2 "$Id: api-filter.shtml 7677 2008-06-19 23:22:19Z mike $"
4 Filter and backend programming introduction for CUPS.
6 Copyright 2007-2013 by Apple Inc.
7 Copyright 1997-2006 by Easy Software Products, all rights reserved.
9 These coded instructions, statements, and computer programs are the
10 property of Apple Inc. and are protected by Federal copyright
11 law. Distribution and use rights are outlined in the file "LICENSE.txt"
12 which should have been included with this file. If this file is
13 file is missing or damaged, see the license at "http://www.cups.org/".
16 <h2 class='title'><a name="OVERVIEW">Overview</a></h2>
18 <p>Filters (which include printer drivers and port monitors) and backends
19 are used to convert job files to a printable format and send that data to the
20 printer itself. All of these programs use a common interface for processing
21 print jobs and communicating status information to the scheduler. Each is run
22 with a standard set of command-line arguments:<p>
30 <dd>The user printing the job</dd>
33 <dd>The job name/title</dd>
36 <dd>The number of copies to print</dd>
39 <dd>The options that were provided when the job was submitted</dd>
42 <dd>The file to print (first program only)</dd>
45 <p>The scheduler runs one or more of these programs to print any given job. The
46 first filter reads from the print file and writes to the standard output, while
47 the remaining filters read from the standard input and write to the standard
48 output. The backend is the last filter in the chain and writes to the
51 <p>Filters are always run as a non-privileged user, typically "lp", with no
52 connection to the user's desktop. Backends are run either as a non-privileged
53 user or as root if the file permissions do not allow user or group execution.
54 The <a href="#PERMISSIONS">file permissions</a> section talks about this in
57 <h3><a name="SECURITY">Security Considerations</a></h3>
59 <p>It is always important to use security programming practices. Filters and
60 most backends are run as a non-privileged user, so the major security
61 consideration is resource utilization - filters should not depend on unlimited
62 amounts of CPU, memory, or disk space, and should protect against conditions
63 that could lead to excess usage of any resource like infinite loops and
64 unbounded recursion. In addition, filters must <em>never</em> allow the user to
65 specify an arbitrary file path to a separator page, template, or other file
66 used by the filter since that can lead to an unauthorized disclosure of
67 information. <em>Always</em> treat input as suspect and validate it!</p>
69 <p>If you are developing a backend that runs as root, make sure to check for
70 potential buffer overflows, integer under/overflow conditions, and file
71 accesses since these can lead to privilege escalations. When writing files,
72 always validate the file path and <em>never</em> allow a user to determine
73 where to store a file.</p>
75 <blockquote><b>Note:</b>
77 <p><em>Never</em> write files to a user's home directory. Aside from the
78 security implications, CUPS is a network print service and as such the network
79 user may not be the same as the local user and/or there may not be a local home
80 directory to write to.</p>
82 <p>In addition, some operating systems provide additional security mechanisms
83 that further limit file system access, even for backends running as root. On
84 OS X, for example, no backend may write to a user's home directory.</p>
87 <h3><a name="SIGNALS">Canceled Jobs and Signal Handling</a></h3>
89 <p>The scheduler sends <code>SIGTERM</code> when a printing job is canceled or
90 held. Filters, backends, and port monitors <em>must</em> catch
91 <code>SIGTERM</code> and perform any cleanup necessary to produce a valid output
92 file or return the printer to a known good state. The recommended behavior is to
93 end the output on the current page, preferably on the current line or object
96 <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>
99 #include <signal.h>>
104 main(int argc, char *argv[])
106 signal(SIGPIPE, SIG_IGN);
112 <h3><a name="PERMISSIONS">File Permissions</a></h3>
114 <p>For security reasons, CUPS will only run filters and backends that are owned
115 by root and do not have world or group write permissions. The recommended
116 permissions for filters and backends are 0555 - read and execute but no write.
117 Backends that must run as root should use permissions of 0500 - read and execute
118 by root, no access for other users. Write permissions can be enabled for the
121 <p>To avoid a warning message, the directory containing your filter(s) must also
122 be owned by root and have world and group write disabled - permissions of 0755
123 or 0555 are strongly encouraged.</p>
125 <h3><a name="TEMPFILES">Temporary Files</a></h3>
127 <p>Temporary files should be created in the directory specified by the
128 "TMPDIR" environment variable. The
129 <a href="#cupsTempFile2"><code>cupsTempFile2</code></a> function can be
130 used to safely create temporary files in this directory.</p>
132 <h3><a name="COPIES">Copy Generation</a></h3>
134 <p>The <code>argv[4]</code> argument specifies the number of copies to produce
135 of the input file. In general, you should only generate copies if the
136 <em>filename</em> argument is supplied. The only exception to this are
137 filters that produce device-independent PostScript output, since the PostScript
138 filter <var>pstops</var> is responsible for generating copies of PostScript
141 <h3><a name="EXITCODES">Exit Codes</a></h3>
143 <p>Filters must exit with status 0 when they successfully generate print data
144 or 1 when they encounter an error. Backends can return any of the
145 <a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p>
147 <h3><a name="ENVIRONMENT">Environment Variables</a></h3>
149 <p>The following environment variables are defined by the printing system
150 when running print filters and backends:</p>
154 <dt>APPLE_LANGUAGE</dt>
155 <dd>The Apple language identifier associated with the job
159 <dd>The job character set, typically "utf-8".</dd>
162 <dd>When a job is submitted to a printer class, contains the name of
163 the destination printer class. Otherwise this environment
164 variable will not be set.</dd>
166 <dt>CONTENT_TYPE</dt>
167 <dd>The MIME type associated with the file (e.g.
168 application/postscript).</dd>
170 <dt>CUPS_CACHEDIR</dt>
171 <dd>The directory where cache files can be stored. Cache files can be
172 used to retain information between jobs or files in a job.</dd>
174 <dt>CUPS_DATADIR</dt>
175 <dd>The directory where (read-only) CUPS data files can be found.</dd>
177 <dt>CUPS_FILETYPE</dt>
178 <dd>The type of file being printed: "job-sheet" for a banner page and
179 "document" for a regular print file.</dd>
181 <dt>CUPS_SERVERROOT</dt>
182 <dd>The root directory of the server.</dd>
185 <dd>The device-uri associated with the printer.</dd>
187 <dt>FINAL_CONTENT_TYPE</dt>
188 <dd>The MIME type associated with the printer (e.g.
189 application/vnd.cups-postscript).</dd>
192 <dd>The language locale associated with the job.</dd>
195 <dd>The full pathname of the PostScript Printer Description (PPD)
196 file for this printer.</dd>
199 <dd>The queue name of the class or printer.</dd>
202 <dd>The recommended amount of memory to use for Raster Image
203 Processors (RIPs).</dd>
206 <dd>The directory where temporary files should be created.</dd>
210 <h3><a name="MESSAGES">Communicating with the Scheduler</a></h3>
212 <p>Filters and backends communicate with the scheduler by writing messages
213 to the standard error file. The scheduler reads messages from all filters in
214 a job and processes the message based on its prefix. For example, the following
215 code sets the current printer state message to "Printing page 5":</p>
217 <pre class="example">
220 fprintf(stderr, "INFO: Printing page %d\n", page);
223 <p>Each message is a single line of text starting with one of the following
228 <dt>ALERT: message</dt>
229 <dd>Sets the printer-state-message attribute and adds the specified
230 message to the current error log file using the "alert" log level.</dd>
232 <dt>ATTR: attribute=value [attribute=value]</dt>
233 <dd>Sets the named printer or job attribute(s). Typically this is used
234 to set the <code>marker-colors</code>, <code>marker-high-levels</code>,
235 <code>marker-levels</code>, <code>marker-low-levels</code>,
236 <code>marker-message</code>, <code>marker-names</code>,
237 <code>marker-types</code>, <code>printer-alert</code>, and
238 <code>printer-alert-description</code> printer attributes. Standard
239 <code>marker-types</code> values are listed in <a href='#TABLE1'>Table
242 <dt>CRIT: message</dt>
243 <dd>Sets the printer-state-message attribute and adds the specified
244 message to the current error log file using the "critical" log
247 <dt>DEBUG: message</dt>
248 <dd>Sets the printer-state-message attribute and adds the specified
249 message to the current error log file using the "debug" log level.</dd>
251 <dt>DEBUG2: message</dt>
252 <dd>Sets the printer-state-message attribute and adds the specified
253 message to the current error log file using the "debug2" log level.</dd>
255 <dt>EMERG: message</dt>
256 <dd>Sets the printer-state-message attribute and adds the specified
257 message to the current error log file using the "emergency" log
260 <dt>ERROR: message</dt>
261 <dd>Sets the printer-state-message attribute and adds the specified
262 message to the current error log file using the "error" log level.
263 Use "ERROR:" messages for non-persistent processing errors.</dd>
265 <dt>INFO: message</dt>
266 <dd>Sets the printer-state-message attribute. If the current log level
267 is set to "debug2", also adds the specified message to the current error
268 log file using the "info" log level.</dd>
270 <dt>NOTICE: message</dt>
271 <dd>Sets the printer-state-message attribute and adds the specified
272 message to the current error log file using the "notice" log level.</dd>
274 <dt>PAGE: page-number #-copies</dt>
275 <dt>PAGE: total #-pages</dt>
276 <dd>Adds an entry to the current page log file. The first form adds
277 #-copies to the job-media-sheets-completed attribute. The second
278 form sets the job-media-sheets-completed attribute to #-pages.</dd>
280 <dt>PPD: keyword=value [keyword=value ...]</dt>
281 <dd>Changes or adds keywords to the printer's PPD file. Typically
282 this is used to update installable options or default media settings
283 based on the printer configuration.</dd>
285 <dt>STATE: + printer-state-reason [printer-state-reason ...]</dt>
286 <dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
287 <dd>Sets or clears printer-state-reason keywords for the current queue.
288 Typically this is used to indicate persistent media, ink, toner, and
289 configuration conditions or errors on a printer.
290 <a href='#TABLE2'>Table 2</a> lists the standard state keywords -
291 use vendor-prefixed ("com.example.foo") keywords for custom states. See
292 <a href="#MANAGING_STATE">Managing Printer State in a Filter</a> for more
295 <dt>WARNING: message</dt>
296 <dd>Sets the printer-state-message attribute and adds the specified
297 message to the current error log file using the "warning" log
302 <p>Messages without one of these prefixes are treated as if they began with
303 the "DEBUG:" prefix string.</p>
305 <div class='table'><table width='80%' summary='Table 1: Standard marker-types Values'>
306 <caption>Table 1: <a name='TABLE1'>Standard marker-types Values</a></caption>
316 <td>Developer unit</td>
323 <td>fuserCleaningPad</td>
324 <td>Fuser cleaning pad</td>
336 <td>Photo conductor</td>
344 <td>Staple supply</td>
348 <td>Toner supply</td>
351 <td>transferUnit</td>
352 <td>Transfer unit</td>
356 <td>Waste ink tank</td>
360 <td>Waste toner tank</td>
364 <td>Waste wax tank</td>
371 <div class='table'><table width='80%' summary='Table 2: Standard State Keywords'>
372 <caption>Table 2: <a name='TABLE2'>Standard State Keywords</a></caption>
381 <td>connecting-to-device</td>
382 <td>Connecting to printer but not printing yet.</td>
386 <td>The printer's cover is open.</td>
389 <td>input-tray-missing</td>
390 <td>The paper tray is missing.</td>
393 <td>marker-supply-empty</td>
394 <td>The printer is out of ink.</td>
397 <td>marker-supply-low</td>
398 <td>The printer is almost out of ink.</td>
401 <td>marker-waste-almost-full</td>
402 <td>The printer's waste bin is almost full.</td>
405 <td>marker-waste-full</td>
406 <td>The printer's waste bin is full.</td>
410 <td>The paper tray (any paper tray) is empty.</td>
414 <td>There is a paper jam.</td>
418 <td>The paper tray (any paper tray) is almost empty.</td>
421 <td>media-needed</td>
422 <td>The paper tray needs to be filled (for a job that is printing).</td>
426 <td>Stop the printer.</td>
430 <td>Unable to connect to printer.</td>
434 <td>The printer is out of toner.</td>
438 <td>The printer is low on toner.</td>
443 <h4><a name="MANAGING_STATE">Managing Printer State in a Filter</a></h4>
445 <p>Filters are responsible for managing the state keywords they set using
446 "STATE:" messages. Typically you will update <em>all</em> of the keywords that
447 are used by the filter at startup, for example:</p>
449 <pre class="example">
450 if (foo_condition != 0)
451 fputs("STATE: +com.example.foo\n", stderr);
453 fputs("STATE: -com.example.foo\n", stderr);
455 if (bar_condition != 0)
456 fputs("STATE: +com.example.bar\n", stderr);
458 fputs("STATE: -com.example.bar\n", stderr);
461 <p>Then as conditions change, your filter sends "STATE: +keyword" or "STATE:
462 -keyword" messages as necessary to set or clear the corresponding keyword,
465 <p>State keywords are often used to notify the user of issues that span across
466 jobs, for example "media-empty-warning" that indicates one or more paper trays
467 are empty. These keywords should not be cleared unless the corresponding issue
468 no longer exists.</p>
470 <p>Filters should clear job-related keywords on startup and exit so that they
471 do not remain set between jobs. For example, "connecting-to-device" is a job
472 sub-state and not an issue that applies when a job is not printing.</p>
474 <blockquote><b>Note:</b>
476 <p>"STATE:" messages often provide visible alerts to the user. For example,
477 on OS X setting a printer-state-reason value with an "-error" or
478 "-warning" suffix will cause the printer's dock item to bounce if the
479 corresponding reason is localized with a cupsIPPReason keyword in the
480 printer's PPD file.</p>
482 <p>When providing a vendor-prefixed keyword, <em>always</em> provide the
483 corresponding standard keyword (if any) to allow clients to respond to the
484 condition correctly. For example, if you provide a vendor-prefixed keyword
485 for a low cyan ink condition ("com.example.cyan-ink-low") you must also set the
486 "marker-supply-low-warning" keyword. In such cases you should also refrain
487 from localizing the vendor-prefixed keyword in the PPD file - otherwise both
488 the generic and vendor-specific keyword will be shown in the user
493 <h4><a name="REPORTING_SUPPLIES">Reporting Supply Levels</a></h4>
495 <p>CUPS tracks several "marker-*" attributes for ink/toner supply level
496 reporting. These attributes allow applications to display the current supply
497 levels for a printer without printer-specific software. <a href="#TABLE3">Table 3</a> lists the marker attributes and what they represent.</p>
499 <p>Filters set marker attributes by sending "ATTR:" messages to stderr. For
500 example, a filter supporting an inkjet printer with black and tri-color ink
501 cartridges would use the following to initialize the supply attributes:</p>
503 <pre class="example">
504 fputs("ATTR: marker-colors=#000000,#00FFFF#FF00FF#FFFF00\n", stderr);
505 fputs("ATTR: marker-low-levels=5,10\n", stderr);
506 fputs("ATTR: marker-names=Black,Tri-Color\n", stderr);
507 fputs("ATTR: marker-types=ink,ink\n", stderr);
510 <p>Then periodically the filter queries the printer for its current supply
511 levels and updates them with a separate "ATTR:" message:</p>
513 <pre class="example">
514 int black_level, tri_level;
516 fprintf(stderr, "ATTR: marker-levels=%d,%d\n", black_level, tri_level);
519 <div class='table'><table width='80%' summary='Table 3: Supply Level Attributes'>
520 <caption>Table 3: <a name='TABLE3'>Supply Level Attributes</a></caption>
529 <td>marker-colors</td>
530 <td>A list of comma-separated colors; each color is either "none" or one or
531 more hex-encoded sRGB colors of the form "#RRGGBB".</td>
534 <td>marker-high-levels</td>
535 <td>A list of comma-separated "almost full" level values from 0 to 100; a
536 value of 100 should be used for supplies that are consumed/emptied like ink
540 <td>marker-levels</td>
541 <td>A list of comma-separated level values for each supply. A value of -1
542 indicates the level is unavailable, -2 indicates unknown, and -3 indicates
543 the level is unknown but has not yet reached capacity. Values from 0 to 100
544 indicate the corresponding percentage.</td>
547 <td>marker-low-levels</td>
548 <td>A list of comma-separated "almost empty" level values from 0 to 100; a
549 value of 0 should be used for supplies that are filled like waste ink
553 <td>marker-message</td>
554 <td>A human-readable supply status message for the user like "12 pages of
558 <td>marker-names</td>
559 <td>A list of comma-separated supply names like "Cyan Ink", "Fuser",
563 <td>marker-types</td>
564 <td>A list of comma-separated supply types; the types are listed in
565 <a href="#TABLE1">Table 1</a>.</td>
570 <h3><a name="COMMUNICATING_BACKEND">Communicating with the Backend</a></h3>
572 <p>Filters can communicate with the backend via the
573 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and
574 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
576 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function
577 reads data that has been sent back from the device and is typically used to
578 obtain status and configuration information. For example, the following code
579 polls the backend for back-channel data:</p>
581 <pre class="example">
582 #include <cups/cups.h>
587 /* Use a timeout of 0.0 seconds to poll for back-channel data */
588 bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
591 <p>Filters can also use <code>select()</code> or <code>poll()</code> on the
592 back-channel file descriptor (3 or <code>CUPS_BC_FD</code>) to read data only
593 when it is available.</p>
596 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
597 function allows you to get out-of-band status information and do synchronization
598 with the device. For example, the following code gets the current IEEE-1284
599 device ID string from the backend:</p>
601 <pre class="example">
602 #include <cups/sidechannel.h>
606 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
608 /* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for
609 nul-termination... */
610 datalen = sizeof(data) - 1;
612 /* Get the IEEE-1284 device ID, waiting for up to 1 second */
613 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &datalen, 1.0);
615 /* Use the returned value if OK was returned and the length is non-zero */
616 if (status == CUPS_SC_STATUS_OK && datalen > 0)
617 data[datalen] = '\0';
622 <h4><a name="DRAIN_OUTPUT">Forcing All Output to a Printer</a></h4>
625 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
626 function allows you to tell the backend to send all pending data to the printer.
627 This is most often needed when sending query commands to the printer. For example:</p>
629 <pre class="example">
630 #include <cups/cups.h>
631 #include <cups/sidechannel.h>
634 int datalen = sizeof(data);
635 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
637 /* Flush pending output to stdout */
640 /* Drain output to backend, waiting for up to 30 seconds */
641 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_DRAIN_OUTPUT, data, &datalen, 30.0);
643 /* Read the response if the output was sent */
644 if (status == CUPS_SC_STATUS_OK)
648 /* Wait up to 10.0 seconds for back-channel data */
649 bytes = cupsBackChannelRead(data, sizeof(data), 10.0);
650 /* do something with the data from the printer */
654 <h3><a name="COMMUNICATING_FILTER">Communicating with Filters</a></h3>
656 <p>Backends communicate with filters using the reciprocal functions
657 <a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>,
658 <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and
659 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We
660 recommend writing back-channel data using a timeout of 1.0 seconds:</p>
662 <pre class="example">
663 #include <cups/cups.h>
668 /* Obtain data from printer/device */
671 /* Use a timeout of 1.0 seconds to give filters a chance to read */
672 cupsBackChannelWrite(buffer, bytes, 1.0);
675 <p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>
676 function reads a side-channel command from a filter, driver, or port monitor.
677 Backends can either poll for commands using a <code>timeout</code> of 0.0, wait
678 indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a
679 separate thread for that purpose), or use <code>select</code> or
680 <code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle
681 input and output on several file descriptors at the same time.</p>
683 <p>Once a command is processed, the backend uses the
684 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function
685 to send its response. For example, the following code shows how to poll for a
686 side-channel command and respond to it:</p>
688 <pre class="example">
689 #include <cups/sidechannel.h>
691 <a href="#cups_sc_command_t">cups_sc_command_t</a> command;
692 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
694 int datalen = sizeof(data);
696 /* Poll for a command... */
697 if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&command, &status, data, &datalen, 0.0))
701 /* handle supported commands, fill data/datalen/status with values as needed */
704 status = CUPS_SC_STATUS_NOT_IMPLEMENTED;
709 /* Send a response... */
710 <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0);
714 <h3><a name="SNMP">Doing SNMP Queries with Network Printers</a></h3>
716 <p>The Simple Network Management Protocol (SNMP) allows you to get the current
717 status, page counter, and supply levels from most network printers. Every
718 piece of information is associated with an Object Identifier (OID), and
719 every printer has a <em>community</em> name associated with it. OIDs can be
720 queried directly or by "walking" over a range of OIDs with a common prefix.</p>
722 <p>The two CUPS SNMP functions provide a simple API for querying network
723 printers through the side-channel interface. Each accepts a string containing
724 an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID)
725 along with a timeout for the query.</p>
727 <p>The <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>
728 function queries a single OID and returns the value as a string in a buffer
731 <pre class="example">
732 #include <cups/sidechannel.h>
735 int datalen = sizeof(data);
737 if (<a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a>(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &datalen, 5.0)
738 == CUPS_SC_STATUS_OK)
740 /* Do something with the value */
741 printf("Page counter is: %s\n", data);
746 <a href="#cupsSideChannelSNMPWalk"><code>cupsSideChannelSNMPWalk</code></a>
747 function allows you to query a whole group of OIDs, calling a function of your
748 choice for each OID that is found:</p>
750 <pre class="example">
751 #include <cups/sidechannel.h>
754 my_callback(const char *oid, const char *data, int datalen, void *context)
756 /* Do something with the value */
757 printf("%s=%s\n", oid, data);
764 <a href="#cupsSideChannelSNMPWalk">cupsSNMPSideChannelWalk</a>(".1.3.6.1.2.1.43", 5.0, my_callback, my_data);