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  Filter and backend programming introduction for CUPS.

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<h2 class='title'><a name="OVERVIEW">Overview</a></h2>

<p>Filters (which include printer drivers and port monitors) and backends
are used to convert job files to a printable format and send that data to the
printer itself. All of these programs use a common interface for processing
print jobs and communicating status information to the scheduler. Each is run
with a standard set of command-line arguments:<p>

<dl class="code">

	<dt>argv[1]</dt>
	<dd>The job ID</dd>

	<dt>argv[2]</dt>
	<dd>The user printing the job</dd>

	<dt>argv[3]</dt>
	<dd>The job name/title</dd>

	<dt>argv[4]</dt>
	<dd>The number of copies to print</dd>

	<dt>argv[5]</dt>
	<dd>The options that were provided when the job was submitted</dd>

	<dt>argv[6]</dt>
	<dd>The file to print (first program only)</dd>
</dl>

<p>The scheduler runs one or more of these programs to print any given job. The
first filter reads from the print file and writes to the standard output, while
the remaining filters read from the standard input and write to the standard
output. The backend is the last filter in the chain and writes to the
device.</p>

<p>Filters are always run as a non-privileged user, typically "lp", with no
connection to the user's desktop. Backends are run either as a non-privileged
user or as root if the file permissions do not allow user or group execution.
The <a href="#PERMISSIONS">file permissions</a> section talks about this in
more detail.</p>

<h3><a name="SECURITY">Security Considerations</a></h3>

<p>It is always important to use security programming practices. Filters and
most backends are run as a non-privileged user, so the major security
consideration is resource utilization - filters should not depend on unlimited
amounts of CPU, memory, or disk space, and should protect against conditions
that could lead to excess usage of any resource like infinite loops and
unbounded recursion. In addition, filters must <em>never</em> allow the user to
specify an arbitrary file path to a separator page, template, or other file
used by the filter since that can lead to an unauthorized disclosure of
information. <em>Always</em> treat input as suspect and validate it!</p>

<p>If you are developing a backend that runs as root, make sure to check for
potential buffer overflows, integer under/overflow conditions, and file
accesses since these can lead to privilege escalations. When writing files,
always validate the file path and <em>never</em> allow a user to determine
where to store a file.</p>

<blockquote><b>Note:</b>

<p><em>Never</em> write files to a user's home directory. Aside from the
security implications, CUPS is a network print service and as such the network
user may not be the same as the local user and/or there may not be a local home
directory to write to.</p>

<p>In addition, some operating systems provide additional security mechanisms
that further limit file system access, even for backends running as root. On
macOS, for example, no backend may write to a user's home directory. See the <a href="#SANDBOXING">Sandboxing on macOS</a> section for more information.</p>
</blockquote>

<h3><a name="SIGNALS">Canceled Jobs and Signal Handling</a></h3>

<p>The scheduler sends <code>SIGTERM</code> when a printing job is canceled or
held. Filters, backends, and port monitors <em>must</em> catch
<code>SIGTERM</code> and perform any cleanup necessary to produce a valid output
file or return the printer to a known good state. The recommended behavior is to
end the output on the current page, preferably on the current line or object
being printed.</p>

<p>Filters and backends may also receive <code>SIGPIPE</code> when an upstream or downstream filter/backend exits with a non-zero status. Developers should generally ignore <code>SIGPIPE</code> at the beginning of <code>main()</code> with the following function call:</p>

<pre class="example">
#include &lt;signal.h&gt;

...

int
main(int argc, char *argv[])
{
  signal(SIGPIPE, SIG_IGN);

  ...
}
</pre>

<h3><a name="PERMISSIONS">File Permissions</a></h3>

<p>For security reasons, CUPS will only run filters and backends that are owned
by root and do not have world or group write permissions. The recommended
permissions for filters and backends are 0555 - read and execute but no write.
Backends that must run as root should use permissions of 0500 - read and execute
by root, no access for other users. Write permissions can be enabled for the
root user only.</p>

<p>To avoid a warning message, the directory containing your filter(s) must also
be owned by root and have world and group write disabled - permissions of 0755
or 0555 are strongly encouraged.</p>

<h3><a name="TEMPFILES">Temporary Files</a></h3>

<p>Temporary files should be created in the directory specified by the
"TMPDIR" environment variable. The
<a href="#cupsTempFile2"><code>cupsTempFile2</code></a> function can be
used to safely create temporary files in this directory.</p>

<h3><a name="COPIES">Copy Generation</a></h3>

<p>The <code>argv[4]</code> argument specifies the number of copies to produce
of the input file. In general, you should only generate copies if the
<em>filename</em> argument is supplied. The only exception to this are
filters that produce device-independent PostScript output, since the PostScript
filter <var>pstops</var> is responsible for generating copies of PostScript
files.</p>

<h3><a name="EXITCODES">Exit Codes</a></h3>

<p>Filters must exit with status 0 when they successfully generate print data
or 1 when they encounter an error. Backends can return any of the
<a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p>

<h3><a name="ENVIRONMENT">Environment Variables</a></h3>

<p>The following environment variables are defined by the printing system
when running print filters and backends:</p>

<dl class="code">

	<dt>APPLE_LANGUAGE</dt>
	<dd>The Apple language identifier associated with the job
	(macOS only).</dd>

	<dt>CHARSET</dt>
	<dd>The job character set, typically "utf-8".</dd>

	<dt>CLASS</dt>
	<dd>When a job is submitted to a printer class, contains the name of
	the destination printer class. Otherwise this environment
	variable will not be set.</dd>

	<dt>CONTENT_TYPE</dt>
	<dd>The MIME type associated with the file (e.g.
	application/postscript).</dd>

	<dt>CUPS_CACHEDIR</dt>
	<dd>The directory where cache files can be stored. Cache files can be
	used to retain information between jobs or files in a job.</dd>

	<dt>CUPS_DATADIR</dt>
	<dd>The directory where (read-only) CUPS data files can be found.</dd>

	<dt>CUPS_FILETYPE</dt>
	<dd>The type of file being printed: "job-sheet" for a banner page and
	"document" for a regular print file.</dd>

	<dt>CUPS_SERVERROOT</dt>
	<dd>The root directory of the server.</dd>

	<dt>DEVICE_URI</dt>
	<dd>The device-uri associated with the printer.</dd>

	<dt>FINAL_CONTENT_TYPE</dt>
	<dd>The MIME type associated with the printer (e.g.
	application/vnd.cups-postscript).</dd>

	<dt>LANG</dt>
	<dd>The language locale associated with the job.</dd>

	<dt>PPD</dt>
	<dd>The full pathname of the PostScript Printer Description (PPD)
	file for this printer.</dd>

	<dt>PRINTER</dt>
	<dd>The queue name of the class or printer.</dd>

	<dt>RIP_CACHE</dt>
	<dd>The recommended amount of memory to use for Raster Image
	Processors (RIPs).</dd>

	<dt>TMPDIR</dt>
	<dd>The directory where temporary files should be created.</dd>

</dl>

<h3><a name="MESSAGES">Communicating with the Scheduler</a></h3>

<p>Filters and backends communicate with the scheduler by writing messages
to the standard error file. The scheduler reads messages from all filters in
a job and processes the message based on its prefix. For example, the following
code sets the current printer state message to "Printing page 5":</p>

<pre class="example">
int page = 5;

fprintf(stderr, "INFO: Printing page %d\n", page);
</pre>

<p>Each message is a single line of text starting with one of the following
prefix strings:</p>

<dl class="code">

	<dt>ALERT: message</dt>
	<dd>Sets the printer-state-message attribute and adds the specified
	message to the current error log file using the "alert" log level.</dd>

	<dt>ATTR: attribute=value [attribute=value]</dt>
	<dd>Sets the named printer or job attribute(s). Typically this is used
	to set the <code>marker-colors</code>, <code>marker-high-levels</code>,
	<code>marker-levels</code>, <code>marker-low-levels</code>,
	<code>marker-message</code>, <code>marker-names</code>,
	<code>marker-types</code>, <code>printer-alert</code>, and
	<code>printer-alert-description</code> printer attributes. Standard
	<code>marker-types</code> values are listed in <a href='#TABLE1'>Table
	1</a>. String values need special handling - see <a href="#ATTR_STRINGS">Reporting Attribute String Values</a> below.</dd>

	<dt>CRIT: message</dt>
	<dd>Sets the printer-state-message attribute and adds the specified
	message to the current error log file using the "critical" log
	level.</dd>

	<dt>DEBUG: message</dt>
	<dd>Sets the printer-state-message attribute and adds the specified
	message to the current error log file using the "debug" log level.</dd>

	<dt>DEBUG2: message</dt>
	<dd>Sets the printer-state-message attribute and adds the specified
	message to the current error log file using the "debug2" log level.</dd>

	<dt>EMERG: message</dt>
	<dd>Sets the printer-state-message attribute and adds the specified
	message to the current error log file using the "emergency" log
	level.</dd>

	<dt>ERROR: message</dt>
	<dd>Sets the printer-state-message attribute and adds the specified
	message to the current error log file using the "error" log level.
	Use "ERROR:" messages for non-persistent processing errors.</dd>

	<dt>INFO: message</dt>
	<dd>Sets the printer-state-message attribute. If the current log level
	is set to "debug2", also adds the specified message to the current error
	log file using the "info" log level.</dd>

	<dt>NOTICE: message</dt>
	<dd>Sets the printer-state-message attribute and adds the specified
	message to the current error log file using the "notice" log level.</dd>

	<dt>PAGE: page-number #-copies</dt>
	<dt>PAGE: total #-pages</dt>
	<dd>Adds an entry to the current page log file. The first form adds
	#-copies to the job-media-sheets-completed attribute. The second
	form sets the job-media-sheets-completed attribute to #-pages.</dd>

	<dt>PPD: keyword=value [keyword=value ...]</dt>
	<dd>Changes or adds keywords to the printer's PPD file. Typically
	this is used to update installable options or default media settings
	based on the printer configuration.</dd>

	<dt>STATE: + printer-state-reason [printer-state-reason ...]</dt>
	<dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
	<dd>Sets or clears printer-state-reason keywords for the current queue.
	Typically this is used to indicate persistent media, ink, toner, and
	configuration conditions or errors on a printer.
	<a href='#TABLE2'>Table 2</a> lists some of the standard "printer-state-reasons" keywords from the <a href="http://www.iana.org/assignments/ipp-registrations/ipp-registrations.xhtml#ipp-registrations-4">IANA IPP Registry</a> -
	use vendor-prefixed ("com.example.foo") keywords for custom states. See
	<a href="#MANAGING_STATE">Managing Printer State in a Filter</a> for more
	information.

	<dt>WARNING: message</dt>
	<dd>Sets the printer-state-message attribute and adds the specified
	message to the current error log file using the "warning" log
	level.</dd>

</dl>

<p>Messages without one of these prefixes are treated as if they began with
the "DEBUG:" prefix string.</p>

<div class='table'><table width='80%' summary='Table 1: Standard marker-types Values'>
<caption>Table 1: <a name='TABLE1'>Standard marker-types Values</a></caption>
<thead>
<tr>
	<th>marker-type</th>
	<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
	<td>developer</td>
	<td>Developer unit</td>
</tr>
<tr>
	<td>fuser</td>
	<td>Fuser unit</td>
</tr>
<tr>
	<td>fuser-cleaning-pad</td>
	<td>Fuser cleaning pad</td>
</tr>
<tr>
	<td>fuser-oil</td>
	<td>Fuser oil</td>
</tr>
<tr>
	<td>ink</td>
	<td>Ink supply</td>
</tr>
<tr>
	<td>opc</td>
	<td>Photo conductor</td>
</tr>
<tr>
	<td>solid-wax</td>
	<td>Wax supply</td>
</tr>
<tr>
	<td>staples</td>
	<td>Staple supply</td>
</tr>
<tr>
	<td>toner</td>
	<td>Toner supply</td>
</tr>
<tr>
	<td>transfer-unit</td>
	<td>Transfer unit</td>
</tr>
<tr>
	<td>waste-ink</td>
	<td>Waste ink tank</td>
</tr>
<tr>
	<td>waste-toner</td>
	<td>Waste toner tank</td>
</tr>
<tr>
	<td>waste-wax</td>
	<td>Waste wax tank</td>
</tr>
</tbody>
</table></div>

<br>

<div class='table'><table width='80%' summary='Table 2: Standard State Keywords'>
<caption>Table 2: <a name='TABLE2'>Standard State Keywords</a></caption>
<thead>
<tr>
	<th>Keyword</th>
	<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
	<td>connecting-to-device</td>
	<td>Connecting to printer but not printing yet.</td>
</tr>
<tr>
	<td>cover-open</td>
	<td>The printer's cover is open.</td>
</tr>
<tr>
	<td>input-tray-missing</td>
	<td>The paper tray is missing.</td>
</tr>
<tr>
	<td>marker-supply-empty</td>
	<td>The printer is out of ink.</td>
</tr>
<tr>
	<td>marker-supply-low</td>
	<td>The printer is almost out of ink.</td>
</tr>
<tr>
	<td>marker-waste-almost-full</td>
	<td>The printer's waste bin is almost full.</td>
</tr>
<tr>
	<td>marker-waste-full</td>
	<td>The printer's waste bin is full.</td>
</tr>
<tr>
	<td>media-empty</td>
	<td>The paper tray (any paper tray) is empty.</td>
</tr>
<tr>
	<td>media-jam</td>
	<td>There is a paper jam.</td>
</tr>
<tr>
	<td>media-low</td>
	<td>The paper tray (any paper tray) is almost empty.</td>
</tr>
<tr>
	<td>media-needed</td>
	<td>The paper tray needs to be filled (for a job that is printing).</td>
</tr>
<tr>
	<td>paused</td>
	<td>Stop the printer.</td>
</tr>
<tr>
	<td>timed-out</td>
	<td>Unable to connect to printer.</td>
</tr>
<tr>
	<td>toner-empty</td>
	<td>The printer is out of toner.</td>
</tr>
<tr>
	<td>toner-low</td>
	<td>The printer is low on toner.</td>
</tr>
</tbody>
</table></div>


<h4><a name="ATTR_STRINGS">Reporting Attribute String Values</a></h4>

<p>When reporting string values using "ATTR:" messages, a filter or backend must take special care to appropriately quote those values. The scheduler uses the CUPS option parsing code for attributes, so the general syntax is:</p>

<pre class="example">
name=simple
name=simple,simple,...
name='complex value'
name="complex value"
name='"complex value"','"complex value"',...
</pre>

<p>Simple values are strings that do not contain spaces, quotes, backslashes, or the comma and can be placed verbatim in the "ATTR:" message, for example:</p>

<pre class="example">
int levels[4] = { 40, 50, 60, 70 }; /* CMYK */

fputs("ATTR: marker-colors=#00FFFF,#FF00FF,#FFFF00,#000000\n", stderr);
fputs("ATTR: marker-high-levels=100,100,100,100\n", stderr);
fprintf(stderr, "ATTR: marker-levels=%d,%d,%d,%d\n", levels[0], levels[1],
        levels[2], levels[3], levels[4]);
fputs("ATTR: marker-low-levels=5,5,5,5\n", stderr);
fputs("ATTR: marker-types=toner,toner,toner,toner\n", stderr);
</pre>

<p>Complex values that contains spaces, quotes, backslashes, or the comma must be quoted. For a single value a single set of quotes is sufficient:</p>

<pre class="example">
fputs("ATTR: marker-message='Levels shown are approximate.'\n", stderr);
</pre>

<p>When multiple values are reported, each value must be enclosed by a set of single and double quotes:</p>

<pre class="example">
fputs("ATTR: marker-names='\"Cyan Toner\"','\"Magenta Toner\"',"
      "'\"Yellow Toner\"','\"Black Toner\"'\n", stderr);
</pre>

<p>The IPP backend includes a <var>quote_string</var> function that may be used to properly quote a complex value in an "ATTR:" message:</p>

<pre class="example">
static const char *                     /* O - Quoted string */
quote_string(const char *s,             /* I - String */
             char       *q,             /* I - Quoted string buffer */
             size_t     qsize)          /* I - Size of quoted string buffer */
{
  char  *qptr,                          /* Pointer into string buffer */
        *qend;                          /* End of string buffer */


  qptr = q;
  qend = q + qsize - 5;

  if (qend &lt; q)
  {
    *q = '\0';
    return (q);
  }

  *qptr++ = '\'';
  *qptr++ = '\"';

  while (*s && qptr &lt; qend)
  {
    if (*s == '\\' || *s == '\"' || *s == '\'')
    {
      if (qptr &lt; (qend - 4))
      {
        *qptr++ = '\\';
        *qptr++ = '\\';
        *qptr++ = '\\';
      }
      else
        break;
    }

    *qptr++ = *s++;
  }

  *qptr++ = '\"';
  *qptr++ = '\'';
  *qptr   = '\0';

  return (q);
}
</pre>


<h4><a name="MANAGING_STATE">Managing Printer State in a Filter</a></h4>

<p>Filters are responsible for managing the state keywords they set using
"STATE:" messages. Typically you will update <em>all</em> of the keywords that
are used by the filter at startup, for example:</p>

<pre class="example">
if (foo_condition != 0)
  fputs("STATE: +com.example.foo\n", stderr);
else
  fputs("STATE: -com.example.foo\n", stderr);

if (bar_condition != 0)
  fputs("STATE: +com.example.bar\n", stderr);
else
  fputs("STATE: -com.example.bar\n", stderr);
</pre>

<p>Then as conditions change, your filter sends "STATE: +keyword" or "STATE:
-keyword" messages as necessary to set or clear the corresponding keyword,
respectively.</p>

<p>State keywords are often used to notify the user of issues that span across
jobs, for example "media-empty-warning" that indicates one or more paper trays
are empty. These keywords should not be cleared unless the corresponding issue
no longer exists.</p>

<p>Filters should clear job-related keywords on startup and exit so that they
do not remain set between jobs.  For example, "connecting-to-device" is a job
sub-state and not an issue that applies when a job is not printing.</p>

<blockquote><b>Note:</b>

<p>"STATE:" messages often provide visible alerts to the user. For example,
on macOS setting a printer-state-reason value with an "-error" or
"-warning" suffix will cause the printer's dock item to bounce if the
corresponding reason is localized with a cupsIPPReason keyword in the
printer's PPD file.</p>

<p>When providing a vendor-prefixed keyword, <em>always</em> provide the
corresponding standard keyword (if any) to allow clients to respond to the
condition correctly. For example, if you provide a vendor-prefixed keyword
for a low cyan ink condition ("com.example.cyan-ink-low") you must also set the
"marker-supply-low-warning" keyword. In such cases you should also refrain
from localizing the vendor-prefixed keyword in the PPD file - otherwise both
the generic and vendor-specific keyword will be shown in the user
interface.</p>

</blockquote>

<h4><a name="REPORTING_SUPPLIES">Reporting Supply Levels</a></h4>

<p>CUPS tracks several "marker-*" attributes for ink/toner supply level
reporting. These attributes allow applications to display the current supply
levels for a printer without printer-specific software. <a href="#TABLE3">Table 3</a> lists the marker attributes and what they represent.</p>

<p>Filters set marker attributes by sending "ATTR:" messages to stderr. For
example, a filter supporting an inkjet printer with black and tri-color ink
cartridges would use the following to initialize the supply attributes:</p>

<pre class="example">
fputs("ATTR: marker-colors=#000000,#00FFFF#FF00FF#FFFF00\n", stderr);
fputs("ATTR: marker-low-levels=5,10\n", stderr);
fputs("ATTR: marker-names=Black,Tri-Color\n", stderr);
fputs("ATTR: marker-types=ink,ink\n", stderr);
</pre>

<p>Then periodically the filter queries the printer for its current supply
levels and updates them with a separate "ATTR:" message:</p>

<pre class="example">
int black_level, tri_level;
...
fprintf(stderr, "ATTR: marker-levels=%d,%d\n", black_level, tri_level);
</pre>

<div class='table'><table width='80%' summary='Table 3: Supply Level Attributes'>
<caption>Table 3: <a name='TABLE3'>Supply Level Attributes</a></caption>
<thead>
<tr>
	<th>Attribute</th>
	<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
	<td>marker-colors</td>
	<td>A list of comma-separated colors; each color is either "none" or one or
	more hex-encoded sRGB colors of the form "#RRGGBB".</td>
</tr>
<tr>
	<td>marker-high-levels</td>
	<td>A list of comma-separated "almost full" level values from 0 to 100; a
	value of 100 should be used for supplies that are consumed/emptied like ink
	cartridges.</td>
</tr>
<tr>
	<td>marker-levels</td>
	<td>A list of comma-separated level values for each supply. A value of -1
	indicates the level is unavailable, -2 indicates unknown, and -3 indicates
	the level is unknown but has not yet reached capacity. Values from 0 to 100
	indicate the corresponding percentage.</td>
</tr>
<tr>
	<td>marker-low-levels</td>
	<td>A list of comma-separated "almost empty" level values from 0 to 100; a
	value of 0 should be used for supplies that are filled like waste ink
	tanks.</td>
</tr>
<tr>
	<td>marker-message</td>
	<td>A human-readable supply status message for the user like "12 pages of
	ink remaining."</td>
</tr>
<tr>
	<td>marker-names</td>
	<td>A list of comma-separated supply names like "Cyan Ink", "Fuser",
	etc.</td>
</tr>
<tr>
	<td>marker-types</td>
	<td>A list of comma-separated supply types; the types are listed in
	<a href="#TABLE1">Table 1</a>.</td>
</tr>
</tbody>
</table></div>

<h3><a name="COMMUNICATING_BACKEND">Communicating with the Backend</a></h3>

<p>Filters can communicate with the backend via the
<a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and
<a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
functions. The
<a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function
reads data that has been sent back from the device and is typically used to
obtain status and configuration information. For example, the following code
polls the backend for back-channel data:</p>

<pre class="example">
#include &lt;cups/cups.h&gt;

char buffer[8192];
ssize_t bytes;

/* Use a timeout of 0.0 seconds to poll for back-channel data */
bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
</pre>

<p>Filters can also use <code>select()</code> or <code>poll()</code> on the
back-channel file descriptor (3 or <code>CUPS_BC_FD</code>) to read data only
when it is available.</p>

<p>The
<a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
function allows you to get out-of-band status information and do synchronization
with the device. For example, the following code gets the current IEEE-1284
device ID string from the backend:</p>

<pre class="example">
#include &lt;cups/sidechannel.h&gt;

char data[2049];
int datalen;
<a href="#cups_sc_status_t">cups_sc_status_t</a> status;

/* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for
   nul-termination... */
datalen = sizeof(data) - 1;

/* Get the IEEE-1284 device ID, waiting for up to 1 second */
status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &amp;datalen, 1.0);

/* Use the returned value if OK was returned and the length is non-zero */
if (status == CUPS_SC_STATUS_OK &amp;&amp; datalen > 0)
  data[datalen] = '\0';
else
  data[0] = '\0';
</pre>

<h4><a name="DRAIN_OUTPUT">Forcing All Output to a Printer</a></h4>

<p>The
<a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
function allows you to tell the backend to send all pending data to the printer.
This is most often needed when sending query commands to the printer. For example:</p>

<pre class="example">
#include &lt;cups/cups.h&gt;
#include &lt;cups/sidechannel.h&gt;

char data[1024];
int datalen = sizeof(data);
<a href="#cups_sc_status_t">cups_sc_status_t</a> status;

/* Flush pending output to stdout */
fflush(stdout);

/* Drain output to backend, waiting for up to 30 seconds */
status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_DRAIN_OUTPUT, data, &amp;datalen, 30.0);

/* Read the response if the output was sent */
if (status == CUPS_SC_STATUS_OK)
{
  ssize_t bytes;

  /* Wait up to 10.0 seconds for back-channel data */
  bytes = cupsBackChannelRead(data, sizeof(data), 10.0);
  /* do something with the data from the printer */
}
</pre>

<h3><a name="COMMUNICATING_FILTER">Communicating with Filters</a></h3>

<p>Backends communicate with filters using the reciprocal functions
<a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>,
<a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and
<a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We
recommend writing back-channel data using a timeout of 1.0 seconds:</p>

<pre class="example">
#include &lt;cups/cups.h&gt;

char buffer[8192];
ssize_t bytes;

/* Obtain data from printer/device */
...

/* Use a timeout of 1.0 seconds to give filters a chance to read */
cupsBackChannelWrite(buffer, bytes, 1.0);
</pre>

<p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>
function reads a side-channel command from a filter, driver, or port monitor.
Backends can either poll for commands using a <code>timeout</code> of 0.0, wait
indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a
separate thread for that purpose), or use <code>select</code> or
<code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle
input and output on several file descriptors at the same time.</p>

<p>Once a command is processed, the backend uses the
<a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function
to send its response. For example, the following code shows how to poll for a
side-channel command and respond to it:</p>

<pre class="example">
#include &lt;cups/sidechannel.h&gt;

<a href="#cups_sc_command_t">cups_sc_command_t</a> command;
<a href="#cups_sc_status_t">cups_sc_status_t</a> status;
char data[2048];
int datalen = sizeof(data);

/* Poll for a command... */
if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&amp;command, &amp;status, data, &amp;datalen, 0.0))
{
  switch (command)
  {
    /* handle supported commands, fill data/datalen/status with values as needed */

    default :
        status  = CUPS_SC_STATUS_NOT_IMPLEMENTED;
	datalen = 0;
	break;
  }

  /* Send a response... */
  <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0);
}
</pre>

<h3><a name="SNMP">Doing SNMP Queries with Network Printers</a></h3>

<p>The Simple Network Management Protocol (SNMP) allows you to get the current
status, page counter, and supply levels from most network printers. Every
piece of information is associated with an Object Identifier (OID), and
every printer has a <em>community</em> name associated with it. OIDs can be
queried directly or by "walking" over a range of OIDs with a common prefix.</p>

<p>The two CUPS SNMP functions provide a simple API for querying network
printers through the side-channel interface. Each accepts a string containing
an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID)
along with a timeout for the query.</p>

<p>The <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>
function queries a single OID and returns the value as a string in a buffer
you supply:</p>

<pre class="example">
#include &lt;cups/sidechannel.h&gt;

char data[512];
int datalen = sizeof(data);

if (<a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a>(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &amp;datalen, 5.0)
        == CUPS_SC_STATUS_OK)
{
  /* Do something with the value */
  printf("Page counter is: %s\n", data);
}
</pre>

<p>The
<a href="#cupsSideChannelSNMPWalk"><code>cupsSideChannelSNMPWalk</code></a>
function allows you to query a whole group of OIDs, calling a function of your
choice for each OID that is found:</p>

<pre class="example">
#include &lt;cups/sidechannel.h&gt;

void
my_callback(const char *oid, const char *data, int datalen, void *context)
{
  /* Do something with the value */
  printf("%s=%s\n", oid, data);
}

...

void *my_data;

<a href="#cupsSideChannelSNMPWalk">cupsSNMPSideChannelWalk</a>(".1.3.6.1.2.1.43", 5.0, my_callback, my_data);
</pre>

<h2><a name="SANDBOXING">Sandboxing on macOS</a></h2>

<p>Starting with macOS 10.6, filters and backends are run inside a security "sandbox" which further limits (beyond the normal UNIX user/group permissions) what a filter or backend can do. This helps to both secure the printing system from malicious software and enforce the functional separation of components in the CUPS filter chain. What follows is a list of actions that are explicitly allowed for all filters and backends:</p>

<ol>

	<li>Reading of files: pursuant to normal UNIX file permissions, filters and backends can read files for the current job from the <var>/private/var/spool/cups</var> directory and other files on mounted filesystems <em>except</em> for user home directories under <var>/Users</var>.</li>

	<li>Writing of files: pursuant to normal UNIX file permissions, filters and backends can read/write files to the cache directory specified by the <code>CUPS_CACHEDIR</code> environment variable, to the state directory specified by the <code>CUPS_STATEDIR</code> environment variable, to the temporary directory specified by the <code>TMPDIR</code> environment variable, and under the <var>/private/var/db</var>, <var>/private/var/folders</var>, <var>/private/var/lib</var>, <var>/private/var/mysql</var>, <var>/private/var/run</var>, <var>/private/var/spool</var> (except <var>/private/var/spool/cups</var>), <var>/Library/Application&nbsp;Support</var>, <var>/Library/Caches</var>, <var>/Library/Logs</var>, <var>/Library/Preferences</var>, <var>/Library/WebServer</var>, and <var>/Users/Shared</var> directories.</li>

	<li>Execution of programs: pursuant to normal UNIX file permissions, filters and backends can execute any program not located under the <var>/Users</var> directory. Child processes inherit the sandbox and are subject to the same restrictions as the parent.</li>

	<li>Bluetooth and USB: backends can access Bluetooth and USB printers through IOKit. <em>Filters cannot access Bluetooth and USB printers directly.</em></li>

	<li>Network: filters and backends can access UNIX domain sockets under the <var>/private/tmp</var>, <var>/private/var/run</var>, and <var>/private/var/tmp</var> directories. Backends can also create IPv4 and IPv6 TCP (outgoing) and UDP (incoming and outgoing) socket, and bind to local source ports. <em>Filters cannot directly create IPv4 and IPv6 TCP or UDP sockets.</em></li>

	<li>Notifications: filters and backends can send notifications via the Darwin <code>notify_post()</code> API.</li>

</ol>

<blockquote><b>Note:</b> The sandbox profile used in CUPS 2.0 still allows some actions that are not listed above - these privileges will be removed over time until the profile matches the list above.</blockquote>