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Filter and backend programming introduction for the Common UNIX Printing
System (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-priviledged 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="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 <cups/cups.h>
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 <cups/sidechannel.h>
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, &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 <cups/cups.h>
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 <cups/sidechannel.h>
<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>(&command, &status, data, &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 <cups/sidechannel.h>
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, &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 <cups/sidechannel.h>
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>