doc/help/api-filter.html

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 255 <!--
 256   "$Id: api-filter.header 7285 2008-02-01 23:57:39Z mike $"
 257
 258   Filter and backend programming header for the Common UNIX Printing System
 259   (CUPS).
 260
 261   Copyright 2008 by Apple Inc.
 262
 263   These coded instructions, statements, and computer programs are the
 264   property of Apple Inc. and are protected by Federal copyright
 265   law.  Distribution and use rights are outlined in the file "LICENSE.txt"
 266   which should have been included with this file.  If this file is
 267   file is missing or damaged, see the license at "http://www.cups.org/".
 268 -->
 269
 270 <h1 class="title">Filter and Backend Programming</h1>
 271
 272 <div class='summary'><table summary='General Information'>
 273 <thead>
 274 <tr>
 275         <th>Headers</th>
 276         <th>cups/backend.h<br>
 277         cups/sidechannel.h<br>
 278         cups/snmp.h</th>
 279 </tr>
 280 </thead>
 281 <tbody>
 282 <tr>
 283         <th>Library</th>
 284         <td>-lcups</td>
 285 </tr>
 286 <tr>
 287         <th>See Also</th>
 288         <td>Programming: <a href='api-overview.html' target='_top'>Introduction to CUPS Programming</a><br>
 289         Programming: <a href='api-cups.html' target='_top'>CUPS API</a><br>
 290         Programming: <a href='api-ppd.html' target='_top'>PPD API</a><br>
 291         Programming: <a href='api-raster.html' target='_top'>Raster API</a></td>
 292 </tr>
 293 </tbody>
 294 </table></div>
 295 <h2 class="title">Contents</h2>
 296 <ul class="contents">
 297 </li>
 298 <li><a href="#OVERVIEW">Overview</a><ul class="subcontents">
 299 <li><a href="#SECURITY">Security Considerations</a></li>
 300 <li><a href="#TEMPFILES">Temporary Files</a></li>
 301 <li><a href="#COPIES">Copy Generation</a></li>
 302 <li><a href="#EXITCODES">Exit Codes</a></li>
 303 <li><a href="#ENVIRONMENT">Environment Variables</a></li>
 304 <li><a href="#MESSAGES">Communicating with the Scheduler</a></li>
 305 <li><a href="#COMMUNICATING_BACKEND">Communicating with the Backend</a></li>
 306 <li><a href="#COMMUNICATING_FILTER">Communicating with Filters</a></li>
 307 <li><a href="#SNMP">Doing SNMP Queries with Network Printers</a></li>
 308 </ul></li>
 309 <li><a href="#FUNCTIONS">Functions</a><ul class="code">
 310 <li><a href="#cupsBackChannelRead" title="Read data from the backchannel.">cupsBackChannelRead</a></li>
 311 <li><a href="#cupsBackChannelWrite" title="Write data to the backchannel.">cupsBackChannelWrite</a></li>
 312 <li><a href="#cupsBackendDeviceURI" title="Get the device URI for a backend.">cupsBackendDeviceURI</a></li>
 313 <li><a href="#cupsSideChannelDoRequest" title="Send a side-channel command to a backend and wait for a response.">cupsSideChannelDoRequest</a></li>
 314 <li><a href="#cupsSideChannelRead" title="Read a side-channel message.">cupsSideChannelRead</a></li>
 315 <li><a href="#cupsSideChannelSNMPGet" title="Query a SNMP OID's value.">cupsSideChannelSNMPGet</a></li>
 316 <li><a href="#cupsSideChannelSNMPWalk" title="Query multiple SNMP OID values.">cupsSideChannelSNMPWalk</a></li>
 317 <li><a href="#cupsSideChannelWrite" title="Write a side-channel message.">cupsSideChannelWrite</a></li>
 318 </ul>
 319 <li><a href="#TYPES">Data Types</a><ul class="code">
 320         <li><a href="#cups_backend_t" title="Backend exit codes">cups_backend_t</a></li>
 321         <li><a href="#cups_sc_bidi_t" title="Bidirectional capabilities">cups_sc_bidi_t</a></li>
 322         <li><a href="#cups_sc_command_t" title="Request command codes">cups_sc_command_t</a></li>
 323         <li><a href="#cups_sc_state_t" title="Printer state bits">cups_sc_state_t</a></li>
 324         <li><a href="#cups_sc_status_t" title="Response status codes">cups_sc_status_t</a></li>
 325         <li><a href="#cups_sc_walk_func_t" title="SNMP walk callback">cups_sc_walk_func_t</a></li>
 326 </ul></li>
 327 <li><a href="#ENUMERATIONS">Constants</a><ul class="code">
 328         <li><a href="#cups_backend_e" title="Backend exit codes">cups_backend_e</a></li>
 329         <li><a href="#cups_sc_bidi_e" title="Bidirectional capabilities">cups_sc_bidi_e</a></li>
 330         <li><a href="#cups_sc_command_e" title="Request command codes">cups_sc_command_e</a></li>
 331         <li><a href="#cups_sc_state_e" title="Printer state bits">cups_sc_state_e</a></li>
 332         <li><a href="#cups_sc_status_e" title="Response status codes">cups_sc_status_e</a></li>
 333 </ul></li>
 334 </ul>
 335 <!--
 336   "$Id: api-filter.shtml 7502 2008-04-28 21:30:12Z mike $"
 337
 338   Filter and backend programming introduction for the Common UNIX Printing
 339   System (CUPS).
 340
 341   Copyright 2007-2008 by Apple Inc.
 342   Copyright 1997-2006 by Easy Software Products, all rights reserved.
 343
 344   These coded instructions, statements, and computer programs are the
 345   property of Apple Inc. and are protected by Federal copyright
 346   law.  Distribution and use rights are outlined in the file "LICENSE.txt"
 347   which should have been included with this file.  If this file is
 348   file is missing or damaged, see the license at "http://www.cups.org/".
 349 -->
 350
 351 <h2 class='title'><a name="OVERVIEW">Overview</a></h2>
 352
 353 <p>Filters, printer drivers, port monitors, and backends use a common interface
 354 for processing print jobs and communicating status information to the scheduler.
 355 Each filter is run with a standard set of command-line arguments:<p>
 356
 357 <dl class="code">
 358
 359         <dt>argv[1]</dt>
 360         <dd>The job ID</dd>
 361
 362         <dt>argv[2]</dt>
 363         <dd>The user printing the job</dd>
 364
 365         <dt>argv[3]</dt>
 366         <dd>The job name/title</dd>
 367
 368         <dt>argv[4]</dt>
 369         <dd>The number of copies to print</dd>
 370
 371         <dt>argv[5]</dt>
 372         <dd>The options that were provided when the job was submitted</dd>
 373
 374         <dt>argv[6]</dt>
 375         <dd>The file to print (first filter only)</dd>
 376 </dl>
 377
 378 <p>The scheduler runs one or more of these programs to print any given job. The
 379 first filter reads from the print file and writes to the standard output, while
 380 the remaining filters read from the standard input and write to the standard
 381 output. The backend is the last filter in the chain and writes to the
 382 device.</p>
 383
 384 <h3><a name="SECURITY">Security Considerations</a></h3>
 385
 386 <p>It is always important to use security programming practices. Filters and
 387 most backends are run as a non-priviledged user, so the major security
 388 consideration is resource utilization - filters should not depend on unlimited
 389 amounts of CPU, memory, or disk space, and should protect against conditions
 390 that could lead to excess usage of any resource like infinite loops and
 391 unbounded recursion. In addition, filters must <em>never</em> allow the user to
 392 specify an arbitrary file path to a separator page, template, or other file
 393 used by the filter since that can lead to an unauthorized disclosure of
 394 information. <em>Always</em> treat input as suspect and validate it!</p>
 395
 396 <p>If you are developing a backend that runs as root, make sure to check for
 397 potential buffer overflows, integer under/overflow conditions, and file
 398 accesses since these can lead to privilege escalations. When writing files,
 399 always validate the file path and <em>never</em> allow a user to determine
 400 where to store a file.</p>
 401
 402 <blockquote><b>Note:</b>
 403
 404 <p><em>Never</em> write files to a user's home directory. Aside from the
 405 security implications, CUPS is a network print service and as such the network
 406 user may not be the same as the local user and/or there may not be a local home
 407 directory to write to.</p>
 408
 409 <p>In addition, some operating systems provide additional security mechanisms
 410 that further limit file system access, even for backends running as root.  On
 411 Mac OS X, for example, no backend may write to a user's home directory.</p>
 412 </blockquote>
 413
 414 <h3><a name="TEMPFILES">Temporary Files</a></h3>
 415
 416 <p>Temporary files should be created in the directory specified by the
 417 "TMPDIR" environment variable. The
 418 <a href="#cupsTempFile2"><code>cupsTempFile2</code></a> function can be
 419 used to safely create temporary files in this directory.</p>
 420
 421 <h3><a name="COPIES">Copy Generation</a></h3>
 422
 423 <p>The <code>argv[4]</code> argument specifies the number of copies to produce
 424 of the input file. In general, you should only generate copies if the
 425 <em>filename</em> argument is supplied. The only exception to this are
 426 filters that produce device-independent PostScript output, since the PostScript
 427 filter <var>pstops</var> is responsible for generating copies of PostScript
 428 files.</p>
 429
 430 <h3><a name="EXITCODES">Exit Codes</a></h3>
 431
 432 <p>Filters must exit with status 0 when they successfully generate print data
 433 or 1 when they encounter an error. Backends can return any of the
 434 <a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p>
 435
 436 <h3><a name="ENVIRONMENT">Environment Variables</a></h3>
 437
 438 <p>The following environment variables are defined by the printing system:</p>
 439
 440 <dl class="code">
 441
 442         <dt>APPLE_LANGUAGES</dt>
 443         <dd>The Apple language identifier associated with the job
 444         (Mac OS X only).</dd>
 445
 446         <dt>CHARSET</dt>
 447         <dd>The job character set, typically "utf-8".</dd>
 448
 449         <dt>CLASS</dt>
 450         <dd>When a job is submitted to a printer class, contains the name of
 451         the destination printer class. Otherwise this environment
 452         variable will not be set.</dd>
 453
 454         <dt>CONTENT_TYPE</dt>
 455         <dd>The MIME type associated with the file (e.g.
 456         application/postscript).</dd>
 457
 458         <dt>CUPS_CACHEDIR</dt>
 459         <dd>The directory where cache files can be stored.</dd>
 460
 461         <dt>CUPS_DATADIR</dt>
 462         <dd>The directory where data files can be found.</dd>
 463
 464         <dt>CUPS_SERVERROOT</dt>
 465         <dd>The root directory of the server.</dd>
 466
 467         <dt>DEVICE_URI</dt>
 468         <dd>The device-uri associated with the printer.</dd>
 469
 470         <dt>FINAL_CONTENT_TYPE</dt>
 471         <dd>The MIME type associated with the printer (e.g.
 472         application/vnd.cups-postscript).</dd>
 473
 474         <dt>LANG</dt>
 475         <dd>The language locale associated with the job.</dd>
 476
 477         <dt>PPD</dt>
 478         <dd>The full pathname of the PostScript Printer Description (PPD)
 479         file for this printer.</dd>
 480
 481         <dt>PRINTER</dt>
 482         <dd>The name of the printer.</dd>
 483
 484         <dt>RIP_CACHE</dt>
 485         <dd>The recommended amount of memory to use for Raster Image
 486         Processors (RIPs).</dd>
 487
 488 </dl>
 489
 490 <h3><a name="MESSAGES">Communicating with the Scheduler</a></h3>
 491
 492 <p>Filters and backends communicate wih the scheduler by writing messages
 493 to the standard error file. For example, the following code sets the current
 494 printer state message to "Printing page 5":</p>
 495
 496 <pre class="example">
 497 int page = 5;
 498
 499 fprintf(stderr, "INFO: Printing page %d\n", page);
 500 </pre>
 501
 502 <p>Each message is a single line of text starting with one of the following
 503 prefix strings:</p>
 504
 505 <dl class="code">
 506
 507         <dt>ALERT: message</dt>
 508         <dd>Sets the printer-state-message attribute and adds the specified
 509         message to the current error log file using the "alert" log level.</dd>
 510
 511         <dt>ATTR: attribute=value [attribute=value]</dt>
 512         <dd>Sets the named printer or job attribute(s). Typically this is used
 513         to set the <code>marker-colors</code>, <code>marker-levels</code>,
 514         <code>marker-names</code>, <code>marker-types</code>,
 515         <code>printer-alert</code>, and <code>printer-alert-description</code>
 516         printer attributes.</dd>
 517
 518         <dt>CRIT: message</dt>
 519         <dd>Sets the printer-state-message attribute and adds the specified
 520         message to the current error log file using the "critical" log
 521         level.</dd>
 522
 523         <dt>DEBUG: message</dt>
 524         <dd>Sets the printer-state-message attribute and adds the specified
 525         message to the current error log file using the "debug" log level.</dd>
 526
 527         <dt>DEBUG2: message</dt>
 528         <dd>Sets the printer-state-message attribute and adds the specified
 529         message to the current error log file using the "debug2" log level.</dd>
 530
 531         <dt>EMERG: message</dt>
 532         <dd>Sets the printer-state-message attribute and adds the specified
 533         message to the current error log file using the "emergency" log
 534         level.</dd>
 535
 536         <dt>ERROR: message</dt>
 537         <dd>Sets the printer-state-message attribute and adds the specified
 538         message to the current error log file using the "error" log level.</dd>
 539
 540         <dt>INFO: message</dt>
 541         <dd>Sets the printer-state-message attribute. If the current log level
 542         is set to "debug2", also adds the specified message to the current error
 543         log file using the "info" log level.</dd>
 544
 545         <dt>NOTICE: message</dt>
 546         <dd>Sets the printer-state-message attribute and adds the specified
 547         message to the current error log file using the "notice" log level.</dd>
 548
 549         <dt>PAGE: page-number #-copies</dt>
 550         <dt>PAGE: total #-pages</dt>
 551         <dd>Adds an entry to the current page log file. The first form adds
 552         #-copies to the job-media-sheets-completed attribute. The second
 553         form sets the job-media-sheets-completed attribute to #-pages.</dd>
 554
 555         <dt>PPD: keyword=value [keyword=value ...]</dt>
 556         <dd>Changes or adds keywords to the printer's PPD file. Typically
 557         this is used to update installable options or default media settings
 558         based on the printer configuration.</dd>
 559
 560         <dt>STATE: printer-state-reason [printer-state-reason ...]</dt>
 561         <dt>STATE: + printer-state-reason [printer-state-reason ...]</dt>
 562         <dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
 563         <dd>Sets, adds, or removes printer-state-reason keywords to the
 564         current queue. Typically this is used to indicate media, ink, and
 565         toner conditions on a printer.</dd>
 566
 567         <dt>WARNING: message</dt>
 568         <dd>Sets the printer-state-message attribute and adds the specified
 569         message to the current error log file using the "warning" log
 570         level.</dd>
 571
 572 </dl>
 573
 574 <p>Messages without one of these prefixes are treated as if they began with
 575 the "DEBUG:" prefix string.</p>
 576
 577 <h3><a name="COMMUNICATING_BACKEND">Communicating with the Backend</a></h3>
 578
 579 <p>Filters can communicate with the backend via the
 580 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and
 581 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
 582 functions. The
 583 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function
 584 reads data that has been sent back from the device and is typically used to
 585 obtain status and configuration information. For example, the following code
 586 polls the backend for back-channel data:</p>
 587
 588 <pre class="example">
 589 #include &lt;cups/cups.h&gt;
 590
 591 char buffer[8192];
 592 ssize_t bytes;
 593
 594 /* Use a timeout of 0.0 seconds to poll for back-channel data */
 595 bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
 596 </pre>
 597
 598 The
 599 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
 600 function allows you to get out-of-band status information and do synchronization
 601 with the device. For example, the following code gets the current IEEE-1284
 602 device ID string from the backend:</p>
 603
 604 <pre class="example">
 605 #include &lt;cups/sidechannel.h&gt;
 606
 607 char data[2049];
 608 int datalen;
 609 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
 610
 611 /* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for nul-termination... */
 612 datalen = sizeof(data) - 1;
 613
 614 /* Get the IEEE-1284 device ID, waiting for up to 1 second */
 615 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &amp;datalen, 1.0);
 616
 617 /* Use the returned value if OK was returned and the length is non-zero */
 618 if (status == CUPS_SC_STATUS_OK && datalen > 0)
 619   data[datalen] = '\0';
 620 else
 621   data[0] = '\0';
 622 </pre>
 623
 624 <h3><a name="COMMUNICATING_FILTER">Communicating with Filters</a></h3>
 625
 626 <p>Backends communicate with filters using the reciprocal functions
 627 <a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>,
 628 <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and
 629 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We
 630 recommend writing back-channel data using a timeout of 1.0 seconds:</p>
 631
 632 <pre class="example">
 633 #include &lt;cups/cups.h&gt;
 634
 635 char buffer[8192];
 636 ssize_t bytes;
 637
 638 /* Use a timeout of 1.0 seconds to give filters a chance to read */
 639 cupsBackChannelWrite(buffer, bytes, 1.0);
 640 </pre>
 641
 642 <p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>
 643 function reads a side-channel command from a filter, driver, or port monitor.
 644 Backends can either poll for commands using a <code>timeout</code> of 0.0, wait
 645 indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a
 646 separate thread for that purpose), or use <code>select</code> or
 647 <code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle
 648 input and output on several file descriptors at the same time.</p>
 649
 650 <p>Once a command is processed, the backend uses the
 651 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function
 652 to send its response. For example, the following code shows how to poll for a
 653 side-channel command and respond to it:</p>
 654
 655 <pre class="example">
 656 #include &lt;cups/sidechannel.h&gt;
 657
 658 <a href="#cups_sc_command_t">cups_sc_command_t</a> command;
 659 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
 660 char data[2048];
 661 int datalen = sizeof(data);
 662
 663 /* Poll for a command... */
 664 if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&amp;command, &amp;status, data, &amp;datalen, 0.0))
 665 {
 666   switch (command)
 667   {
 668     /* handle supported commands, fill data/datalen/status with values as needed */
 669
 670     default :
 671         status  = CUPS_SC_STATUS_NOT_IMPLEMENTED;
 672         datalen = 0;
 673         break;
 674   }
 675
 676   /* Send a response... */
 677   <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0);
 678 }
 679 </pre>
 680
 681 <h3><a name="SNMP">Doing SNMP Queries with Network Printers</a></h3>
 682
 683 <p>The Simple Network Management Protocol (SNMP) allows you to get the current
 684 status, page counter, and supply levels from most network printers. Every
 685 piece of information is associated with an Object Identifier (OID), and
 686 every printer has a <em>community</em> name associated with it. OIDs can be
 687 queried directly or by "walking" over a range of OIDs with a common prefix.</p>
 688
 689 <p>The two CUPS SNMP functions provide a simple API for querying network
 690 printers through the side-channel interface. Each accepts a string containing
 691 an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID)
 692 along with a timeout for the query.</p>
 693
 694 <p>The <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>
 695 function queries a single OID and returns the value as a string in a buffer
 696 you supply:</p>
 697
 698 <pre class="example">
 699 #include &lt;cups/sidechannel.h&gt;
 700
 701 char data[512];
 702 int datalen = sizeof(data);
 703
 704 if (<a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a>(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &amp;datalen, 5.0)
 705         == CUPS_SC_STATUS_OK)
 706 {
 707   /* Do something with the value */
 708   printf("Page counter is: %s\n", data);
 709 }
 710 </pre>
 711
 712 <p>The
 713 <a href="#cupsSideChannelSNMPWalk"><code>cupsSideChannelSNMPWalk</code></a>
 714 function allows you to query a whole group of OIDs, calling a function of your
 715 choice for each OID that is found:</p>
 716
 717 <pre class="example">
 718 #include &lt;cups/sidechannel.h&gt;
 719
 720 void
 721 my_callback(const char *oid, const char *data, int datalen, void *context)
 722 {
 723   /* Do something with the value */
 724   printf("%s=%s\n", oid, data);
 725 }
 726
 727 ...
 728
 729 void *my_data;
 730
 731 <a href="#cupsSideChannelSNMPWalk">cupsSNMPSideChannelWalk</a>(".1.3.6.1.2.1.43", 5.0, my_callback, my_data);
 732 </pre>
 733 <h2 class="title"><a name="FUNCTIONS">Functions</a></h2>
 734 <h3 class="function"><span class="info">&nbsp;CUPS 1.2&nbsp;</span><a name="cupsBackChannelRead">cupsBackChannelRead</a></h3>
 735 <p class="description">Read data from the backchannel.</p>
 736 <p class="code">
 737 ssize_t cupsBackChannelRead (<br>
 738 &nbsp;&nbsp;&nbsp;&nbsp;char *buffer,<br>
 739 &nbsp;&nbsp;&nbsp;&nbsp;size_t bytes,<br>
 740 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
 741 );</p>
 742 <h4 class="parameters">Parameters</h4>
 743 <dl>
 744 <dt>buffer</dt>
 745 <dd class="description">Buffer to read</dd>
 746 <dt>bytes</dt>
 747 <dd class="description">Bytes to read</dd>
 748 <dt>timeout</dt>
 749 <dd class="description">Timeout in seconds</dd>
 750 </dl>
 751 <h4 class="returnvalue">Return Value</h4>
 752 <p class="description">Bytes read or -1 on error</p>
 753 <h4 class="discussion">Discussion</h4>
 754 <p class="discussion">Reads up to &quot;bytes&quot; bytes from the backchannel. The &quot;timeout&quot;
 755 parameter controls how many seconds to wait for the data - use
 756 0.0 to return immediately if there is no data, -1.0 to wait
 757 for data indefinitely.
 758
 759 </p>
 760 <h3 class="function"><span class="info">&nbsp;CUPS 1.2&nbsp;</span><a name="cupsBackChannelWrite">cupsBackChannelWrite</a></h3>
 761 <p class="description">Write data to the backchannel.</p>
 762 <p class="code">
 763 ssize_t cupsBackChannelWrite (<br>
 764 &nbsp;&nbsp;&nbsp;&nbsp;const char *buffer,<br>
 765 &nbsp;&nbsp;&nbsp;&nbsp;size_t bytes,<br>
 766 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
 767 );</p>
 768 <h4 class="parameters">Parameters</h4>
 769 <dl>
 770 <dt>buffer</dt>
 771 <dd class="description">Buffer to write</dd>
 772 <dt>bytes</dt>
 773 <dd class="description">Bytes to write</dd>
 774 <dt>timeout</dt>
 775 <dd class="description">Timeout in seconds</dd>
 776 </dl>
 777 <h4 class="returnvalue">Return Value</h4>
 778 <p class="description">Bytes written or -1 on error</p>
 779 <h4 class="discussion">Discussion</h4>
 780 <p class="discussion">Writes &quot;bytes&quot; bytes to the backchannel. The &quot;timeout&quot; parameter
 781 controls how many seconds to wait for the data to be written - use
 782 0.0 to return immediately if the data cannot be written, -1.0 to wait
 783 indefinitely.
 784
 785 </p>
 786 <h3 class="function"><a name="cupsBackendDeviceURI">cupsBackendDeviceURI</a></h3>
 787 <p class="description">Get the device URI for a backend.</p>
 788 <p class="code">
 789 const char *cupsBackendDeviceURI (<br>
 790 &nbsp;&nbsp;&nbsp;&nbsp;char **argv<br>
 791 );</p>
 792 <h4 class="parameters">Parameters</h4>
 793 <dl>
 794 <dt>argv</dt>
 795 <dd class="description">Command-line arguments</dd>
 796 </dl>
 797 <h4 class="returnvalue">Return Value</h4>
 798 <p class="description">Device URI or <code>NULL</code></p>
 799 <h4 class="discussion">Discussion</h4>
 800 <p class="discussion">The &quot;argv&quot; argument is the argv argument passed to main(). This
 801 function returns the device URI passed in the DEVICE_URI environment
 802 variable or the device URI passed in argv[0], whichever is found
 803 first.</p>
 804 <h3 class="function"><span class="info">&nbsp;CUPS 1.3&nbsp;</span><a name="cupsSideChannelDoRequest">cupsSideChannelDoRequest</a></h3>
 805 <p class="description">Send a side-channel command to a backend and wait for a response.</p>
 806 <p class="code">
 807 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelDoRequest (<br>
 808 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> command,<br>
 809 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
 810 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
 811 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
 812 );</p>
 813 <h4 class="parameters">Parameters</h4>
 814 <dl>
 815 <dt>command</dt>
 816 <dd class="description">Command to send</dd>
 817 <dt>data</dt>
 818 <dd class="description">Response data buffer pointer</dd>
 819 <dt>datalen</dt>
 820 <dd class="description">Size of data buffer on entry, number of bytes in buffer on return</dd>
 821 <dt>timeout</dt>
 822 <dd class="description">Timeout in seconds</dd>
 823 </dl>
 824 <h4 class="returnvalue">Return Value</h4>
 825 <p class="description">Status of command</p>
 826 <h4 class="discussion">Discussion</h4>
 827 <p class="discussion">This function is normally only called by filters, drivers, or port
 828 monitors in order to communicate with the backend used by the current
 829 printer.  Programs must be prepared to handle timeout or &quot;not
 830 implemented&quot; status codes, which indicate that the backend or device
 831 do not support the specified side-channel command.<br>
 832 <br>
 833 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
 834 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
 835 update the value to contain the number of data bytes in the buffer.
 836
 837 </p>
 838 <h3 class="function"><span class="info">&nbsp;CUPS 1.3&nbsp;</span><a name="cupsSideChannelRead">cupsSideChannelRead</a></h3>
 839 <p class="description">Read a side-channel message.</p>
 840 <p class="code">
 841 int cupsSideChannelRead (<br>
 842 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> *command,<br>
 843 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_status_t">cups_sc_status_t</a> *status,<br>
 844 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
 845 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
 846 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
 847 );</p>
 848 <h4 class="parameters">Parameters</h4>
 849 <dl>
 850 <dt>command</dt>
 851 <dd class="description">Command code</dd>
 852 <dt>status</dt>
 853 <dd class="description">Status code</dd>
 854 <dt>data</dt>
 855 <dd class="description">Data buffer pointer</dd>
 856 <dt>datalen</dt>
 857 <dd class="description">Size of data buffer on entry, number of bytes in buffer on return</dd>
 858 <dt>timeout</dt>
 859 <dd class="description">Timeout in seconds</dd>
 860 </dl>
 861 <h4 class="returnvalue">Return Value</h4>
 862 <p class="description">0 on success, -1 on error</p>
 863 <h4 class="discussion">Discussion</h4>
 864 <p class="discussion">This function is normally only called by backend programs to read
 865 commands from a filter, driver, or port monitor program.  The
 866 caller must be prepared to handle incomplete or invalid messages
 867 and return the corresponding status codes.<br>
 868 <br>
 869 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
 870 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
 871 update the value to contain the number of data bytes in the buffer.
 872
 873 </p>
 874 <h3 class="function"><span class="info">&nbsp;CUPS 1.4&nbsp;</span><a name="cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a></h3>
 875 <p class="description">Query a SNMP OID's value.</p>
 876 <p class="code">
 877 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPGet (<br>
 878 &nbsp;&nbsp;&nbsp;&nbsp;const char *oid,<br>
 879 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
 880 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
 881 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
 882 );</p>
 883 <h4 class="parameters">Parameters</h4>
 884 <dl>
 885 <dt>oid</dt>
 886 <dd class="description">OID to query</dd>
 887 <dt>data</dt>
 888 <dd class="description">Buffer for OID value</dd>
 889 <dt>datalen</dt>
 890 <dd class="description">Size of OID buffer on entry, size of value on return</dd>
 891 <dt>timeout</dt>
 892 <dd class="description">Timeout in seconds</dd>
 893 </dl>
 894 <h4 class="returnvalue">Return Value</h4>
 895 <p class="description">Query status</p>
 896 <h4 class="discussion">Discussion</h4>
 897 <p class="discussion">This function asks the backend to do a SNMP OID query on behalf of the
 898 filter, port monitor, or backend using the default community name.<br>
 899 <br>
 900 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
 901 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
 902 supported and must be converted to their numeric forms.<br>
 903 <br>
 904 On input, &quot;data&quot; and &quot;datalen&quot; provide the location and size of the
 905 buffer to hold the OID value as a string. HEX-String (binary) values are
 906 converted to hexadecimal strings representing the binary data, while
 907 NULL-Value and unknown OID types are returned as the empty string.
 908 The returned &quot;datalen&quot; does not include the trailing nul.
 909
 910 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
 911 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
 912 the printer does not respond to the SNMP query.
 913
 914 </p>
 915 <h3 class="function"><span class="info">&nbsp;CUPS 1.4&nbsp;</span><a name="cupsSideChannelSNMPWalk">cupsSideChannelSNMPWalk</a></h3>
 916 <p class="description">Query multiple SNMP OID values.</p>
 917 <p class="code">
 918 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPWalk (<br>
 919 &nbsp;&nbsp;&nbsp;&nbsp;const char *oid,<br>
 920 &nbsp;&nbsp;&nbsp;&nbsp;double timeout,<br>
 921 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_walk_func_t">cups_sc_walk_func_t</a> cb,<br>
 922 &nbsp;&nbsp;&nbsp;&nbsp;void *context<br>
 923 );</p>
 924 <h4 class="parameters">Parameters</h4>
 925 <dl>
 926 <dt>oid</dt>
 927 <dd class="description">First numeric OID to query</dd>
 928 <dt>timeout</dt>
 929 <dd class="description">Timeout for each query in seconds</dd>
 930 <dt>cb</dt>
 931 <dd class="description">Function to call with each value</dd>
 932 <dt>context</dt>
 933 <dd class="description">Application-defined pointer to send to callback</dd>
 934 </dl>
 935 <h4 class="returnvalue">Return Value</h4>
 936 <p class="description">Status of first query of <code>CUPS_SC_STATUS_OK</code> on success</p>
 937 <h4 class="discussion">Discussion</h4>
 938 <p class="discussion">This function asks the backend to do multiple SNMP OID queries on behalf
 939 of the filter, port monitor, or backend using the default community name.
 940 All OIDs under the &quot;parent&quot; OID are queried and the results are sent to
 941 the callback function you provide.<br>
 942 <br>
 943 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
 944 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
 945 supported and must be converted to their numeric forms.<br>
 946 <br>
 947 &quot;timeout&quot; specifies the timeout for each OID query. The total amount of
 948 time will depend on the number of OID values found and the time required
 949 for each query.<br>
 950 <br>
 951 &quot;cb&quot; provides a function to call for every value that is found. &quot;context&quot;
 952 is an application-defined pointer that is sent to the callback function
 953 along with the OID and current data. The data passed to the callback is the
 954 same as returned by <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>.
 955
 956 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
 957 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
 958 the printer does not respond to the first SNMP query.
 959
 960 </p>
 961 <h3 class="function"><span class="info">&nbsp;CUPS 1.3&nbsp;</span><a name="cupsSideChannelWrite">cupsSideChannelWrite</a></h3>
 962 <p class="description">Write a side-channel message.</p>
 963 <p class="code">
 964 int cupsSideChannelWrite (<br>
 965 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> command,<br>
 966 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_status_t">cups_sc_status_t</a> status,<br>
 967 &nbsp;&nbsp;&nbsp;&nbsp;const char *data,<br>
 968 &nbsp;&nbsp;&nbsp;&nbsp;int datalen,<br>
 969 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
 970 );</p>
 971 <h4 class="parameters">Parameters</h4>
 972 <dl>
 973 <dt>command</dt>
 974 <dd class="description">Command code</dd>
 975 <dt>status</dt>
 976 <dd class="description">Status code</dd>
 977 <dt>data</dt>
 978 <dd class="description">Data buffer pointer</dd>
 979 <dt>datalen</dt>
 980 <dd class="description">Number of bytes of data</dd>
 981 <dt>timeout</dt>
 982 <dd class="description">Timeout in seconds</dd>
 983 </dl>
 984 <h4 class="returnvalue">Return Value</h4>
 985 <p class="description">0 on success, -1 on error</p>
 986 <h4 class="discussion">Discussion</h4>
 987 <p class="discussion">This function is normally only called by backend programs to send
 988 responses to a filter, driver, or port monitor program.
 989
 990 </p>
 991 <h2 class="title"><a name="TYPES">Data Types</a></h2>
 992 <h3 class="typedef"><a name="cups_backend_t">cups_backend_t</a></h3>
 993 <p class="description">Backend exit codes</p>
 994 <p class="code">
 995 typedef enum <a href="#cups_backend_e">cups_backend_e</a> cups_backend_t;
 996 </p>
 997 <h3 class="typedef"><a name="cups_sc_bidi_t">cups_sc_bidi_t</a></h3>
 998 <p class="description">Bidirectional capabilities</p>
 999 <p class="code">
1000 typedef enum <a href="#cups_sc_bidi_e">cups_sc_bidi_e</a> cups_sc_bidi_t;
1001 </p>
1002 <h3 class="typedef"><a name="cups_sc_command_t">cups_sc_command_t</a></h3>
1003 <p class="description">Request command codes</p>
1004 <p class="code">
1005 typedef enum <a href="#cups_sc_command_e">cups_sc_command_e</a> cups_sc_command_t;
1006 </p>
1007 <h3 class="typedef"><a name="cups_sc_state_t">cups_sc_state_t</a></h3>
1008 <p class="description">Printer state bits</p>
1009 <p class="code">
1010 typedef enum <a href="#cups_sc_state_e">cups_sc_state_e</a> cups_sc_state_t;
1011 </p>
1012 <h3 class="typedef"><a name="cups_sc_status_t">cups_sc_status_t</a></h3>
1013 <p class="description">Response status codes</p>
1014 <p class="code">
1015 typedef enum <a href="#cups_sc_status_e">cups_sc_status_e</a> cups_sc_status_t;
1016 </p>
1017 <h3 class="typedef"><a name="cups_sc_walk_func_t">cups_sc_walk_func_t</a></h3>
1018 <p class="description">SNMP walk callback</p>
1019 <p class="code">
1020 typedef void (*cups_sc_walk_func_t)(const char *oid, const char *data, int datalen, void *context);
1021 </p>
1022 <h2 class="title"><a name="ENUMERATIONS">Constants</a></h2>
1023 <h3 class="enumeration"><a name="cups_backend_e">cups_backend_e</a></h3>
1024 <p class="description">Backend exit codes</p>
1025 <h4 class="constants">Constants</h4>
1026 <dl>
1027 <dt>CUPS_BACKEND_AUTH_REQUIRED </dt>
1028 <dd class="description">Job failed, authentication required</dd>
1029 <dt>CUPS_BACKEND_CANCEL </dt>
1030 <dd class="description">Job failed, cancel job</dd>
1031 <dt>CUPS_BACKEND_FAILED </dt>
1032 <dd class="description">Job failed, use error-policy</dd>
1033 <dt>CUPS_BACKEND_HOLD </dt>
1034 <dd class="description">Job failed, hold job</dd>
1035 <dt>CUPS_BACKEND_OK </dt>
1036 <dd class="description">Job completed successfully</dd>
1037 <dt>CUPS_BACKEND_STOP </dt>
1038 <dd class="description">Job failed, stop queue</dd>
1039 </dl>
1040 <h3 class="enumeration"><a name="cups_sc_bidi_e">cups_sc_bidi_e</a></h3>
1041 <p class="description">Bidirectional capabilities</p>
1042 <h4 class="constants">Constants</h4>
1043 <dl>
1044 <dt>CUPS_SC_BIDI_NOT_SUPPORTED </dt>
1045 <dd class="description">Bidirectional I/O is not supported</dd>
1046 <dt>CUPS_SC_BIDI_SUPPORTED </dt>
1047 <dd class="description">Bidirectional I/O is supported</dd>
1048 </dl>
1049 <h3 class="enumeration"><a name="cups_sc_command_e">cups_sc_command_e</a></h3>
1050 <p class="description">Request command codes</p>
1051 <h4 class="constants">Constants</h4>
1052 <dl>
1053 <dt>CUPS_SC_CMD_DRAIN_OUTPUT </dt>
1054 <dd class="description">Drain all pending output</dd>
1055 <dt>CUPS_SC_CMD_GET_BIDI </dt>
1056 <dd class="description">Return bidirectional capabilities</dd>
1057 <dt>CUPS_SC_CMD_GET_DEVICE_ID </dt>
1058 <dd class="description">Return the IEEE-1284 device ID</dd>
1059 <dt>CUPS_SC_CMD_GET_STATE </dt>
1060 <dd class="description">Return the device state</dd>
1061 <dt>CUPS_SC_CMD_SNMP_GET <span class="info">&nbsp;CUPS 1.4&nbsp;</span></dt>
1062 <dd class="description">Query an SNMP OID </dd>
1063 <dt>CUPS_SC_CMD_SNMP_GET_NEXT <span class="info">&nbsp;CUPS 1.4&nbsp;</span></dt>
1064 <dd class="description">Query the next SNMP OID </dd>
1065 <dt>CUPS_SC_CMD_SOFT_RESET </dt>
1066 <dd class="description">Do a soft reset</dd>
1067 </dl>
1068 <h3 class="enumeration"><a name="cups_sc_state_e">cups_sc_state_e</a></h3>
1069 <p class="description">Printer state bits</p>
1070 <h4 class="constants">Constants</h4>
1071 <dl>
1072 <dt>CUPS_SC_STATE_BUSY </dt>
1073 <dd class="description">Device is busy</dd>
1074 <dt>CUPS_SC_STATE_ERROR </dt>
1075 <dd class="description">Other error condition</dd>
1076 <dt>CUPS_SC_STATE_MARKER_EMPTY </dt>
1077 <dd class="description">Toner/ink out condition</dd>
1078 <dt>CUPS_SC_STATE_MARKER_LOW </dt>
1079 <dd class="description">Toner/ink low condition</dd>
1080 <dt>CUPS_SC_STATE_MEDIA_EMPTY </dt>
1081 <dd class="description">Paper out condition</dd>
1082 <dt>CUPS_SC_STATE_MEDIA_LOW </dt>
1083 <dd class="description">Paper low condition</dd>
1084 <dt>CUPS_SC_STATE_OFFLINE </dt>
1085 <dd class="description">Device is off-line</dd>
1086 <dt>CUPS_SC_STATE_ONLINE </dt>
1087 <dd class="description">Device is on-line</dd>
1088 </dl>
1089 <h3 class="enumeration"><a name="cups_sc_status_e">cups_sc_status_e</a></h3>
1090 <p class="description">Response status codes</p>
1091 <h4 class="constants">Constants</h4>
1092 <dl>
1093 <dt>CUPS_SC_STATUS_BAD_MESSAGE </dt>
1094 <dd class="description">The command/response message was invalid</dd>
1095 <dt>CUPS_SC_STATUS_IO_ERROR </dt>
1096 <dd class="description">An I/O error occurred</dd>
1097 <dt>CUPS_SC_STATUS_NONE </dt>
1098 <dd class="description">No status</dd>
1099 <dt>CUPS_SC_STATUS_NOT_IMPLEMENTED </dt>
1100 <dd class="description">Command not implemented</dd>
1101 <dt>CUPS_SC_STATUS_NO_RESPONSE </dt>
1102 <dd class="description">The device did not respond</dd>
1103 <dt>CUPS_SC_STATUS_OK </dt>
1104 <dd class="description">Operation succeeded</dd>
1105 <dt>CUPS_SC_STATUS_TIMEOUT </dt>
1106 <dd class="description">The backend did not respond</dd>
1107 <dt>CUPS_SC_STATUS_TOO_BIG </dt>
1108 <dd class="description">Response too big</dd>
1109 </dl>
1110 </div>
1111 </body>
1112 </html>