<!--
- "$Id: api-filter.shtml 6649 2007-07-11 21:46:42Z mike $"
+ "$Id: api-filter.shtml 7677 2008-06-19 23:22:19Z mike $"
- Filter and backend programming introduction for the Common UNIX Printing
- System (CUPS).
+ Filter and backend programming introduction for CUPS.
- Copyright 2007-2008 by Apple Inc.
+ Copyright 2007-2011 by Apple Inc.
Copyright 1997-2006 by Easy Software Products, all rights reserved.
These coded instructions, statements, and computer programs are the
<h2 class='title'><a name="OVERVIEW">Overview</a></h2>
-<p>Filters, printer drivers, port monitors, and backends use a common interface
-for processing print jobs and communicating status information to the scheduler.
-Each filter is run with a standard set of command-line arguments:<p>
+<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">
<dd>The options that were provided when the job was submitted</dd>
<dt>argv[6]</dt>
- <dd>The file to print (first filter only)</dd>
+ <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
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-priviledged user, so the major security
+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
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
+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 <signal.h>>
+
+...
+
+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
<h3><a name="ENVIRONMENT">Environment Variables</a></h3>
-<p>The following environment variables are defined by the printing system:</p>
+<p>The following environment variables are defined by the printing system
+when running print filters and backends:</p>
<dl class="code">
- <dt>APPLE_LANGUAGES</dt>
+ <dt>APPLE_LANGUAGE</dt>
<dd>The Apple language identifier associated with the job
(Mac OS X only).</dd>
application/postscript).</dd>
<dt>CUPS_CACHEDIR</dt>
- <dd>The directory where cache files can be stored.</dd>
+ <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 data files can be found.</dd>
+ <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>
file for this printer.</dd>
<dt>PRINTER</dt>
- <dd>The name of the printer.</dd>
+ <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 wih the scheduler by writing messages
-to the standard error file. For example, the following code sets the current
-printer state message to "Printing page 5":</p>
+<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;
<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-names</code>, <code>marker-types</code>,
- <code>printer-alert</code>, and <code>printer-alert-description</code>
- printer attributes.</dd>
+ <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
<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.</dd>
+ 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
<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 media, ink, and
- toner conditions on a printer.</dd>
+ 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
<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
+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
bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
</pre>
-The
+<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
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... */
+/* 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 */
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>
projects / thirdparty / cups.git / blobdiff