2 Filter and backend programming introduction for CUPS.
4 Copyright © 2007-2016 by Apple Inc.
5 Copyright © 1997-2006 by Easy Software Products, all rights reserved.
7 Licensed under Apache License v2.0. See the file "LICENSE" for more
11 <h2 class='title'><a name="OVERVIEW">Overview</a></h2>
13 <p>Filters (which include printer drivers and port monitors) and backends
14 are used to convert job files to a printable format and send that data to the
15 printer itself. All of these programs use a common interface for processing
16 print jobs and communicating status information to the scheduler. Each is run
17 with a standard set of command-line arguments:<p>
25 <dd>The user printing the job</dd>
28 <dd>The job name/title</dd>
31 <dd>The number of copies to print</dd>
34 <dd>The options that were provided when the job was submitted</dd>
37 <dd>The file to print (first program only)</dd>
40 <p>The scheduler runs one or more of these programs to print any given job. The
41 first filter reads from the print file and writes to the standard output, while
42 the remaining filters read from the standard input and write to the standard
43 output. The backend is the last filter in the chain and writes to the
46 <p>Filters are always run as a non-privileged user, typically "lp", with no
47 connection to the user's desktop. Backends are run either as a non-privileged
48 user or as root if the file permissions do not allow user or group execution.
49 The <a href="#PERMISSIONS">file permissions</a> section talks about this in
52 <h3><a name="SECURITY">Security Considerations</a></h3>
54 <p>It is always important to use security programming practices. Filters and
55 most backends are run as a non-privileged user, so the major security
56 consideration is resource utilization - filters should not depend on unlimited
57 amounts of CPU, memory, or disk space, and should protect against conditions
58 that could lead to excess usage of any resource like infinite loops and
59 unbounded recursion. In addition, filters must <em>never</em> allow the user to
60 specify an arbitrary file path to a separator page, template, or other file
61 used by the filter since that can lead to an unauthorized disclosure of
62 information. <em>Always</em> treat input as suspect and validate it!</p>
64 <p>If you are developing a backend that runs as root, make sure to check for
65 potential buffer overflows, integer under/overflow conditions, and file
66 accesses since these can lead to privilege escalations. When writing files,
67 always validate the file path and <em>never</em> allow a user to determine
68 where to store a file.</p>
70 <blockquote><b>Note:</b>
72 <p><em>Never</em> write files to a user's home directory. Aside from the
73 security implications, CUPS is a network print service and as such the network
74 user may not be the same as the local user and/or there may not be a local home
75 directory to write to.</p>
77 <p>In addition, some operating systems provide additional security mechanisms
78 that further limit file system access, even for backends running as root. On
79 macOS, for example, no backend may write to a user's home directory. See the <a href="#SANDBOXING">Sandboxing on macOS</a> section for more information.</p>
82 <h3><a name="SIGNALS">Canceled Jobs and Signal Handling</a></h3>
84 <p>The scheduler sends <code>SIGTERM</code> when a printing job is canceled or
85 held. Filters, backends, and port monitors <em>must</em> catch
86 <code>SIGTERM</code> and perform any cleanup necessary to produce a valid output
87 file or return the printer to a known good state. The recommended behavior is to
88 end the output on the current page, preferably on the current line or object
91 <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>
94 #include <signal.h>
99 main(int argc, char *argv[])
101 signal(SIGPIPE, SIG_IGN);
107 <h3><a name="PERMISSIONS">File Permissions</a></h3>
109 <p>For security reasons, CUPS will only run filters and backends that are owned
110 by root and do not have world or group write permissions. The recommended
111 permissions for filters and backends are 0555 - read and execute but no write.
112 Backends that must run as root should use permissions of 0500 - read and execute
113 by root, no access for other users. Write permissions can be enabled for the
116 <p>To avoid a warning message, the directory containing your filter(s) must also
117 be owned by root and have world and group write disabled - permissions of 0755
118 or 0555 are strongly encouraged.</p>
120 <h3><a name="TEMPFILES">Temporary Files</a></h3>
122 <p>Temporary files should be created in the directory specified by the
123 "TMPDIR" environment variable. The
124 <a href="#cupsTempFile2"><code>cupsTempFile2</code></a> function can be
125 used to safely create temporary files in this directory.</p>
127 <h3><a name="COPIES">Copy Generation</a></h3>
129 <p>The <code>argv[4]</code> argument specifies the number of copies to produce
130 of the input file. In general, you should only generate copies if the
131 <em>filename</em> argument is supplied. The only exception to this are
132 filters that produce device-independent PostScript output, since the PostScript
133 filter <var>pstops</var> is responsible for generating copies of PostScript
136 <h3><a name="EXITCODES">Exit Codes</a></h3>
138 <p>Filters must exit with status 0 when they successfully generate print data
139 or 1 when they encounter an error. Backends can return any of the
140 <a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p>
142 <h3><a name="ENVIRONMENT">Environment Variables</a></h3>
144 <p>The following environment variables are defined by the printing system
145 when running print filters and backends:</p>
149 <dt>APPLE_LANGUAGE</dt>
150 <dd>The Apple language identifier associated with the job
154 <dd>The job character set, typically "utf-8".</dd>
157 <dd>When a job is submitted to a printer class, contains the name of
158 the destination printer class. Otherwise this environment
159 variable will not be set.</dd>
161 <dt>CONTENT_TYPE</dt>
162 <dd>The MIME type associated with the file (e.g.
163 application/postscript).</dd>
165 <dt>CUPS_CACHEDIR</dt>
166 <dd>The directory where cache files can be stored. Cache files can be
167 used to retain information between jobs or files in a job.</dd>
169 <dt>CUPS_DATADIR</dt>
170 <dd>The directory where (read-only) CUPS data files can be found.</dd>
172 <dt>CUPS_FILETYPE</dt>
173 <dd>The type of file being printed: "job-sheet" for a banner page and
174 "document" for a regular print file.</dd>
176 <dt>CUPS_SERVERROOT</dt>
177 <dd>The root directory of the server.</dd>
180 <dd>The device-uri associated with the printer.</dd>
182 <dt>FINAL_CONTENT_TYPE</dt>
183 <dd>The MIME type associated with the printer (e.g.
184 application/vnd.cups-postscript).</dd>
187 <dd>The language locale associated with the job.</dd>
190 <dd>The full pathname of the PostScript Printer Description (PPD)
191 file for this printer.</dd>
194 <dd>The queue name of the class or printer.</dd>
197 <dd>The recommended amount of memory to use for Raster Image
198 Processors (RIPs).</dd>
201 <dd>The directory where temporary files should be created.</dd>
205 <h3><a name="MESSAGES">Communicating with the Scheduler</a></h3>
207 <p>Filters and backends communicate with the scheduler by writing messages
208 to the standard error file. The scheduler reads messages from all filters in
209 a job and processes the message based on its prefix. For example, the following
210 code sets the current printer state message to "Printing page 5":</p>
212 <pre class="example">
215 fprintf(stderr, "INFO: Printing page %d\n", page);
218 <p>Each message is a single line of text starting with one of the following
223 <dt>ALERT: message</dt>
224 <dd>Sets the printer-state-message attribute and adds the specified
225 message to the current error log file using the "alert" log level.</dd>
227 <dt>ATTR: attribute=value [attribute=value]</dt>
228 <dd>Sets the named printer or job attribute(s). Typically this is used
229 to set the <code>marker-colors</code>, <code>marker-high-levels</code>,
230 <code>marker-levels</code>, <code>marker-low-levels</code>,
231 <code>marker-message</code>, <code>marker-names</code>,
232 <code>marker-types</code>, <code>printer-alert</code>, and
233 <code>printer-alert-description</code> printer attributes. Standard
234 <code>marker-types</code> values are listed in <a href='#TABLE1'>Table
235 1</a>. String values need special handling - see <a href="#ATTR_STRINGS">Reporting Attribute String Values</a> below.</dd>
237 <dt>CRIT: message</dt>
238 <dd>Sets the printer-state-message attribute and adds the specified
239 message to the current error log file using the "critical" log
242 <dt>DEBUG: message</dt>
243 <dd>Sets the printer-state-message attribute and adds the specified
244 message to the current error log file using the "debug" log level.</dd>
246 <dt>DEBUG2: message</dt>
247 <dd>Sets the printer-state-message attribute and adds the specified
248 message to the current error log file using the "debug2" log level.</dd>
250 <dt>EMERG: message</dt>
251 <dd>Sets the printer-state-message attribute and adds the specified
252 message to the current error log file using the "emergency" log
255 <dt>ERROR: message</dt>
256 <dd>Sets the printer-state-message attribute and adds the specified
257 message to the current error log file using the "error" log level.
258 Use "ERROR:" messages for non-persistent processing errors.</dd>
260 <dt>INFO: message</dt>
261 <dd>Sets the printer-state-message attribute. If the current log level
262 is set to "debug2", also adds the specified message to the current error
263 log file using the "info" log level.</dd>
265 <dt>NOTICE: message</dt>
266 <dd>Sets the printer-state-message attribute and adds the specified
267 message to the current error log file using the "notice" log level.</dd>
269 <dt>PAGE: page-number #-copies</dt>
270 <dt>PAGE: total #-pages</dt>
271 <dd>Adds an entry to the current page log file. The first form adds
272 #-copies to the job-media-sheets-completed attribute. The second
273 form sets the job-media-sheets-completed attribute to #-pages.</dd>
275 <dt>PPD: keyword=value [keyword=value ...]</dt>
276 <dd>Changes or adds keywords to the printer's PPD file. Typically
277 this is used to update installable options or default media settings
278 based on the printer configuration.</dd>
280 <dt>STATE: + printer-state-reason [printer-state-reason ...]</dt>
281 <dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
282 <dd>Sets or clears printer-state-reason keywords for the current queue.
283 Typically this is used to indicate persistent media, ink, toner, and
284 configuration conditions or errors on a printer.
285 <a href='#TABLE2'>Table 2</a> lists some of the standard "printer-state-reasons" keywords from the <a href="http://www.iana.org/assignments/ipp-registrations/ipp-registrations.xhtml#ipp-registrations-4">IANA IPP Registry</a> -
286 use vendor-prefixed ("com.example.foo") keywords for custom states. See
287 <a href="#MANAGING_STATE">Managing Printer State in a Filter</a> for more
290 <dt>WARNING: message</dt>
291 <dd>Sets the printer-state-message attribute and adds the specified
292 message to the current error log file using the "warning" log
297 <p>Messages without one of these prefixes are treated as if they began with
298 the "DEBUG:" prefix string.</p>
300 <div class='table'><table width='80%' summary='Table 1: Standard marker-types Values'>
301 <caption>Table 1: <a name='TABLE1'>Standard marker-types Values</a></caption>
311 <td>Developer unit</td>
318 <td>fuser-cleaning-pad</td>
319 <td>Fuser cleaning pad</td>
331 <td>Photo conductor</td>
339 <td>Staple supply</td>
343 <td>Toner supply</td>
346 <td>transfer-unit</td>
347 <td>Transfer unit</td>
351 <td>Waste ink tank</td>
355 <td>Waste toner tank</td>
359 <td>Waste wax tank</td>
366 <div class='table'><table width='80%' summary='Table 2: Standard State Keywords'>
367 <caption>Table 2: <a name='TABLE2'>Standard State Keywords</a></caption>
376 <td>connecting-to-device</td>
377 <td>Connecting to printer but not printing yet.</td>
381 <td>The printer's cover is open.</td>
384 <td>input-tray-missing</td>
385 <td>The paper tray is missing.</td>
388 <td>marker-supply-empty</td>
389 <td>The printer is out of ink.</td>
392 <td>marker-supply-low</td>
393 <td>The printer is almost out of ink.</td>
396 <td>marker-waste-almost-full</td>
397 <td>The printer's waste bin is almost full.</td>
400 <td>marker-waste-full</td>
401 <td>The printer's waste bin is full.</td>
405 <td>The paper tray (any paper tray) is empty.</td>
409 <td>There is a paper jam.</td>
413 <td>The paper tray (any paper tray) is almost empty.</td>
416 <td>media-needed</td>
417 <td>The paper tray needs to be filled (for a job that is printing).</td>
421 <td>Stop the printer.</td>
425 <td>Unable to connect to printer.</td>
429 <td>The printer is out of toner.</td>
433 <td>The printer is low on toner.</td>
439 <h4><a name="ATTR_STRINGS">Reporting Attribute String Values</a></h4>
441 <p>When reporting string values using "ATTR:" messages, a filter or backend must take special care to appropriately quote those values. The scheduler uses the CUPS option parsing code for attributes, so the general syntax is:</p>
443 <pre class="example">
445 name=simple,simple,...
448 name='"complex value"','"complex value"',...
451 <p>Simple values are strings that do not contain spaces, quotes, backslashes, or the comma and can be placed verbatim in the "ATTR:" message, for example:</p>
453 <pre class="example">
454 int levels[4] = { 40, 50, 60, 70 }; /* CMYK */
456 fputs("ATTR: marker-colors=#00FFFF,#FF00FF,#FFFF00,#000000\n", stderr);
457 fputs("ATTR: marker-high-levels=100,100,100,100\n", stderr);
458 fprintf(stderr, "ATTR: marker-levels=%d,%d,%d,%d\n", levels[0], levels[1],
459 levels[2], levels[3], levels[4]);
460 fputs("ATTR: marker-low-levels=5,5,5,5\n", stderr);
461 fputs("ATTR: marker-types=toner,toner,toner,toner\n", stderr);
464 <p>Complex values that contains spaces, quotes, backslashes, or the comma must be quoted. For a single value a single set of quotes is sufficient:</p>
466 <pre class="example">
467 fputs("ATTR: marker-message='Levels shown are approximate.'\n", stderr);
470 <p>When multiple values are reported, each value must be enclosed by a set of single and double quotes:</p>
472 <pre class="example">
473 fputs("ATTR: marker-names='\"Cyan Toner\"','\"Magenta Toner\"',"
474 "'\"Yellow Toner\"','\"Black Toner\"'\n", stderr);
477 <p>The IPP backend includes a <var>quote_string</var> function that may be used to properly quote a complex value in an "ATTR:" message:</p>
479 <pre class="example">
480 static const char * /* O - Quoted string */
481 quote_string(const char *s, /* I - String */
482 char *q, /* I - Quoted string buffer */
483 size_t qsize) /* I - Size of quoted string buffer */
485 char *qptr, /* Pointer into string buffer */
486 *qend; /* End of string buffer */
490 qend = q + qsize - 5;
501 while (*s && qptr < qend)
503 if (*s == '\\' || *s == '\"' || *s == '\'')
505 if (qptr < (qend - 4))
527 <h4><a name="MANAGING_STATE">Managing Printer State in a Filter</a></h4>
529 <p>Filters are responsible for managing the state keywords they set using
530 "STATE:" messages. Typically you will update <em>all</em> of the keywords that
531 are used by the filter at startup, for example:</p>
533 <pre class="example">
534 if (foo_condition != 0)
535 fputs("STATE: +com.example.foo\n", stderr);
537 fputs("STATE: -com.example.foo\n", stderr);
539 if (bar_condition != 0)
540 fputs("STATE: +com.example.bar\n", stderr);
542 fputs("STATE: -com.example.bar\n", stderr);
545 <p>Then as conditions change, your filter sends "STATE: +keyword" or "STATE:
546 -keyword" messages as necessary to set or clear the corresponding keyword,
549 <p>State keywords are often used to notify the user of issues that span across
550 jobs, for example "media-empty-warning" that indicates one or more paper trays
551 are empty. These keywords should not be cleared unless the corresponding issue
552 no longer exists.</p>
554 <p>Filters should clear job-related keywords on startup and exit so that they
555 do not remain set between jobs. For example, "connecting-to-device" is a job
556 sub-state and not an issue that applies when a job is not printing.</p>
558 <blockquote><b>Note:</b>
560 <p>"STATE:" messages often provide visible alerts to the user. For example,
561 on macOS setting a printer-state-reason value with an "-error" or
562 "-warning" suffix will cause the printer's dock item to bounce if the
563 corresponding reason is localized with a cupsIPPReason keyword in the
564 printer's PPD file.</p>
566 <p>When providing a vendor-prefixed keyword, <em>always</em> provide the
567 corresponding standard keyword (if any) to allow clients to respond to the
568 condition correctly. For example, if you provide a vendor-prefixed keyword
569 for a low cyan ink condition ("com.example.cyan-ink-low") you must also set the
570 "marker-supply-low-warning" keyword. In such cases you should also refrain
571 from localizing the vendor-prefixed keyword in the PPD file - otherwise both
572 the generic and vendor-specific keyword will be shown in the user
577 <h4><a name="REPORTING_SUPPLIES">Reporting Supply Levels</a></h4>
579 <p>CUPS tracks several "marker-*" attributes for ink/toner supply level
580 reporting. These attributes allow applications to display the current supply
581 levels for a printer without printer-specific software. <a href="#TABLE3">Table 3</a> lists the marker attributes and what they represent.</p>
583 <p>Filters set marker attributes by sending "ATTR:" messages to stderr. For
584 example, a filter supporting an inkjet printer with black and tri-color ink
585 cartridges would use the following to initialize the supply attributes:</p>
587 <pre class="example">
588 fputs("ATTR: marker-colors=#000000,#00FFFF#FF00FF#FFFF00\n", stderr);
589 fputs("ATTR: marker-low-levels=5,10\n", stderr);
590 fputs("ATTR: marker-names=Black,Tri-Color\n", stderr);
591 fputs("ATTR: marker-types=ink,ink\n", stderr);
594 <p>Then periodically the filter queries the printer for its current supply
595 levels and updates them with a separate "ATTR:" message:</p>
597 <pre class="example">
598 int black_level, tri_level;
600 fprintf(stderr, "ATTR: marker-levels=%d,%d\n", black_level, tri_level);
603 <div class='table'><table width='80%' summary='Table 3: Supply Level Attributes'>
604 <caption>Table 3: <a name='TABLE3'>Supply Level Attributes</a></caption>
613 <td>marker-colors</td>
614 <td>A list of comma-separated colors; each color is either "none" or one or
615 more hex-encoded sRGB colors of the form "#RRGGBB".</td>
618 <td>marker-high-levels</td>
619 <td>A list of comma-separated "almost full" level values from 0 to 100; a
620 value of 100 should be used for supplies that are consumed/emptied like ink
624 <td>marker-levels</td>
625 <td>A list of comma-separated level values for each supply. A value of -1
626 indicates the level is unavailable, -2 indicates unknown, and -3 indicates
627 the level is unknown but has not yet reached capacity. Values from 0 to 100
628 indicate the corresponding percentage.</td>
631 <td>marker-low-levels</td>
632 <td>A list of comma-separated "almost empty" level values from 0 to 100; a
633 value of 0 should be used for supplies that are filled like waste ink
637 <td>marker-message</td>
638 <td>A human-readable supply status message for the user like "12 pages of
642 <td>marker-names</td>
643 <td>A list of comma-separated supply names like "Cyan Ink", "Fuser",
647 <td>marker-types</td>
648 <td>A list of comma-separated supply types; the types are listed in
649 <a href="#TABLE1">Table 1</a>.</td>
654 <h3><a name="COMMUNICATING_BACKEND">Communicating with the Backend</a></h3>
656 <p>Filters can communicate with the backend via the
657 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and
658 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
660 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function
661 reads data that has been sent back from the device and is typically used to
662 obtain status and configuration information. For example, the following code
663 polls the backend for back-channel data:</p>
665 <pre class="example">
666 #include <cups/cups.h>
671 /* Use a timeout of 0.0 seconds to poll for back-channel data */
672 bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
675 <p>Filters can also use <code>select()</code> or <code>poll()</code> on the
676 back-channel file descriptor (3 or <code>CUPS_BC_FD</code>) to read data only
677 when it is available.</p>
680 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
681 function allows you to get out-of-band status information and do synchronization
682 with the device. For example, the following code gets the current IEEE-1284
683 device ID string from the backend:</p>
685 <pre class="example">
686 #include <cups/sidechannel.h>
690 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
692 /* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for
693 nul-termination... */
694 datalen = sizeof(data) - 1;
696 /* Get the IEEE-1284 device ID, waiting for up to 1 second */
697 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &datalen, 1.0);
699 /* Use the returned value if OK was returned and the length is non-zero */
700 if (status == CUPS_SC_STATUS_OK && datalen > 0)
701 data[datalen] = '\0';
706 <h4><a name="DRAIN_OUTPUT">Forcing All Output to a Printer</a></h4>
709 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
710 function allows you to tell the backend to send all pending data to the printer.
711 This is most often needed when sending query commands to the printer. For example:</p>
713 <pre class="example">
714 #include <cups/cups.h>
715 #include <cups/sidechannel.h>
718 int datalen = sizeof(data);
719 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
721 /* Flush pending output to stdout */
724 /* Drain output to backend, waiting for up to 30 seconds */
725 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_DRAIN_OUTPUT, data, &datalen, 30.0);
727 /* Read the response if the output was sent */
728 if (status == CUPS_SC_STATUS_OK)
732 /* Wait up to 10.0 seconds for back-channel data */
733 bytes = cupsBackChannelRead(data, sizeof(data), 10.0);
734 /* do something with the data from the printer */
738 <h3><a name="COMMUNICATING_FILTER">Communicating with Filters</a></h3>
740 <p>Backends communicate with filters using the reciprocal functions
741 <a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>,
742 <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and
743 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We
744 recommend writing back-channel data using a timeout of 1.0 seconds:</p>
746 <pre class="example">
747 #include <cups/cups.h>
752 /* Obtain data from printer/device */
755 /* Use a timeout of 1.0 seconds to give filters a chance to read */
756 cupsBackChannelWrite(buffer, bytes, 1.0);
759 <p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>
760 function reads a side-channel command from a filter, driver, or port monitor.
761 Backends can either poll for commands using a <code>timeout</code> of 0.0, wait
762 indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a
763 separate thread for that purpose), or use <code>select</code> or
764 <code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle
765 input and output on several file descriptors at the same time.</p>
767 <p>Once a command is processed, the backend uses the
768 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function
769 to send its response. For example, the following code shows how to poll for a
770 side-channel command and respond to it:</p>
772 <pre class="example">
773 #include <cups/sidechannel.h>
775 <a href="#cups_sc_command_t">cups_sc_command_t</a> command;
776 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
778 int datalen = sizeof(data);
780 /* Poll for a command... */
781 if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&command, &status, data, &datalen, 0.0))
785 /* handle supported commands, fill data/datalen/status with values as needed */
788 status = CUPS_SC_STATUS_NOT_IMPLEMENTED;
793 /* Send a response... */
794 <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0);
798 <h3><a name="SNMP">Doing SNMP Queries with Network Printers</a></h3>
800 <p>The Simple Network Management Protocol (SNMP) allows you to get the current
801 status, page counter, and supply levels from most network printers. Every
802 piece of information is associated with an Object Identifier (OID), and
803 every printer has a <em>community</em> name associated with it. OIDs can be
804 queried directly or by "walking" over a range of OIDs with a common prefix.</p>
806 <p>The two CUPS SNMP functions provide a simple API for querying network
807 printers through the side-channel interface. Each accepts a string containing
808 an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID)
809 along with a timeout for the query.</p>
811 <p>The <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>
812 function queries a single OID and returns the value as a string in a buffer
815 <pre class="example">
816 #include <cups/sidechannel.h>
819 int datalen = sizeof(data);
821 if (<a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a>(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &datalen, 5.0)
822 == CUPS_SC_STATUS_OK)
824 /* Do something with the value */
825 printf("Page counter is: %s\n", data);
830 <a href="#cupsSideChannelSNMPWalk"><code>cupsSideChannelSNMPWalk</code></a>
831 function allows you to query a whole group of OIDs, calling a function of your
832 choice for each OID that is found:</p>
834 <pre class="example">
835 #include <cups/sidechannel.h>
838 my_callback(const char *oid, const char *data, int datalen, void *context)
840 /* Do something with the value */
841 printf("%s=%s\n", oid, data);
848 <a href="#cupsSideChannelSNMPWalk">cupsSNMPSideChannelWalk</a>(".1.3.6.1.2.1.43", 5.0, my_callback, my_data);
851 <h2><a name="SANDBOXING">Sandboxing on macOS</a></h2>
853 <p>Starting with macOS 10.6, filters and backends are run inside a security "sandbox" which further limits (beyond the normal UNIX user/group permissions) what a filter or backend can do. This helps to both secure the printing system from malicious software and enforce the functional separation of components in the CUPS filter chain. What follows is a list of actions that are explicitly allowed for all filters and backends:</p>
857 <li>Reading of files: pursuant to normal UNIX file permissions, filters and backends can read files for the current job from the <var>/private/var/spool/cups</var> directory and other files on mounted filesystems <em>except</em> for user home directories under <var>/Users</var>.</li>
859 <li>Writing of files: pursuant to normal UNIX file permissions, filters and backends can read/write files to the cache directory specified by the <code>CUPS_CACHEDIR</code> environment variable, to the state directory specified by the <code>CUPS_STATEDIR</code> environment variable, to the temporary directory specified by the <code>TMPDIR</code> environment variable, and under the <var>/private/var/db</var>, <var>/private/var/folders</var>, <var>/private/var/lib</var>, <var>/private/var/mysql</var>, <var>/private/var/run</var>, <var>/private/var/spool</var> (except <var>/private/var/spool/cups</var>), <var>/Library/Application Support</var>, <var>/Library/Caches</var>, <var>/Library/Logs</var>, <var>/Library/Preferences</var>, <var>/Library/WebServer</var>, and <var>/Users/Shared</var> directories.</li>
861 <li>Execution of programs: pursuant to normal UNIX file permissions, filters and backends can execute any program not located under the <var>/Users</var> directory. Child processes inherit the sandbox and are subject to the same restrictions as the parent.</li>
863 <li>Bluetooth and USB: backends can access Bluetooth and USB printers through IOKit. <em>Filters cannot access Bluetooth and USB printers directly.</em></li>
865 <li>Network: filters and backends can access UNIX domain sockets under the <var>/private/tmp</var>, <var>/private/var/run</var>, and <var>/private/var/tmp</var> directories. Backends can also create IPv4 and IPv6 TCP (outgoing) and UDP (incoming and outgoing) socket, and bind to local source ports. <em>Filters cannot directly create IPv4 and IPv6 TCP or UDP sockets.</em></li>
867 <li>Notifications: filters and backends can send notifications via the Darwin <code>notify_post()</code> API.</li>
871 <blockquote><b>Note:</b> The sandbox profile used in CUPS 2.0 still allows some actions that are not listed above - these privileges will be removed over time until the profile matches the list above.</blockquote>