>
  66     <p>Back-channel data from the device should be relayed to the job filters using the <em>cupsBackChannelWrite</em> function.
  67 </p>
  68     <p>Backends are responsible for reading side-channel requests using the
  69 <strong>cupsSideChannelRead</strong>()
  70
  71 function and responding with the
  72 <strong>cupsSideChannelWrite</strong>()
  73
  74 function. The
  75 <strong>CUPS_SC_FD</strong>
  76 constant defines the file descriptor that should be monitored for incoming requests.
  77 </p>
  78     <h3 id="backend-7.description.device-discovery">Device Discovery</h3>
  79 <p>When run with no arguments, the backend should list the devices and schemes it supports or is advertising to the standard output.
  80 The output consists of zero or more lines consisting of any of the following forms:
  81 </p>
  82     <pre>
  83     device-class scheme &quot;Unknown&quot; &quot;device-info&quot;
  84     device-class device-uri &quot;device-make-and-model&quot; &quot;device-info&quot;
  85     device-class device-uri &quot;device-make-and-model&quot; &quot;device-info&quot; &quot;device-id&quot;
  86     device-class device-uri &quot;device-make-and-model&quot; &quot;device-info&quot; &quot;device-id&quot; &quot;device-location&quot;
  87 </pre>
  88     <p>The
  89 <strong>cupsBackendReport</strong>()
  90
  91 function can be used to generate these lines and handle any necessary escaping of characters in the various strings.
  92 </p>
  93     <p>The
  94 <em>device-class</em>
  95 field is one of the following values:
  96 </p>
  97     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>direct</strong><br>
  98 The device-uri refers to a specific direct-access device with no options, such as a parallel, USB, or SCSI device.
  99 </p>
 100     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>file</strong><br>
 101 The device-uri refers to a file on disk.
 102 </p>
 103     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>network</strong><br>
 104 The device-uri refers to a networked device and conforms to the general form for
 105 network URIs.
 106 </p>
 107     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>serial</strong><br>
 108 The device-uri refers to a serial device with configurable baud rate and other options.
 109 If the device-uri contains a baud value, it represents the maximum baud rate supported by the device.
 110 </p>
 111     <p>The
 112 <em>scheme</em>
 113 field provides the URI scheme that is supported by the backend.
 114 Backends should use this form only when the backend supports any URI using that scheme.
 115 The
 116 <em>device-uri</em>
 117 field specifies the full URI to use when communicating with the device.
 118 </p>
 119     <p>The
 120 <em>device-make-and-model</em>
 121 field specifies the make and model of the device, e.g. &quot;Example Foojet 2000&quot;.
 122 If the make and model is not known, you must report &quot;Unknown&quot;.
 123 </p>
 124     <p>The
 125 <em>device-info</em>
 126 field specifies additional information about the device.
 127 Typically this includes the make and model along with the port number or network address, e.g. &quot;Example Foojet 2000 USB #1&quot;.
 128 </p>
 129     <p>The optional
 130 <em>device-id</em>
 131 field specifies the IEEE-1284 device ID string for the device, which is used to select a matching driver.
 132 </p>
 133     <p>The optional
 134 <em>device-location</em>
 135 field specifies the physical location of the device, which is often used to pre-populate the printer-location attribute when adding a printer.
 136 </p>
 137     <h3 id="backend-7.description.permissions">Permissions</h3>
 138 <p>Backends without world read and execute permissions are run as the root user.
 139 Otherwise, the backend is run using an unprivileged user account, typically &quot;lp&quot;.
 140 </p>
 141     <h2 id="backend-7.exit-status">Exit Status</h2>
 142 <p>The following exit codes are defined for backends:
 143 </p>
 144     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>CUPS_BACKEND_OK</strong><br>
 145 The print file was successfully transmitted to the device or remote server.
 146 </p>
 147     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>CUPS_BACKEND_FAILED</strong><br>
 148 <br>
 149 The print file was not successfully transmitted to the device or remote server.
 150 The scheduler will respond to this by canceling the job, retrying the job, or stopping the queue depending on the state of the
 151 <em>printer-error-policy</em>
 152 attribute.
 153 </p>
 154     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>CUPS_BACKEND_AUTH_REQUIRED</strong><br>
 155 The print file was not successfully transmitted because valid authentication information is required.
 156 The scheduler will respond to this by holding the job and adding the 'cups-held-for-authentication' keyword to the &quot;job-reasons&quot; Job Description attribute.
 157 </p>
 158     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>CUPS_BACKEND_HOLD</strong><br>
 159 The print file was not successfully transmitted because it cannot be printed at this time.
 160 The scheduler will respond to this by holding the job.
 161 </p>
 162     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>CUPS_BACKEND_STOP</strong><br>
 163 The print file was not successfully transmitted because it cannot be printed at this time.
 164 The scheduler will respond to this by stopping the queue.
 165 </p>
 166     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>CUPS_BACKEND_CANCEL</strong><br>
 167 The print file was not successfully transmitted because one or more attributes are not supported or the job was canceled at the printer.
 168 The scheduler will respond to this by canceling the job.
 169 </p>
 170     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>CUPS_BACKEND_RETRY</strong><br>
 171 The print file was not successfully transmitted because of a temporary issue.
 172 The scheduler will retry the job at a future time - other jobs may print before this one.
 173 </p>
 174     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>CUPS_BACKEND_RETRY_CURRENT</strong><br>
 175 The print file was not successfully transmitted because of a temporary issue.
 176 The scheduler will retry the job immediately without allowing intervening jobs.
 177 </p>
 178     <p>All other exit code values are reserved.
 179 </p>
 180     <h2 id="backend-7.environment">Environment</h2>
 181 <p>In addition to the environment variables listed in
 182 <a href="cups.html"><strong>cups</strong>(1)</a>
 183
 184 and
 185 <a href="filter.html"><strong>filter</strong>(7),</a>
 186
 187 CUPS backends can expect the following environment variable:
 188 </p>
 189     <p style="margin-left: 2.5em; text-indent: -2.5em;"><strong>DEVICE_URI</strong><br>
 190 The device URI associated with the printer.
 191 </p>
 192     <h2 id="backend-7.files">Files</h2>
 193 <p><em>/etc/cups/cups-files.conf</em>
 194 </p>
 195     <h2 id="backend-7.notes">Notes</h2>
 196 <p>CUPS backends are not generally designed to be run directly by the user.
 197 Aside from the device URI issue (
 198 <em>argv[0]</em>
 199 and
 200 <strong>DEVICE_URI</strong>
 201 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.
 202 Backends can also be installed with restricted permissions (0500 or 0700) that tell the scheduler to run them as the &quot;root&quot; user instead of an unprivileged user (typically &quot;lp&quot;) on the system.
 203 </p>
 204     <p>Unless you are a developer and know what you are doing, please do not run backends directly.
 205 Instead, use the
 206 <a href="lp.html"><strong>lp</strong>(1)</a>
 207
 208 or
 209 <a href="lpr.html"><strong>lpr</strong>(1)</a>
 210
 211 programs to send print jobs or
 212 <a href="lpinfo.html"><strong>lpinfo</strong>(8)</a>
 213
 214 to query for available printers using the backend.
 215 The one exception is the SNMP backend - see
 216 <a href="cups-snmp.html"><strong>cups-snmp</strong>(8)</a>
 217
 218 for more information.
 219 </p>
 220     <h2 id="backend-7.notes">Notes</h2>
 221 <p>CUPS printer drivers and backends are deprecated and will no longer be supported in a future feature release of CUPS.
 222 Printers that do not support IPP can be supported using applications such as
 223 <a href="ippeveprinter.html"><strong>ippeveprinter</strong>(1).</a>
 224
 225 </p>
 226     <h2 id="backend-7.see-also">See Also</h2>
 227 <p><em>cups</em>(1),
 228
 229 <em>cups-files.conf</em>(5),
 230
 231 <em>cups-snmp</em>(8),
 232
 233 <em>cupsd</em>(8),
 234
 235 <em>filter</em>(7),
 236
 237 <em>lp</em>(1),
 238
 239 <em>lpinfo</em>(8),
 240
 241 <em>lpr</em>(1),
 242
 243 <br>
 244 CUPS Online Help (<a href="http://localhost:631/help">http://localhost:631/help</a>)
 245 </p>
 246     <h2 id="backend-7.copyright">Copyright</h2>
 247 <p>Copyright &copy; 2020-2024 by OpenPrinting.
 248   </body>
 249 </html>