-Backends are a special type of <a href='man-filter.html?TOPIC=Man+Pages'>filter(7)</a> which is used to send print data
-to and discover different devices on the system.
-<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.
-<p>The command name (argv[0]) is set to the device URI of the destination printer.
-Starting with CUPS 1.1.22, any authentication information in argv[0] is removed,
-so backend developers are urged to use the DEVICE_URI environment variable
-whenever authentication information is required. The CUPS API includes a
-<i>cupsBackendDeviceURI</i> function for retrieving the correct device URI.
-<p>Back-channel data from the device should be relayed to the job
-filters by writing to file descriptor 3. The CUPS API includes
-the <i>cupsBackChannelWrite</i> function for this purpose.
-<h2 class="title"><a name="WARNING">Warning</a></h2>
-CUPS backends are not generally design to be run directly by the user. Aside
-from the device URI issue (argv[0] and DEVICE_URI 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 OS X) 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.
-<p>Unless you are a developer and know what you are doing, please do not run
-backends directly. Instead, use the <a href='man-lp.html?TOPIC=Man+Pages'>lp(1)</a> or <a href='man-lpr.html?TOPIC=Man+Pages'>lpr(1)</a> programs to send
-a print job or <a href='man-lpinfo.html?TOPIC=Man+Pages'>lpinfo(8)</a> program to query for available printers using the
-backend. The one exception is the SNMP backend - see <i>snmpbackend(8)</i> for
-more information.
-<h2 class="title"><a name="DEVICE_DISCOVERY">Device Discovery</a></h2>
-When run with no arguments, the backend should list the devices and schemes it
-supports or is advertising to stdout. The output consists of zero or more lines
-consisting of any of the following forms:
+Backends are a special type of
+<a href="man-filter.html?TOPIC=Man+Pages"><b>filter</b>(7)</a>
+which is used to send print data to and discover different devices on the system.
+<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.
+<p>The command name (<i>argv[0]</i>) is set to the device URI of the destination printer. Authentication information in
+<i>argv[0]</i>
+is removed, so backend developers are urged to use the
+<b>DEVICE_URI</b>
+environment variable whenever authentication information is required. The
+<b>cupsBackendDeviceURI</b>()
+function may be used to retrieve the correct device URI.
+<p>Back-channel data from the device should be relayed to the job filters using the <i>cupsBackChannelWrite</i> function.
+<p>Backends are responsible for reading side-channel requests using the
+<b>cupsSideChannelRead</b>()
+function and responding with the
+<b>cupsSideChannelWrite()</b>
+function. The
+<b>CUPS_SC_FD</b>
+constant defines the file descriptor that should be monitored for incoming requests.
+<h3><a name="DEVICE_DISCOVERY">Device Discovery</a></h3>
+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:
+<pre class="man">