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ef416fc2 | 1 | <!-- |
75bd9771 | 2 | "$Id: api-filter.shtml 7677 2008-06-19 23:22:19Z mike $" |
ef416fc2 | 3 | |
5a738aea MS |
4 | Filter and backend programming introduction for the Common UNIX Printing |
5 | System (CUPS). | |
ef416fc2 | 6 | |
178cb736 | 7 | Copyright 2007-2009 by Apple Inc. |
bc44d920 | 8 | Copyright 1997-2006 by Easy Software Products, all rights reserved. |
ef416fc2 | 9 | |
10 | These coded instructions, statements, and computer programs are the | |
bc44d920 | 11 | property of Apple Inc. and are protected by Federal copyright |
12 | law. Distribution and use rights are outlined in the file "LICENSE.txt" | |
13 | which should have been included with this file. If this file is | |
14 | file is missing or damaged, see the license at "http://www.cups.org/". | |
ef416fc2 | 15 | --> |
16 | ||
5a738aea | 17 | <h2 class='title'><a name="OVERVIEW">Overview</a></h2> |
ef416fc2 | 18 | |
79e1d494 MS |
19 | <p>Filters (which include printer drivers and port monitors) and backends |
20 | are used to convert job files to a printable format and send that data to the | |
21 | printer itself. All of these programs use a common interface for processing | |
22 | print jobs and communicating status information to the scheduler. Each is run | |
23 | with a standard set of command-line arguments:<p> | |
ef416fc2 | 24 | |
5a738aea | 25 | <dl class="code"> |
ef416fc2 | 26 | |
5a738aea MS |
27 | <dt>argv[1]</dt> |
28 | <dd>The job ID</dd> | |
ef416fc2 | 29 | |
5a738aea MS |
30 | <dt>argv[2]</dt> |
31 | <dd>The user printing the job</dd> | |
f7deaa1a | 32 | |
5a738aea MS |
33 | <dt>argv[3]</dt> |
34 | <dd>The job name/title</dd> | |
ef416fc2 | 35 | |
5a738aea MS |
36 | <dt>argv[4]</dt> |
37 | <dd>The number of copies to print</dd> | |
ef416fc2 | 38 | |
5a738aea MS |
39 | <dt>argv[5]</dt> |
40 | <dd>The options that were provided when the job was submitted</dd> | |
f7deaa1a | 41 | |
5a738aea | 42 | <dt>argv[6]</dt> |
79e1d494 | 43 | <dd>The file to print (first program only)</dd> |
5a738aea | 44 | </dl> |
ef416fc2 | 45 | |
5a738aea MS |
46 | <p>The scheduler runs one or more of these programs to print any given job. The |
47 | first filter reads from the print file and writes to the standard output, while | |
48 | the remaining filters read from the standard input and write to the standard | |
49 | output. The backend is the last filter in the chain and writes to the | |
50 | device.</p> | |
f7deaa1a | 51 | |
178cb736 MS |
52 | <p>Filters are always run as a non-privileged user, typically "lp", with no |
53 | connection to the user's desktop. Backends are run either as a non-privileged | |
54 | user or as root if the file permissions do not allow user or group execution. | |
55 | The <a href="#PERMISSIONS">file permissions</a> section talks about this in | |
56 | more detail.</p> | |
57 | ||
ac884b6a MS |
58 | <h3><a name="SECURITY">Security Considerations</a></h3> |
59 | ||
60 | <p>It is always important to use security programming practices. Filters and | |
61 | most backends are run as a non-priviledged user, so the major security | |
62 | consideration is resource utilization - filters should not depend on unlimited | |
63 | amounts of CPU, memory, or disk space, and should protect against conditions | |
64 | that could lead to excess usage of any resource like infinite loops and | |
65 | unbounded recursion. In addition, filters must <em>never</em> allow the user to | |
66 | specify an arbitrary file path to a separator page, template, or other file | |
67 | used by the filter since that can lead to an unauthorized disclosure of | |
68 | information. <em>Always</em> treat input as suspect and validate it!</p> | |
69 | ||
4d301e69 | 70 | <p>If you are developing a backend that runs as root, make sure to check for |
ac884b6a MS |
71 | potential buffer overflows, integer under/overflow conditions, and file |
72 | accesses since these can lead to privilege escalations. When writing files, | |
73 | always validate the file path and <em>never</em> allow a user to determine | |
74 | where to store a file.</p> | |
75 | ||
76 | <blockquote><b>Note:</b> | |
77 | ||
78 | <p><em>Never</em> write files to a user's home directory. Aside from the | |
79 | security implications, CUPS is a network print service and as such the network | |
80 | user may not be the same as the local user and/or there may not be a local home | |
81 | directory to write to.</p> | |
82 | ||
83 | <p>In addition, some operating systems provide additional security mechanisms | |
178cb736 | 84 | that further limit file system access, even for backends running as root. On |
ac884b6a MS |
85 | Mac OS X, for example, no backend may write to a user's home directory.</p> |
86 | </blockquote> | |
87 | ||
178cb736 MS |
88 | <h3><a name="PERMISSIONS">File Permissions</a></h3> |
89 | ||
90 | <p>For security reasons, CUPS will only run filters and backends that are owned | |
4d301e69 MS |
91 | by root and do not have world or group write permissions. The recommended |
92 | permissions for filters and backends are 0555 - read and execute but no write. | |
93 | Backends that must run as root should use permissions of 0500 - read and execute | |
94 | by root, no access for other users. Write permissions can be enabled for the | |
95 | root user only.</p> | |
178cb736 MS |
96 | |
97 | <p>To avoid a warning message, the directory containing your filter(s) must also | |
4d301e69 MS |
98 | be owned by root and have world and group write disabled - permissions of 0755 |
99 | or 0555 are strongly encouraged.</p> | |
178cb736 | 100 | |
ac884b6a MS |
101 | <h3><a name="TEMPFILES">Temporary Files</a></h3> |
102 | ||
103 | <p>Temporary files should be created in the directory specified by the | |
104 | "TMPDIR" environment variable. The | |
105 | <a href="#cupsTempFile2"><code>cupsTempFile2</code></a> function can be | |
106 | used to safely create temporary files in this directory.</p> | |
107 | ||
108 | <h3><a name="COPIES">Copy Generation</a></h3> | |
109 | ||
110 | <p>The <code>argv[4]</code> argument specifies the number of copies to produce | |
111 | of the input file. In general, you should only generate copies if the | |
112 | <em>filename</em> argument is supplied. The only exception to this are | |
113 | filters that produce device-independent PostScript output, since the PostScript | |
114 | filter <var>pstops</var> is responsible for generating copies of PostScript | |
115 | files.</p> | |
116 | ||
5a738aea | 117 | <h3><a name="EXITCODES">Exit Codes</a></h3> |
f7deaa1a | 118 | |
5a738aea MS |
119 | <p>Filters must exit with status 0 when they successfully generate print data |
120 | or 1 when they encounter an error. Backends can return any of the | |
121 | <a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p> | |
f7deaa1a | 122 | |
5a738aea | 123 | <h3><a name="ENVIRONMENT">Environment Variables</a></h3> |
f7deaa1a | 124 | |
79e1d494 MS |
125 | <p>The following environment variables are defined by the printing system |
126 | when running print filters and backends:</p> | |
f7deaa1a | 127 | |
5a738aea | 128 | <dl class="code"> |
f7deaa1a | 129 | |
acb056cb | 130 | <dt>APPLE_LANGUAGE</dt> |
5a738aea MS |
131 | <dd>The Apple language identifier associated with the job |
132 | (Mac OS X only).</dd> | |
f7deaa1a | 133 | |
5a738aea MS |
134 | <dt>CHARSET</dt> |
135 | <dd>The job character set, typically "utf-8".</dd> | |
f7deaa1a | 136 | |
5a738aea MS |
137 | <dt>CLASS</dt> |
138 | <dd>When a job is submitted to a printer class, contains the name of | |
139 | the destination printer class. Otherwise this environment | |
140 | variable will not be set.</dd> | |
f7deaa1a | 141 | |
5a738aea MS |
142 | <dt>CONTENT_TYPE</dt> |
143 | <dd>The MIME type associated with the file (e.g. | |
144 | application/postscript).</dd> | |
f7deaa1a | 145 | |
5a738aea | 146 | <dt>CUPS_CACHEDIR</dt> |
79e1d494 MS |
147 | <dd>The directory where cache files can be stored. Cache files can be |
148 | used to retain information between jobs or files in a job.</dd> | |
f7deaa1a | 149 | |
5a738aea | 150 | <dt>CUPS_DATADIR</dt> |
79e1d494 | 151 | <dd>The directory where (read-only) CUPS data files can be found.</dd> |
f7deaa1a | 152 | |
0af14961 MS |
153 | <dt>CUPS_FILETYPE</dt> |
154 | <dd>The type of file being printed: "job-sheet" for a banner page and | |
155 | "document" for a regular print file.</dd> | |
156 | ||
5a738aea MS |
157 | <dt>CUPS_SERVERROOT</dt> |
158 | <dd>The root directory of the server.</dd> | |
f7deaa1a | 159 | |
5a738aea MS |
160 | <dt>DEVICE_URI</dt> |
161 | <dd>The device-uri associated with the printer.</dd> | |
f7deaa1a | 162 | |
5a738aea MS |
163 | <dt>FINAL_CONTENT_TYPE</dt> |
164 | <dd>The MIME type associated with the printer (e.g. | |
165 | application/vnd.cups-postscript).</dd> | |
f7deaa1a | 166 | |
5a738aea MS |
167 | <dt>LANG</dt> |
168 | <dd>The language locale associated with the job.</dd> | |
f7deaa1a | 169 | |
5a738aea MS |
170 | <dt>PPD</dt> |
171 | <dd>The full pathname of the PostScript Printer Description (PPD) | |
172 | file for this printer.</dd> | |
f7deaa1a | 173 | |
5a738aea | 174 | <dt>PRINTER</dt> |
79e1d494 | 175 | <dd>The queue name of the class or printer.</dd> |
f7deaa1a | 176 | |
5a738aea MS |
177 | <dt>RIP_CACHE</dt> |
178 | <dd>The recommended amount of memory to use for Raster Image | |
179 | Processors (RIPs).</dd> | |
f7deaa1a | 180 | |
79e1d494 MS |
181 | <dt>TMPDIR</dt> |
182 | <dd>The directory where temporary files should be created.</dd> | |
183 | ||
5a738aea | 184 | </dl> |
f7deaa1a | 185 | |
5a738aea | 186 | <h3><a name="MESSAGES">Communicating with the Scheduler</a></h3> |
f7deaa1a | 187 | |
79e1d494 MS |
188 | <p>Filters and backends communicate with the scheduler by writing messages |
189 | to the standard error file. The scheduler reads messages from all filters in | |
190 | a job and processes the message based on its prefix. For example, the following | |
191 | code sets the current printer state message to "Printing page 5":</p> | |
f7deaa1a | 192 | |
5a738aea MS |
193 | <pre class="example"> |
194 | int page = 5; | |
f7deaa1a | 195 | |
5a738aea MS |
196 | fprintf(stderr, "INFO: Printing page %d\n", page); |
197 | </pre> | |
f7deaa1a | 198 | |
5a738aea MS |
199 | <p>Each message is a single line of text starting with one of the following |
200 | prefix strings:</p> | |
201 | ||
202 | <dl class="code"> | |
203 | ||
204 | <dt>ALERT: message</dt> | |
205 | <dd>Sets the printer-state-message attribute and adds the specified | |
206 | message to the current error log file using the "alert" log level.</dd> | |
207 | ||
208 | <dt>ATTR: attribute=value [attribute=value]</dt> | |
209 | <dd>Sets the named printer or job attribute(s). Typically this is used | |
210 | to set the <code>marker-colors</code>, <code>marker-levels</code>, | |
75bd9771 MS |
211 | <code>marker-message</code>, <code>marker-names</code>, |
212 | <code>marker-types</code>, <code>printer-alert</code>, and | |
213 | <code>printer-alert-description</code> printer attributes. Standard | |
214 | <code>marker-types</code> values are listed in <a href='#TABLE1'>Table | |
215 | 1</a>.</dd> | |
5a738aea MS |
216 | |
217 | <dt>CRIT: message</dt> | |
218 | <dd>Sets the printer-state-message attribute and adds the specified | |
219 | message to the current error log file using the "critical" log | |
220 | level.</dd> | |
221 | ||
222 | <dt>DEBUG: message</dt> | |
223 | <dd>Sets the printer-state-message attribute and adds the specified | |
224 | message to the current error log file using the "debug" log level.</dd> | |
225 | ||
226 | <dt>DEBUG2: message</dt> | |
227 | <dd>Sets the printer-state-message attribute and adds the specified | |
228 | message to the current error log file using the "debug2" log level.</dd> | |
229 | ||
230 | <dt>EMERG: message</dt> | |
231 | <dd>Sets the printer-state-message attribute and adds the specified | |
232 | message to the current error log file using the "emergency" log | |
233 | level.</dd> | |
234 | ||
235 | <dt>ERROR: message</dt> | |
236 | <dd>Sets the printer-state-message attribute and adds the specified | |
79e1d494 MS |
237 | message to the current error log file using the "error" log level. |
238 | Use "ERROR:" messages for non-persistent processing errors.</dd> | |
5a738aea MS |
239 | |
240 | <dt>INFO: message</dt> | |
241 | <dd>Sets the printer-state-message attribute. If the current log level | |
242 | is set to "debug2", also adds the specified message to the current error | |
243 | log file using the "info" log level.</dd> | |
244 | ||
245 | <dt>NOTICE: message</dt> | |
246 | <dd>Sets the printer-state-message attribute and adds the specified | |
247 | message to the current error log file using the "notice" log level.</dd> | |
248 | ||
249 | <dt>PAGE: page-number #-copies</dt> | |
250 | <dt>PAGE: total #-pages</dt> | |
251 | <dd>Adds an entry to the current page log file. The first form adds | |
252 | #-copies to the job-media-sheets-completed attribute. The second | |
253 | form sets the job-media-sheets-completed attribute to #-pages.</dd> | |
254 | ||
20fbc903 MS |
255 | <dt>PPD: keyword=value [keyword=value ...]</dt> |
256 | <dd>Changes or adds keywords to the printer's PPD file. Typically | |
257 | this is used to update installable options or default media settings | |
258 | based on the printer configuration.</dd> | |
259 | ||
5a738aea MS |
260 | <dt>STATE: printer-state-reason [printer-state-reason ...]</dt> |
261 | <dt>STATE: + printer-state-reason [printer-state-reason ...]</dt> | |
262 | <dt>STATE: - printer-state-reason [printer-state-reason ...]</dt> | |
263 | <dd>Sets, adds, or removes printer-state-reason keywords to the | |
79e1d494 MS |
264 | current queue. Typically this is used to indicate persistent media, |
265 | ink, toner, and configuration conditions or errors on a printer. | |
266 | <a href='#TABLE2'>Table 2</a> lists the standard state keywords - | |
acb056cb MS |
267 | use vendor-prefixed ("com.acme.foo") keywords for custom states. |
268 | ||
269 | <blockquote><b>Note:</b> | |
270 | ||
271 | <p>"STATE:" messages often provide visible alerts to the user. For example, on | |
272 | Mac OS X setting a printer-state-reason value with an "-error" or "-warning" | |
273 | suffix will cause the printer's dock item to bounce if the corresponding reason | |
274 | is localized with a cupsIPPReason keyword in the printer's PPD file.</p> | |
275 | ||
276 | </blockquote></dd> | |
5a738aea MS |
277 | |
278 | <dt>WARNING: message</dt> | |
279 | <dd>Sets the printer-state-message attribute and adds the specified | |
280 | message to the current error log file using the "warning" log | |
281 | level.</dd> | |
282 | ||
283 | </dl> | |
284 | ||
285 | <p>Messages without one of these prefixes are treated as if they began with | |
286 | the "DEBUG:" prefix string.</p> | |
287 | ||
79e1d494 MS |
288 | <div class='table'><table width='80%' summary='Table 1: Standard marker-types Values'> |
289 | <caption>Table 1: <a name='TABLE1'>Standard marker-types Values</a></caption> | |
290 | <thead> | |
291 | <tr> | |
292 | <th>marker-type</th> | |
293 | <th>Description</th> | |
294 | </tr> | |
295 | </thead> | |
296 | <tbody> | |
297 | <tr> | |
298 | <td>developer</td> | |
299 | <td>Developer unit</td> | |
300 | </tr> | |
301 | <tr> | |
302 | <td>fuser</td> | |
303 | <td>Fuser unit</td> | |
304 | </tr> | |
305 | <tr> | |
306 | <td>fuserCleaningPad</td> | |
307 | <td>Fuser cleaning pad</td> | |
308 | </tr> | |
309 | <tr> | |
310 | <td>fuserOil</td> | |
311 | <td>Fuser oil</td> | |
312 | </tr> | |
313 | <tr> | |
314 | <td>ink</td> | |
315 | <td>Ink supply</td> | |
316 | </tr> | |
317 | <tr> | |
318 | <td>opc</td> | |
319 | <td>Photo conductor</td> | |
320 | </tr> | |
321 | <tr> | |
322 | <td>solidWax</td> | |
323 | <td>Wax supply</td> | |
324 | </tr> | |
325 | <tr> | |
326 | <td>staples</td> | |
327 | <td>Staple supply</td> | |
328 | </tr> | |
329 | <tr> | |
330 | <td>toner</td> | |
331 | <td>Toner supply</td> | |
332 | </tr> | |
333 | <tr> | |
334 | <td>transferUnit</td> | |
335 | <td>Transfer unit</td> | |
336 | </tr> | |
337 | <tr> | |
338 | <td>wasteInk</td> | |
339 | <td>Waste ink tank</td> | |
340 | </tr> | |
341 | <tr> | |
342 | <td>wasteToner</td> | |
343 | <td>Waste toner tank</td> | |
344 | </tr> | |
345 | <tr> | |
346 | <td>wasteWax</td> | |
347 | <td>Waste wax tank</td> | |
348 | </tr> | |
349 | </tbody> | |
350 | </table></div> | |
351 | ||
352 | <br> | |
353 | ||
354 | <div class='table'><table width='80%' summary='Table 2: Standard State Keywords'> | |
355 | <caption>Table 2: <a name='TABLE2'>Standard State Keywords</a></caption> | |
356 | <thead> | |
357 | <tr> | |
358 | <th>Keyword</th> | |
359 | <th>Description</th> | |
360 | </tr> | |
361 | </thead> | |
362 | <tbody> | |
363 | <tr> | |
364 | <td>connecting-to-device</td> | |
365 | <td>Connecting to printer but not printing yet</td> | |
366 | </tr> | |
367 | <tr> | |
368 | <td>cover-open</td> | |
369 | <td>A cover is open on the printer</td> | |
370 | </tr> | |
371 | <tr> | |
372 | <td>input-tray-missing</td> | |
373 | <td>An input tray is missing from the printer</td> | |
374 | </tr> | |
375 | <tr> | |
376 | <td>marker-supply-empty</td> | |
377 | <td>Out of ink</td> | |
378 | </tr> | |
379 | <tr> | |
380 | <td>marker-supply-low</td> | |
381 | <td>Low on ink</td> | |
382 | </tr> | |
383 | <tr> | |
384 | <td>marker-waste-almost-full</td> | |
385 | <td>Waste tank almost full</td> | |
386 | </tr> | |
387 | <tr> | |
388 | <td>marker-waste-full</td> | |
389 | <td>Waste tank full</td> | |
390 | </tr> | |
391 | <tr> | |
392 | <td>media-empty</td> | |
393 | <td>Out of media</td> | |
394 | </tr> | |
395 | <tr> | |
396 | <td>media-jam</td> | |
397 | <td>Media is jammed in the printer</td> | |
398 | </tr> | |
399 | <tr> | |
400 | <td>media-low</td> | |
401 | <td>Low on media</td> | |
402 | </tr> | |
403 | <tr> | |
404 | <td>paused</td> | |
405 | <td>Stop the printer</td> | |
406 | </tr> | |
407 | <tr> | |
408 | <td>timed-out</td> | |
409 | <td>Unable to connect to printer</td> | |
410 | </tr> | |
411 | <tr> | |
412 | <td>toner-empty</td> | |
413 | <td>Out of toner</td> | |
414 | </tr> | |
415 | <tr> | |
416 | <td>toner-low</td> | |
417 | <td>Low on toner</td> | |
418 | </tr> | |
419 | </tbody> | |
420 | </table></div> | |
421 | ||
20fbc903 | 422 | <h3><a name="COMMUNICATING_BACKEND">Communicating with the Backend</a></h3> |
5a738aea MS |
423 | |
424 | <p>Filters can communicate with the backend via the | |
425 | <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and | |
426 | <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a> | |
427 | functions. The | |
428 | <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function | |
429 | reads data that has been sent back from the device and is typically used to | |
430 | obtain status and configuration information. For example, the following code | |
431 | polls the backend for back-channel data:</p> | |
432 | ||
433 | <pre class="example"> | |
434 | #include <cups/cups.h> | |
f7deaa1a | 435 | |
5a738aea MS |
436 | char buffer[8192]; |
437 | ssize_t bytes; | |
f7deaa1a | 438 | |
5a738aea MS |
439 | /* Use a timeout of 0.0 seconds to poll for back-channel data */ |
440 | bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0); | |
f7deaa1a | 441 | </pre> |
442 | ||
79e1d494 MS |
443 | <p>Filters can also use <code>select()</code> or <code>poll()</code> on the |
444 | back-channel file descriptor (3 or <code>CUPS_BC_FD</code>) to read data only | |
445 | when it is available.</p> | |
446 | ||
447 | <p>The | |
5a738aea MS |
448 | <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a> |
449 | function allows you to get out-of-band status information and do synchronization | |
450 | with the device. For example, the following code gets the current IEEE-1284 | |
451 | device ID string from the backend:</p> | |
f7deaa1a | 452 | |
5a738aea | 453 | <pre class="example"> |
f7deaa1a | 454 | #include <cups/sidechannel.h> |
455 | ||
456 | char data[2049]; | |
457 | int datalen; | |
5a738aea | 458 | <a href="#cups_sc_status_t">cups_sc_status_t</a> status; |
f7deaa1a | 459 | |
79e1d494 MS |
460 | /* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for |
461 | nul-termination... */ | |
f7deaa1a | 462 | datalen = sizeof(data) - 1; |
463 | ||
464 | /* Get the IEEE-1284 device ID, waiting for up to 1 second */ | |
5a738aea | 465 | status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &datalen, 1.0); |
f7deaa1a | 466 | |
467 | /* Use the returned value if OK was returned and the length is non-zero */ | |
468 | if (status == CUPS_SC_STATUS_OK && datalen > 0) | |
469 | data[datalen] = '\0'; | |
470 | else | |
471 | data[0] = '\0'; | |
472 | </pre> | |
473 | ||
20fbc903 MS |
474 | <h3><a name="COMMUNICATING_FILTER">Communicating with Filters</a></h3> |
475 | ||
5a738aea MS |
476 | <p>Backends communicate with filters using the reciprocal functions |
477 | <a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>, | |
478 | <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and | |
479 | <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We | |
480 | recommend writing back-channel data using a timeout of 1.0 seconds:</p> | |
f7deaa1a | 481 | |
5a738aea MS |
482 | <pre class="example"> |
483 | #include <cups/cups.h> | |
f7deaa1a | 484 | |
5a738aea MS |
485 | char buffer[8192]; |
486 | ssize_t bytes; | |
f7deaa1a | 487 | |
79e1d494 MS |
488 | /* Obtain data from printer/device */ |
489 | ... | |
490 | ||
5a738aea MS |
491 | /* Use a timeout of 1.0 seconds to give filters a chance to read */ |
492 | cupsBackChannelWrite(buffer, bytes, 1.0); | |
f7deaa1a | 493 | </pre> |
494 | ||
5a738aea MS |
495 | <p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a> |
496 | function reads a side-channel command from a filter, driver, or port monitor. | |
497 | Backends can either poll for commands using a <code>timeout</code> of 0.0, wait | |
498 | indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a | |
499 | separate thread for that purpose), or use <code>select</code> or | |
500 | <code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle | |
20fbc903 | 501 | input and output on several file descriptors at the same time.</p> |
5a738aea MS |
502 | |
503 | <p>Once a command is processed, the backend uses the | |
504 | <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function | |
505 | to send its response. For example, the following code shows how to poll for a | |
506 | side-channel command and respond to it:</p> | |
507 | ||
508 | <pre class="example"> | |
f7deaa1a | 509 | #include <cups/sidechannel.h> |
510 | ||
5a738aea MS |
511 | <a href="#cups_sc_command_t">cups_sc_command_t</a> command; |
512 | <a href="#cups_sc_status_t">cups_sc_status_t</a> status; | |
20fbc903 MS |
513 | char data[2048]; |
514 | int datalen = sizeof(data); | |
f7deaa1a | 515 | |
516 | /* Poll for a command... */ | |
20fbc903 | 517 | if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&command, &status, data, &datalen, 0.0)) |
f7deaa1a | 518 | { |
f7deaa1a | 519 | switch (command) |
520 | { | |
20fbc903 | 521 | /* handle supported commands, fill data/datalen/status with values as needed */ |
f7deaa1a | 522 | |
523 | default : | |
524 | status = CUPS_SC_STATUS_NOT_IMPLEMENTED; | |
525 | datalen = 0; | |
526 | break; | |
527 | } | |
528 | ||
529 | /* Send a response... */ | |
5a738aea | 530 | <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0); |
f7deaa1a | 531 | } |
532 | </pre> | |
ac884b6a MS |
533 | |
534 | <h3><a name="SNMP">Doing SNMP Queries with Network Printers</a></h3> | |
535 | ||
536 | <p>The Simple Network Management Protocol (SNMP) allows you to get the current | |
537 | status, page counter, and supply levels from most network printers. Every | |
538 | piece of information is associated with an Object Identifier (OID), and | |
539 | every printer has a <em>community</em> name associated with it. OIDs can be | |
540 | queried directly or by "walking" over a range of OIDs with a common prefix.</p> | |
541 | ||
20fbc903 MS |
542 | <p>The two CUPS SNMP functions provide a simple API for querying network |
543 | printers through the side-channel interface. Each accepts a string containing | |
544 | an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID) | |
545 | along with a timeout for the query.</p> | |
ac884b6a | 546 | |
20fbc903 MS |
547 | <p>The <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a> |
548 | function queries a single OID and returns the value as a string in a buffer | |
549 | you supply:</p> | |
ac884b6a MS |
550 | |
551 | <pre class="example"> | |
20fbc903 | 552 | #include <cups/sidechannel.h> |
ac884b6a | 553 | |
20fbc903 MS |
554 | char data[512]; |
555 | int datalen = sizeof(data); | |
ac884b6a | 556 | |
20fbc903 MS |
557 | if (<a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a>(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &datalen, 5.0) |
558 | == CUPS_SC_STATUS_OK) | |
ac884b6a MS |
559 | { |
560 | /* Do something with the value */ | |
20fbc903 | 561 | printf("Page counter is: %s\n", data); |
ac884b6a MS |
562 | } |
563 | </pre> | |
564 | ||
20fbc903 MS |
565 | <p>The |
566 | <a href="#cupsSideChannelSNMPWalk"><code>cupsSideChannelSNMPWalk</code></a> | |
567 | function allows you to query a whole group of OIDs, calling a function of your | |
568 | choice for each OID that is found:</p> | |
ac884b6a MS |
569 | |
570 | <pre class="example"> | |
20fbc903 | 571 | #include <cups/sidechannel.h> |
ac884b6a MS |
572 | |
573 | void | |
20fbc903 | 574 | my_callback(const char *oid, const char *data, int datalen, void *context) |
ac884b6a MS |
575 | { |
576 | /* Do something with the value */ | |
20fbc903 | 577 | printf("%s=%s\n", oid, data); |
ac884b6a MS |
578 | } |
579 | ||
20fbc903 MS |
580 | ... |
581 | ||
ac884b6a MS |
582 | void *my_data; |
583 | ||
20fbc903 | 584 | <a href="#cupsSideChannelSNMPWalk">cupsSNMPSideChannelWalk</a>(".1.3.6.1.2.1.43", 5.0, my_callback, my_data); |
ac884b6a | 585 | </pre> |