[thirdparty/cups.git] / doc / help / api-filter.html

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<!--
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  Copyright 2008-2011 by Apple Inc.

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<h1 class='title'>Filter and Backend Programming</h1>

<div class='summary'><table summary='General Information'>
<thead>
<tr>
	<th>Headers</th>
	<th>cups/backend.h<br>
	cups/sidechannel.h</th>
</tr>
</thead>
<tbody>
<tr>
	<th>Library</th>
	<td>-lcups</td>
</tr>
<tr>
	<th>See Also</th>
	<td>Programming: <a href='api-overview.html' target='_top'>Introduction to CUPS Programming</a><br>
	Programming: <a href='api-cups.html' target='_top'>CUPS API</a><br>
	Programming: <a href='api-ppd.html' target='_top'>PPD API</a><br>
	Programming: <a href='api-raster.html' target='_top'>Raster API</a><br>
	Programming: <a href='postscript-driver.html' target='_top'>Developing PostScript Printer Drivers</a><br>
	Programming: <a href='raster-driver.html' target='_top'>Developing Raster Printer Drivers</a><br>
	Specifications: <a href='spec-design.html' target='_top'>CUPS Design Description</a></td>
</tr>
</tbody>
</table></div>
<h2 class="title">Contents</h2>
<ul class="contents">
<li><a href="#OVERVIEW">Overview</a><ul class="subcontents">
	<li><a href="#SECURITY">Security Considerations</a></li>
	<li><a href="#SIGNALS">Signal Handling</a></li>
	<li><a href="#TEMPFILES">Temporary Files</a></li>
	<li><a href="#COPIES">Copy Generation</a></li>
	<li><a href="#EXITCODES">Exit Codes</a></li>
	<li><a href="#ENVIRONMENT">Environment Variables</a></li>
	<li><a href="#MESSAGES">Communicating with the Scheduler</a></li>
	<li><a href="#COMMUNICATING_BACKEND">Communicating with the Backend</a></li>
	<li><a href="#COMMUNICATING_FILTER">Communicating with Filters</a></li>
	<li><a href="#SNMP">Doing SNMP Queries with Network Printers</a></li>
</ul></li>
<li><a href="#FUNCTIONS">Functions</a><ul class="code">
	<li><a href="#cupsBackChannelRead" title="Read data from the backchannel.">cupsBackChannelRead</a></li>
	<li><a href="#cupsBackChannelWrite" title="Write data to the backchannel.">cupsBackChannelWrite</a></li>
	<li><a href="#cupsBackendDeviceURI" title="Get the device URI for a backend.">cupsBackendDeviceURI</a></li>
	<li><a href="#cupsBackendReport" title="Write a device line from a backend.">cupsBackendReport</a></li>
	<li><a href="#cupsSideChannelDoRequest" title="Send a side-channel command to a backend and wait for a response.">cupsSideChannelDoRequest</a></li>
	<li><a href="#cupsSideChannelRead" title="Read a side-channel message.">cupsSideChannelRead</a></li>
	<li><a href="#cupsSideChannelSNMPGet" title="Query a SNMP OID's value.">cupsSideChannelSNMPGet</a></li>
	<li><a href="#cupsSideChannelSNMPWalk" title="Query multiple SNMP OID values.">cupsSideChannelSNMPWalk</a></li>
	<li><a href="#cupsSideChannelWrite" title="Write a side-channel message.">cupsSideChannelWrite</a></li>
</ul></li>
<li><a href="#TYPES">Data Types</a><ul class="code">
	<li><a href="#cups_backend_t" title="Backend exit codes">cups_backend_t</a></li>
	<li><a href="#cups_sc_bidi_t" title="Bidirectional capabilities">cups_sc_bidi_t</a></li>
	<li><a href="#cups_sc_command_t" title="Request command codes">cups_sc_command_t</a></li>
	<li><a href="#cups_sc_connected_t" title="Connectivity values">cups_sc_connected_t</a></li>
	<li><a href="#cups_sc_state_t" title="Printer state bits">cups_sc_state_t</a></li>
	<li><a href="#cups_sc_status_t" title="Response status codes">cups_sc_status_t</a></li>
	<li><a href="#cups_sc_walk_func_t" title="SNMP walk callback">cups_sc_walk_func_t</a></li>
</ul></li>
<li><a href="#ENUMERATIONS">Constants</a><ul class="code">
	<li><a href="#cups_backend_e" title="Backend exit codes">cups_backend_e</a></li>
	<li><a href="#cups_sc_bidi_e" title="Bidirectional capability values">cups_sc_bidi_e</a></li>
	<li><a href="#cups_sc_command_e" title="Request command codes">cups_sc_command_e</a></li>
	<li><a href="#cups_sc_connected_e" title="Connectivity values">cups_sc_connected_e</a></li>
	<li><a href="#cups_sc_state_e" title="Printer state bits">cups_sc_state_e</a></li>
	<li><a href="#cups_sc_status_e" title="Response status codes">cups_sc_status_e</a></li>
</ul></li>
</ul>
<!--
  "$Id$"

  Filter and backend programming introduction for CUPS.

  Copyright 2007-2011 by Apple Inc.
  Copyright 1997-2006 by Easy Software Products, all rights reserved.

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  property of Apple Inc. and are protected by Federal copyright
  law.  Distribution and use rights are outlined in the file "LICENSE.txt"
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  file is missing or damaged, see the license at "http://www.cups.org/".
-->

<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
Mac OS X, for example, no backend may write to a user's home directory.</p>
</blockquote>

<h3><a name="SIGNALS">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.</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 <code>ignore 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
	(Mac OS X 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-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>.</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>
	<dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
	<dd>Sets, adds, or removes printer-state-reason keywords to 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 the standard state keywords -
	use vendor-prefixed ("com.acme.foo") keywords for custom states.

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

	<p>"STATE:" messages often provide visible alerts to the user. For example, on
	Mac OS X 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>

	</blockquote></dd>

	<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>fuserCleaningPad</td>
	<td>Fuser cleaning pad</td>
</tr>
<tr>
	<td>fuserOil</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>solidWax</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>transferUnit</td>
	<td>Transfer unit</td>
</tr>
<tr>
	<td>wasteInk</td>
	<td>Waste ink tank</td>
</tr>
<tr>
	<td>wasteToner</td>
	<td>Waste toner tank</td>
</tr>
<tr>
	<td>wasteWax</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>A cover is open on the printer</td>
</tr>
<tr>
	<td>input-tray-missing</td>
	<td>An input tray is missing from the printer</td>
</tr>
<tr>
	<td>marker-supply-empty</td>
	<td>Out of ink</td>
</tr>
<tr>
	<td>marker-supply-low</td>
	<td>Low on ink</td>
</tr>
<tr>
	<td>marker-waste-almost-full</td>
	<td>Waste tank almost full</td>
</tr>
<tr>
	<td>marker-waste-full</td>
	<td>Waste tank full</td>
</tr>
<tr>
	<td>media-empty</td>
	<td>Out of media</td>
</tr>
<tr>
	<td>media-jam</td>
	<td>Media is jammed in the printer</td>
</tr>
<tr>
	<td>media-low</td>
	<td>Low on media</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>Out of toner</td>
</tr>
<tr>
	<td>toner-low</td>
	<td>Low on toner</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 && datalen > 0)
  data[datalen] = '\0';
else
  data[0] = '\0';
</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 class="title"><a name="FUNCTIONS">Functions</a></h2>
<h3 class="function"><span class="info">&nbsp;CUPS 1.2/Mac OS X 10.5&nbsp;</span><a name="cupsBackChannelRead">cupsBackChannelRead</a></h3>
<p class="description">Read data from the backchannel.</p>
<p class="code">
ssize_t cupsBackChannelRead (<br>
&nbsp;&nbsp;&nbsp;&nbsp;char *buffer,<br>
&nbsp;&nbsp;&nbsp;&nbsp;size_t bytes,<br>
&nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
);</p>
<h4 class="parameters">Parameters</h4>
<dl>
<dt>buffer</dt>
<dd class="description">Buffer to read into</dd>
<dt>bytes</dt>
<dd class="description">Bytes to read</dd>
<dt>timeout</dt>
<dd class="description">Timeout in seconds, typically 0.0 to poll</dd>
</dl>
<h4 class="returnvalue">Return Value</h4>
<p class="description">Bytes read or -1 on error</p>
<h4 class="discussion">Discussion</h4>
<p class="discussion">Reads up to &quot;bytes&quot; bytes from the backchannel/backend. The &quot;timeout&quot;
parameter controls how many seconds to wait for the data - use 0.0 to
return immediately if there is no data, -1.0 to wait for data indefinitely.

</p>
<h3 class="function"><span class="info">&nbsp;CUPS 1.2/Mac OS X 10.5&nbsp;</span><a name="cupsBackChannelWrite">cupsBackChannelWrite</a></h3>
<p class="description">Write data to the backchannel.</p>
<p class="code">
ssize_t cupsBackChannelWrite (<br>
&nbsp;&nbsp;&nbsp;&nbsp;const char *buffer,<br>
&nbsp;&nbsp;&nbsp;&nbsp;size_t bytes,<br>
&nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
);</p>
<h4 class="parameters">Parameters</h4>
<dl>
<dt>buffer</dt>
<dd class="description">Buffer to write</dd>
<dt>bytes</dt>
<dd class="description">Bytes to write</dd>
<dt>timeout</dt>
<dd class="description">Timeout in seconds, typically 1.0</dd>
</dl>
<h4 class="returnvalue">Return Value</h4>
<p class="description">Bytes written or -1 on error</p>
<h4 class="discussion">Discussion</h4>
<p class="discussion">Writes &quot;bytes&quot; bytes to the backchannel/filter. The &quot;timeout&quot; parameter
controls how many seconds to wait for the data to be written - use
0.0 to return immediately if the data cannot be written, -1.0 to wait
indefinitely.

</p>
<h3 class="function"><span class="info">&nbsp;CUPS 1.2/Mac OS X 10.5&nbsp;</span><a name="cupsBackendDeviceURI">cupsBackendDeviceURI</a></h3>
<p class="description">Get the device URI for a backend.</p>
<p class="code">
const char *cupsBackendDeviceURI (<br>
&nbsp;&nbsp;&nbsp;&nbsp;char **argv<br>
);</p>
<h4 class="parameters">Parameters</h4>
<dl>
<dt>argv</dt>
<dd class="description">Command-line arguments</dd>
</dl>
<h4 class="returnvalue">Return Value</h4>
<p class="description">Device URI or <code>NULL</code></p>
<h4 class="discussion">Discussion</h4>
<p class="discussion">The &quot;argv&quot; argument is the argv argument passed to main(). This
function returns the device URI passed in the DEVICE_URI environment
variable or the device URI passed in argv[0], whichever is found
first.

</p>
<h3 class="function"><span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span><a name="cupsBackendReport">cupsBackendReport</a></h3>
<p class="description">Write a device line from a backend.</p>
<p class="code">
void cupsBackendReport (<br>
&nbsp;&nbsp;&nbsp;&nbsp;const char *device_scheme,<br>
&nbsp;&nbsp;&nbsp;&nbsp;const char *device_uri,<br>
&nbsp;&nbsp;&nbsp;&nbsp;const char *device_make_and_model,<br>
&nbsp;&nbsp;&nbsp;&nbsp;const char *device_info,<br>
&nbsp;&nbsp;&nbsp;&nbsp;const char *device_id,<br>
&nbsp;&nbsp;&nbsp;&nbsp;const char *device_location<br>
);</p>
<h4 class="parameters">Parameters</h4>
<dl>
<dt>device_scheme</dt>
<dd class="description">device-scheme string</dd>
<dt>device_uri</dt>
<dd class="description">device-uri string</dd>
<dt>device_make_and_model</dt>
<dd class="description">device-make-and-model string or <code>NULL</code></dd>
<dt>device_info</dt>
<dd class="description">device-info string or <code>NULL</code></dd>
<dt>device_id</dt>
<dd class="description">device-id string or <code>NULL</code></dd>
<dt>device_location</dt>
<dd class="description">device-location string or <code>NULL</code></dd>
</dl>
<h4 class="discussion">Discussion</h4>
<p class="discussion">This function writes a single device line to stdout for a backend.
It handles quoting of special characters in the device-make-and-model,
device-info, device-id, and device-location strings.

</p>
<h3 class="function"><span class="info">&nbsp;CUPS 1.3/Mac OS X 10.5&nbsp;</span><a name="cupsSideChannelDoRequest">cupsSideChannelDoRequest</a></h3>
<p class="description">Send a side-channel command to a backend and wait for a response.</p>
<p class="code">
<a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelDoRequest (<br>
&nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> command,<br>
&nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
&nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
&nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
);</p>
<h4 class="parameters">Parameters</h4>
<dl>
<dt>command</dt>
<dd class="description">Command to send</dd>
<dt>data</dt>
<dd class="description">Response data buffer pointer</dd>
<dt>datalen</dt>
<dd class="description">Size of data buffer on entry, number of bytes in buffer on return</dd>
<dt>timeout</dt>
<dd class="description">Timeout in seconds</dd>
</dl>
<h4 class="returnvalue">Return Value</h4>
<p class="description">Status of command</p>
<h4 class="discussion">Discussion</h4>
<p class="discussion">This function is normally only called by filters, drivers, or port
monitors in order to communicate with the backend used by the current
printer.  Programs must be prepared to handle timeout or &quot;not
implemented&quot; status codes, which indicate that the backend or device
do not support the specified side-channel command.<br>
<br>
The &quot;datalen&quot; parameter must be initialized to the size of the buffer
pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
update the value to contain the number of data bytes in the buffer.

</p>
<h3 class="function"><span class="info">&nbsp;CUPS 1.3/Mac OS X 10.5&nbsp;</span><a name="cupsSideChannelRead">cupsSideChannelRead</a></h3>
<p class="description">Read a side-channel message.</p>
<p class="code">
int cupsSideChannelRead (<br>
&nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> *command,<br>
&nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_status_t">cups_sc_status_t</a> *status,<br>
&nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
&nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
&nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
);</p>
<h4 class="parameters">Parameters</h4>
<dl>
<dt>command</dt>
<dd class="description">Command code</dd>
<dt>status</dt>
<dd class="description">Status code</dd>
<dt>data</dt>
<dd class="description">Data buffer pointer</dd>
<dt>datalen</dt>
<dd class="description">Size of data buffer on entry, number of bytes in buffer on return</dd>
<dt>timeout</dt>
<dd class="description">Timeout in seconds</dd>
</dl>
<h4 class="returnvalue">Return Value</h4>
<p class="description">0 on success, -1 on error</p>
<h4 class="discussion">Discussion</h4>
<p class="discussion">This function is normally only called by backend programs to read
commands from a filter, driver, or port monitor program.  The
caller must be prepared to handle incomplete or invalid messages
and return the corresponding status codes.<br>
<br>
The &quot;datalen&quot; parameter must be initialized to the size of the buffer
pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
update the value to contain the number of data bytes in the buffer.

</p>
<h3 class="function"><span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span><a name="cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a></h3>
<p class="description">Query a SNMP OID's value.</p>
<p class="code">
<a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPGet (<br>
&nbsp;&nbsp;&nbsp;&nbsp;const char *oid,<br>
&nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
&nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
&nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
);</p>
<h4 class="parameters">Parameters</h4>
<dl>
<dt>oid</dt>
<dd class="description">OID to query</dd>
<dt>data</dt>
<dd class="description">Buffer for OID value</dd>
<dt>datalen</dt>
<dd class="description">Size of OID buffer on entry, size of value on return</dd>
<dt>timeout</dt>
<dd class="description">Timeout in seconds</dd>
</dl>
<h4 class="returnvalue">Return Value</h4>
<p class="description">Query status</p>
<h4 class="discussion">Discussion</h4>
<p class="discussion">This function asks the backend to do a SNMP OID query on behalf of the
filter, port monitor, or backend using the default community name.<br>
<br>
&quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
supported and must be converted to their numeric forms.<br>
<br>
On input, &quot;data&quot; and &quot;datalen&quot; provide the location and size of the
buffer to hold the OID value as a string. HEX-String (binary) values are
converted to hexadecimal strings representing the binary data, while
NULL-Value and unknown OID types are returned as the empty string.
The returned &quot;datalen&quot; does not include the trailing nul.

<code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
the printer does not respond to the SNMP query.

</p>
<h3 class="function"><span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span><a name="cupsSideChannelSNMPWalk">cupsSideChannelSNMPWalk</a></h3>
<p class="description">Query multiple SNMP OID values.</p>
<p class="code">
<a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPWalk (<br>
&nbsp;&nbsp;&nbsp;&nbsp;const char *oid,<br>
&nbsp;&nbsp;&nbsp;&nbsp;double timeout,<br>
&nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_walk_func_t">cups_sc_walk_func_t</a> cb,<br>
&nbsp;&nbsp;&nbsp;&nbsp;void *context<br>
);</p>
<h4 class="parameters">Parameters</h4>
<dl>
<dt>oid</dt>
<dd class="description">First numeric OID to query</dd>
<dt>timeout</dt>
<dd class="description">Timeout for each query in seconds</dd>
<dt>cb</dt>
<dd class="description">Function to call with each value</dd>
<dt>context</dt>
<dd class="description">Application-defined pointer to send to callback</dd>
</dl>
<h4 class="returnvalue">Return Value</h4>
<p class="description">Status of first query of <code>CUPS_SC_STATUS_OK</code> on success</p>
<h4 class="discussion">Discussion</h4>
<p class="discussion">This function asks the backend to do multiple SNMP OID queries on behalf
of the filter, port monitor, or backend using the default community name.
All OIDs under the &quot;parent&quot; OID are queried and the results are sent to
the callback function you provide.<br>
<br>
&quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
supported and must be converted to their numeric forms.<br>
<br>
&quot;timeout&quot; specifies the timeout for each OID query. The total amount of
time will depend on the number of OID values found and the time required
for each query.<br>
<br>
&quot;cb&quot; provides a function to call for every value that is found. &quot;context&quot;
is an application-defined pointer that is sent to the callback function
along with the OID and current data. The data passed to the callback is the
same as returned by <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>.

<code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
the printer does not respond to the first SNMP query.

</p>
<h3 class="function"><span class="info">&nbsp;CUPS 1.3/Mac OS X 10.5&nbsp;</span><a name="cupsSideChannelWrite">cupsSideChannelWrite</a></h3>
<p class="description">Write a side-channel message.</p>
<p class="code">
int cupsSideChannelWrite (<br>
&nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> command,<br>
&nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_status_t">cups_sc_status_t</a> status,<br>
&nbsp;&nbsp;&nbsp;&nbsp;const char *data,<br>
&nbsp;&nbsp;&nbsp;&nbsp;int datalen,<br>
&nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
);</p>
<h4 class="parameters">Parameters</h4>
<dl>
<dt>command</dt>
<dd class="description">Command code</dd>
<dt>status</dt>
<dd class="description">Status code</dd>
<dt>data</dt>
<dd class="description">Data buffer pointer</dd>
<dt>datalen</dt>
<dd class="description">Number of bytes of data</dd>
<dt>timeout</dt>
<dd class="description">Timeout in seconds</dd>
</dl>
<h4 class="returnvalue">Return Value</h4>
<p class="description">0 on success, -1 on error</p>
<h4 class="discussion">Discussion</h4>
<p class="discussion">This function is normally only called by backend programs to send
responses to a filter, driver, or port monitor program.

</p>
<h2 class="title"><a name="TYPES">Data Types</a></h2>
<h3 class="typedef"><a name="cups_backend_t">cups_backend_t</a></h3>
<p class="description">Backend exit codes</p>
<p class="code">
typedef enum <a href="#cups_backend_e">cups_backend_e</a> cups_backend_t;
</p>
<h3 class="typedef"><a name="cups_sc_bidi_t">cups_sc_bidi_t</a></h3>
<p class="description">Bidirectional capabilities</p>
<p class="code">
typedef enum <a href="#cups_sc_bidi_e">cups_sc_bidi_e</a> cups_sc_bidi_t;
</p>
<h3 class="typedef"><a name="cups_sc_command_t">cups_sc_command_t</a></h3>
<p class="description">Request command codes</p>
<p class="code">
typedef enum <a href="#cups_sc_command_e">cups_sc_command_e</a> cups_sc_command_t;
</p>
<h3 class="typedef"><a name="cups_sc_connected_t">cups_sc_connected_t</a></h3>
<p class="description">Connectivity values</p>
<p class="code">
typedef enum <a href="#cups_sc_connected_e">cups_sc_connected_e</a> cups_sc_connected_t;
</p>
<h3 class="typedef"><a name="cups_sc_state_t">cups_sc_state_t</a></h3>
<p class="description">Printer state bits</p>
<p class="code">
typedef enum <a href="#cups_sc_state_e">cups_sc_state_e</a> cups_sc_state_t;
</p>
<h3 class="typedef"><a name="cups_sc_status_t">cups_sc_status_t</a></h3>
<p class="description">Response status codes</p>
<p class="code">
typedef enum <a href="#cups_sc_status_e">cups_sc_status_e</a> cups_sc_status_t;
</p>
<h3 class="typedef"><a name="cups_sc_walk_func_t">cups_sc_walk_func_t</a></h3>
<p class="description">SNMP walk callback</p>
<p class="code">
typedef void (*cups_sc_walk_func_t)(const char *oid, const char *data, int datalen, void *context);
</p>
<h2 class="title"><a name="ENUMERATIONS">Constants</a></h2>
<h3 class="enumeration"><a name="cups_backend_e">cups_backend_e</a></h3>
<p class="description">Backend exit codes</p>
<h4 class="constants">Constants</h4>
<dl>
<dt>CUPS_BACKEND_AUTH_REQUIRED </dt>
<dd class="description">Job failed, authentication required</dd>
<dt>CUPS_BACKEND_CANCEL </dt>
<dd class="description">Job failed, cancel job</dd>
<dt>CUPS_BACKEND_FAILED </dt>
<dd class="description">Job failed, use error-policy</dd>
<dt>CUPS_BACKEND_HOLD </dt>
<dd class="description">Job failed, hold job</dd>
<dt>CUPS_BACKEND_OK </dt>
<dd class="description">Job completed successfully</dd>
<dt>CUPS_BACKEND_RETRY </dt>
<dd class="description">Job failed, retry this job later</dd>
<dt>CUPS_BACKEND_RETRY_CURRENT </dt>
<dd class="description">Job failed, retry this job immediately</dd>
<dt>CUPS_BACKEND_STOP </dt>
<dd class="description">Job failed, stop queue</dd>
</dl>
<h3 class="enumeration"><a name="cups_sc_bidi_e">cups_sc_bidi_e</a></h3>
<p class="description">Bidirectional capability values</p>
<h4 class="constants">Constants</h4>
<dl>
<dt>CUPS_SC_BIDI_NOT_SUPPORTED </dt>
<dd class="description">Bidirectional I/O is not supported</dd>
<dt>CUPS_SC_BIDI_SUPPORTED </dt>
<dd class="description">Bidirectional I/O is supported</dd>
</dl>
<h3 class="enumeration"><a name="cups_sc_command_e">cups_sc_command_e</a></h3>
<p class="description">Request command codes</p>
<h4 class="constants">Constants</h4>
<dl>
<dt>CUPS_SC_CMD_DRAIN_OUTPUT </dt>
<dd class="description">Drain all pending output</dd>
<dt>CUPS_SC_CMD_GET_BIDI </dt>
<dd class="description">Return bidirectional capabilities</dd>
<dt>CUPS_SC_CMD_GET_CONNECTED <span class="info">&nbsp;CUPS 1.5/Mac OS X 10.7&nbsp;</span></dt>
<dd class="description">Return whether the backend is &quot;connected&quot; to the printer </dd>
<dt>CUPS_SC_CMD_GET_DEVICE_ID </dt>
<dd class="description">Return the IEEE-1284 device ID</dd>
<dt>CUPS_SC_CMD_GET_STATE </dt>
<dd class="description">Return the device state</dd>
<dt>CUPS_SC_CMD_SNMP_GET <span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span></dt>
<dd class="description">Query an SNMP OID </dd>
<dt>CUPS_SC_CMD_SNMP_GET_NEXT <span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span></dt>
<dd class="description">Query the next SNMP OID </dd>
<dt>CUPS_SC_CMD_SOFT_RESET </dt>
<dd class="description">Do a soft reset</dd>
</dl>
<h3 class="enumeration"><a name="cups_sc_connected_e">cups_sc_connected_e</a></h3>
<p class="description">Connectivity values</p>
<h4 class="constants">Constants</h4>
<dl>
<dt>CUPS_SC_CONNECTED </dt>
<dd class="description">Backend is &quot;connected&quot; to printer</dd>
<dt>CUPS_SC_NOT_CONNECTED </dt>
<dd class="description">Backend is not &quot;connected&quot; to printer</dd>
</dl>
<h3 class="enumeration"><a name="cups_sc_state_e">cups_sc_state_e</a></h3>
<p class="description">Printer state bits</p>
<h4 class="constants">Constants</h4>
<dl>
<dt>CUPS_SC_STATE_BUSY </dt>
<dd class="description">Device is busy</dd>
<dt>CUPS_SC_STATE_ERROR </dt>
<dd class="description">Other error condition</dd>
<dt>CUPS_SC_STATE_MARKER_EMPTY </dt>
<dd class="description">Toner/ink out condition</dd>
<dt>CUPS_SC_STATE_MARKER_LOW </dt>
<dd class="description">Toner/ink low condition</dd>
<dt>CUPS_SC_STATE_MEDIA_EMPTY </dt>
<dd class="description">Paper out condition</dd>
<dt>CUPS_SC_STATE_MEDIA_LOW </dt>
<dd class="description">Paper low condition</dd>
<dt>CUPS_SC_STATE_OFFLINE </dt>
<dd class="description">Device is offline</dd>
<dt>CUPS_SC_STATE_ONLINE </dt>
<dd class="description">Device is online</dd>
</dl>
<h3 class="enumeration"><a name="cups_sc_status_e">cups_sc_status_e</a></h3>
<p class="description">Response status codes</p>
<h4 class="constants">Constants</h4>
<dl>
<dt>CUPS_SC_STATUS_BAD_MESSAGE </dt>
<dd class="description">The command/response message was invalid</dd>
<dt>CUPS_SC_STATUS_IO_ERROR </dt>
<dd class="description">An I/O error occurred</dd>
<dt>CUPS_SC_STATUS_NONE </dt>
<dd class="description">No status</dd>
<dt>CUPS_SC_STATUS_NOT_IMPLEMENTED </dt>
<dd class="description">Command not implemented</dd>
<dt>CUPS_SC_STATUS_NO_RESPONSE </dt>
<dd class="description">The device did not respond</dd>
<dt>CUPS_SC_STATUS_OK </dt>
<dd class="description">Operation succeeded</dd>
<dt>CUPS_SC_STATUS_TIMEOUT </dt>
<dd class="description">The backend did not respond</dd>
<dt>CUPS_SC_STATUS_TOO_BIG </dt>
<dd class="description">Response too big</dd>
</dl>
</div>
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