doc/help/api-filter.html

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 342 <div class='body'>
 343 <!--
 344   "$Id$"
 345
 346   Filter and backend programming header for the Common UNIX Printing System
 347   (CUPS).
 348
 349   Copyright 2008-2009 by Apple Inc.
 350
 351   These coded instructions, statements, and computer programs are the
 352   property of Apple Inc. and are protected by Federal copyright
 353   law.  Distribution and use rights are outlined in the file "LICENSE.txt"
 354   which should have been included with this file.  If this file is
 355   file is missing or damaged, see the license at "http://www.cups.org/".
 356 -->
 357
 358 <h1 class='title'>Filter and Backend Programming</h1>
 359
 360 <div class='summary'><table summary='General Information'>
 361 <thead>
 362 <tr>
 363         <th>Headers</th>
 364         <th>cups/backend.h<br>
 365         cups/sidechannel.h</th>
 366 </tr>
 367 </thead>
 368 <tbody>
 369 <tr>
 370         <th>Library</th>
 371         <td>-lcups</td>
 372 </tr>
 373 <tr>
 374         <th>See Also</th>
 375         <td>Programming: <a href='api-overview.html' target='_top'>Introduction to CUPS Programming</a><br>
 376         Programming: <a href='api-cups.html' target='_top'>CUPS API</a><br>
 377         Programming: <a href='api-ppd.html' target='_top'>PPD API</a><br>
 378         Programming: <a href='api-raster.html' target='_top'>Raster API</a><br>
 379         Programming: <a href='postscript-driver.html' target='_top'>Developing PostScript Printer Drivers</a><br>
 380         Programming: <a href='raster-driver.html' target='_top'>Developing Raster Printer Drivers</a><br>
 381         Specifications: <a href='spec-design.html' target='_top'>CUPS Design Description</a></td>
 382 </tr>
 383 </tbody>
 384 </table></div>
 385 <h2 class="title">Contents</h2>
 386 <ul class="contents">
 387 <li><a href="#OVERVIEW">Overview</a><ul class="subcontents">
 388         <li><a href="#SECURITY">Security Considerations</a></li>
 389         <li><a href="#SIGNALS">Signal Handling</a></li>
 390         <li><a href="#TEMPFILES">Temporary Files</a></li>
 391         <li><a href="#COPIES">Copy Generation</a></li>
 392         <li><a href="#EXITCODES">Exit Codes</a></li>
 393         <li><a href="#ENVIRONMENT">Environment Variables</a></li>
 394         <li><a href="#MESSAGES">Communicating with the Scheduler</a></li>
 395         <li><a href="#COMMUNICATING_BACKEND">Communicating with the Backend</a></li>
 396         <li><a href="#COMMUNICATING_FILTER">Communicating with Filters</a></li>
 397         <li><a href="#SNMP">Doing SNMP Queries with Network Printers</a></li>
 398 </ul></li>
 399 <li><a href="#FUNCTIONS">Functions</a><ul class="code">
 400         <li><a href="#cupsBackChannelRead" title="Read data from the backchannel.">cupsBackChannelRead</a></li>
 401         <li><a href="#cupsBackChannelWrite" title="Write data to the backchannel.">cupsBackChannelWrite</a></li>
 402         <li><a href="#cupsBackendDeviceURI" title="Get the device URI for a backend.">cupsBackendDeviceURI</a></li>
 403         <li><a href="#cupsBackendReport" title="Write a device line from a backend.">cupsBackendReport</a></li>
 404         <li><a href="#cupsSideChannelDoRequest" title="Send a side-channel command to a backend and wait for a response.">cupsSideChannelDoRequest</a></li>
 405         <li><a href="#cupsSideChannelRead" title="Read a side-channel message.">cupsSideChannelRead</a></li>
 406         <li><a href="#cupsSideChannelSNMPGet" title="Query a SNMP OID's value.">cupsSideChannelSNMPGet</a></li>
 407         <li><a href="#cupsSideChannelSNMPWalk" title="Query multiple SNMP OID values.">cupsSideChannelSNMPWalk</a></li>
 408         <li><a href="#cupsSideChannelWrite" title="Write a side-channel message.">cupsSideChannelWrite</a></li>
 409 </ul></li>
 410 <li><a href="#TYPES">Data Types</a><ul class="code">
 411         <li><a href="#cups_backend_t" title="Backend exit codes">cups_backend_t</a></li>
 412         <li><a href="#cups_sc_bidi_t" title="Bidirectional capabilities">cups_sc_bidi_t</a></li>
 413         <li><a href="#cups_sc_command_t" title="Request command codes">cups_sc_command_t</a></li>
 414         <li><a href="#cups_sc_connected_t" title="Connectivity values">cups_sc_connected_t</a></li>
 415         <li><a href="#cups_sc_state_t" title="Printer state bits">cups_sc_state_t</a></li>
 416         <li><a href="#cups_sc_status_t" title="Response status codes">cups_sc_status_t</a></li>
 417         <li><a href="#cups_sc_walk_func_t" title="SNMP walk callback">cups_sc_walk_func_t</a></li>
 418 </ul></li>
 419 <li><a href="#ENUMERATIONS">Constants</a><ul class="code">
 420         <li><a href="#cups_backend_e" title="Backend exit codes">cups_backend_e</a></li>
 421         <li><a href="#cups_sc_bidi_e" title="Bidirectional capability values">cups_sc_bidi_e</a></li>
 422         <li><a href="#cups_sc_command_e" title="Request command codes">cups_sc_command_e</a></li>
 423         <li><a href="#cups_sc_connected_e" title="Connectivity values">cups_sc_connected_e</a></li>
 424         <li><a href="#cups_sc_state_e" title="Printer state bits">cups_sc_state_e</a></li>
 425         <li><a href="#cups_sc_status_e" title="Response status codes">cups_sc_status_e</a></li>
 426 </ul></li>
 427 </ul>
 428 <!--
 429   "$Id$"
 430
 431   Filter and backend programming introduction for CUPS.
 432
 433   Copyright 2007-2011 by Apple Inc.
 434   Copyright 1997-2006 by Easy Software Products, all rights reserved.
 435
 436   These coded instructions, statements, and computer programs are the
 437   property of Apple Inc. and are protected by Federal copyright
 438   law.  Distribution and use rights are outlined in the file "LICENSE.txt"
 439   which should have been included with this file.  If this file is
 440   file is missing or damaged, see the license at "http://www.cups.org/".
 441 -->
 442
 443 <h2 class='title'><a name="OVERVIEW">Overview</a></h2>
 444
 445 <p>Filters (which include printer drivers and port monitors) and backends
 446 are used to convert job files to a printable format and send that data to the
 447 printer itself. All of these programs use a common interface for processing
 448 print jobs and communicating status information to the scheduler. Each is run
 449 with a standard set of command-line arguments:<p>
 450
 451 <dl class="code">
 452
 453         <dt>argv[1]</dt>
 454         <dd>The job ID</dd>
 455
 456         <dt>argv[2]</dt>
 457         <dd>The user printing the job</dd>
 458
 459         <dt>argv[3]</dt>
 460         <dd>The job name/title</dd>
 461
 462         <dt>argv[4]</dt>
 463         <dd>The number of copies to print</dd>
 464
 465         <dt>argv[5]</dt>
 466         <dd>The options that were provided when the job was submitted</dd>
 467
 468         <dt>argv[6]</dt>
 469         <dd>The file to print (first program only)</dd>
 470 </dl>
 471
 472 <p>The scheduler runs one or more of these programs to print any given job. The
 473 first filter reads from the print file and writes to the standard output, while
 474 the remaining filters read from the standard input and write to the standard
 475 output. The backend is the last filter in the chain and writes to the
 476 device.</p>
 477
 478 <p>Filters are always run as a non-privileged user, typically "lp", with no
 479 connection to the user's desktop. Backends are run either as a non-privileged
 480 user or as root if the file permissions do not allow user or group execution.
 481 The <a href="#PERMISSIONS">file permissions</a> section talks about this in
 482 more detail.</p>
 483
 484 <h3><a name="SECURITY">Security Considerations</a></h3>
 485
 486 <p>It is always important to use security programming practices. Filters and
 487 most backends are run as a non-privileged user, so the major security
 488 consideration is resource utilization - filters should not depend on unlimited
 489 amounts of CPU, memory, or disk space, and should protect against conditions
 490 that could lead to excess usage of any resource like infinite loops and
 491 unbounded recursion. In addition, filters must <em>never</em> allow the user to
 492 specify an arbitrary file path to a separator page, template, or other file
 493 used by the filter since that can lead to an unauthorized disclosure of
 494 information. <em>Always</em> treat input as suspect and validate it!</p>
 495
 496 <p>If you are developing a backend that runs as root, make sure to check for
 497 potential buffer overflows, integer under/overflow conditions, and file
 498 accesses since these can lead to privilege escalations. When writing files,
 499 always validate the file path and <em>never</em> allow a user to determine
 500 where to store a file.</p>
 501
 502 <blockquote><b>Note:</b>
 503
 504 <p><em>Never</em> write files to a user's home directory. Aside from the
 505 security implications, CUPS is a network print service and as such the network
 506 user may not be the same as the local user and/or there may not be a local home
 507 directory to write to.</p>
 508
 509 <p>In addition, some operating systems provide additional security mechanisms
 510 that further limit file system access, even for backends running as root. On
 511 Mac OS X, for example, no backend may write to a user's home directory.</p>
 512 </blockquote>
 513
 514 <h3><a name="SIGNALS">Signal Handling</a><h3>
 515
 516 <p>The scheduler sends <code>SIGTERM</code> when a printing job is canceled or
 517 held. Filters, backends, and port monitors <em>must</em> catch
 518 <code>SIGTERM</code> and perform any cleanup necessary to produce a valid output
 519 file or return the printer to a known good state. The recommended behavior is to
 520 end the output on the current page.</p>
 521
 522 <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 <code>ignore SIGPIPE</code> at the beginning of <code>main()</code> with the following function call:</p>
 523
 524 <pre class="example">
 525 #include &lt;signal.h&gt;>
 526
 527 ...
 528
 529 int
 530 main(int argc, char *argv[])
 531 {
 532   signal(SIGPIPE, SIG_IGN);
 533
 534   ...
 535 }
 536 </pre>
 537
 538 <h3><a name="PERMISSIONS">File Permissions</a></h3>
 539
 540 <p>For security reasons, CUPS will only run filters and backends that are owned
 541 by root and do not have world or group write permissions. The recommended
 542 permissions for filters and backends are 0555 - read and execute but no write.
 543 Backends that must run as root should use permissions of 0500 - read and execute
 544 by root, no access for other users. Write permissions can be enabled for the
 545 root user only.</p>
 546
 547 <p>To avoid a warning message, the directory containing your filter(s) must also
 548 be owned by root and have world and group write disabled - permissions of 0755
 549 or 0555 are strongly encouraged.</p>
 550
 551 <h3><a name="TEMPFILES">Temporary Files</a></h3>
 552
 553 <p>Temporary files should be created in the directory specified by the
 554 "TMPDIR" environment variable. The
 555 <a href="#cupsTempFile2"><code>cupsTempFile2</code></a> function can be
 556 used to safely create temporary files in this directory.</p>
 557
 558 <h3><a name="COPIES">Copy Generation</a></h3>
 559
 560 <p>The <code>argv[4]</code> argument specifies the number of copies to produce
 561 of the input file. In general, you should only generate copies if the
 562 <em>filename</em> argument is supplied. The only exception to this are
 563 filters that produce device-independent PostScript output, since the PostScript
 564 filter <var>pstops</var> is responsible for generating copies of PostScript
 565 files.</p>
 566
 567 <h3><a name="EXITCODES">Exit Codes</a></h3>
 568
 569 <p>Filters must exit with status 0 when they successfully generate print data
 570 or 1 when they encounter an error. Backends can return any of the
 571 <a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p>
 572
 573 <h3><a name="ENVIRONMENT">Environment Variables</a></h3>
 574
 575 <p>The following environment variables are defined by the printing system
 576 when running print filters and backends:</p>
 577
 578 <dl class="code">
 579
 580         <dt>APPLE_LANGUAGE</dt>
 581         <dd>The Apple language identifier associated with the job
 582         (Mac OS X only).</dd>
 583
 584         <dt>CHARSET</dt>
 585         <dd>The job character set, typically "utf-8".</dd>
 586
 587         <dt>CLASS</dt>
 588         <dd>When a job is submitted to a printer class, contains the name of
 589         the destination printer class. Otherwise this environment
 590         variable will not be set.</dd>
 591
 592         <dt>CONTENT_TYPE</dt>
 593         <dd>The MIME type associated with the file (e.g.
 594         application/postscript).</dd>
 595
 596         <dt>CUPS_CACHEDIR</dt>
 597         <dd>The directory where cache files can be stored. Cache files can be
 598         used to retain information between jobs or files in a job.</dd>
 599
 600         <dt>CUPS_DATADIR</dt>
 601         <dd>The directory where (read-only) CUPS data files can be found.</dd>
 602
 603         <dt>CUPS_FILETYPE</dt>
 604         <dd>The type of file being printed: "job-sheet" for a banner page and
 605         "document" for a regular print file.</dd>
 606
 607         <dt>CUPS_SERVERROOT</dt>
 608         <dd>The root directory of the server.</dd>
 609
 610         <dt>DEVICE_URI</dt>
 611         <dd>The device-uri associated with the printer.</dd>
 612
 613         <dt>FINAL_CONTENT_TYPE</dt>
 614         <dd>The MIME type associated with the printer (e.g.
 615         application/vnd.cups-postscript).</dd>
 616
 617         <dt>LANG</dt>
 618         <dd>The language locale associated with the job.</dd>
 619
 620         <dt>PPD</dt>
 621         <dd>The full pathname of the PostScript Printer Description (PPD)
 622         file for this printer.</dd>
 623
 624         <dt>PRINTER</dt>
 625         <dd>The queue name of the class or printer.</dd>
 626
 627         <dt>RIP_CACHE</dt>
 628         <dd>The recommended amount of memory to use for Raster Image
 629         Processors (RIPs).</dd>
 630
 631         <dt>TMPDIR</dt>
 632         <dd>The directory where temporary files should be created.</dd>
 633
 634 </dl>
 635
 636 <h3><a name="MESSAGES">Communicating with the Scheduler</a></h3>
 637
 638 <p>Filters and backends communicate with the scheduler by writing messages
 639 to the standard error file. The scheduler reads messages from all filters in
 640 a job and processes the message based on its prefix. For example, the following
 641 code sets the current printer state message to "Printing page 5":</p>
 642
 643 <pre class="example">
 644 int page = 5;
 645
 646 fprintf(stderr, "INFO: Printing page %d\n", page);
 647 </pre>
 648
 649 <p>Each message is a single line of text starting with one of the following
 650 prefix strings:</p>
 651
 652 <dl class="code">
 653
 654         <dt>ALERT: message</dt>
 655         <dd>Sets the printer-state-message attribute and adds the specified
 656         message to the current error log file using the "alert" log level.</dd>
 657
 658         <dt>ATTR: attribute=value [attribute=value]</dt>
 659         <dd>Sets the named printer or job attribute(s). Typically this is used
 660         to set the <code>marker-colors</code>, <code>marker-levels</code>,
 661         <code>marker-message</code>, <code>marker-names</code>,
 662         <code>marker-types</code>, <code>printer-alert</code>, and
 663         <code>printer-alert-description</code> printer attributes. Standard
 664         <code>marker-types</code> values are listed in <a href='#TABLE1'>Table
 665         1</a>.</dd>
 666
 667         <dt>CRIT: message</dt>
 668         <dd>Sets the printer-state-message attribute and adds the specified
 669         message to the current error log file using the "critical" log
 670         level.</dd>
 671
 672         <dt>DEBUG: message</dt>
 673         <dd>Sets the printer-state-message attribute and adds the specified
 674         message to the current error log file using the "debug" log level.</dd>
 675
 676         <dt>DEBUG2: message</dt>
 677         <dd>Sets the printer-state-message attribute and adds the specified
 678         message to the current error log file using the "debug2" log level.</dd>
 679
 680         <dt>EMERG: message</dt>
 681         <dd>Sets the printer-state-message attribute and adds the specified
 682         message to the current error log file using the "emergency" log
 683         level.</dd>
 684
 685         <dt>ERROR: message</dt>
 686         <dd>Sets the printer-state-message attribute and adds the specified
 687         message to the current error log file using the "error" log level.
 688         Use "ERROR:" messages for non-persistent processing errors.</dd>
 689
 690         <dt>INFO: message</dt>
 691         <dd>Sets the printer-state-message attribute. If the current log level
 692         is set to "debug2", also adds the specified message to the current error
 693         log file using the "info" log level.</dd>
 694
 695         <dt>NOTICE: message</dt>
 696         <dd>Sets the printer-state-message attribute and adds the specified
 697         message to the current error log file using the "notice" log level.</dd>
 698
 699         <dt>PAGE: page-number #-copies</dt>
 700         <dt>PAGE: total #-pages</dt>
 701         <dd>Adds an entry to the current page log file. The first form adds
 702         #-copies to the job-media-sheets-completed attribute. The second
 703         form sets the job-media-sheets-completed attribute to #-pages.</dd>
 704
 705         <dt>PPD: keyword=value [keyword=value ...]</dt>
 706         <dd>Changes or adds keywords to the printer's PPD file. Typically
 707         this is used to update installable options or default media settings
 708         based on the printer configuration.</dd>
 709
 710         <dt>STATE: printer-state-reason [printer-state-reason ...]</dt>
 711         <dt>STATE: + printer-state-reason [printer-state-reason ...]</dt>
 712         <dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
 713         <dd>Sets, adds, or removes printer-state-reason keywords to the
 714         current queue. Typically this is used to indicate persistent media,
 715         ink, toner, and configuration conditions or errors on a printer.
 716         <a href='#TABLE2'>Table 2</a> lists the standard state keywords -
 717         use vendor-prefixed ("com.acme.foo") keywords for custom states.
 718
 719         <blockquote><b>Note:</b>
 720
 721         <p>"STATE:" messages often provide visible alerts to the user. For example, on
 722         Mac OS X setting a printer-state-reason value with an "-error" or "-warning"
 723         suffix will cause the printer's dock item to bounce if the corresponding reason
 724         is localized with a cupsIPPReason keyword in the printer's PPD file.</p>
 725
 726         </blockquote></dd>
 727
 728         <dt>WARNING: message</dt>
 729         <dd>Sets the printer-state-message attribute and adds the specified
 730         message to the current error log file using the "warning" log
 731         level.</dd>
 732
 733 </dl>
 734
 735 <p>Messages without one of these prefixes are treated as if they began with
 736 the "DEBUG:" prefix string.</p>
 737
 738 <div class='table'><table width='80%' summary='Table 1: Standard marker-types Values'>
 739 <caption>Table 1: <a name='TABLE1'>Standard marker-types Values</a></caption>
 740 <thead>
 741 <tr>
 742         <th>marker-type</th>
 743         <th>Description</th>
 744 </tr>
 745 </thead>
 746 <tbody>
 747 <tr>
 748         <td>developer</td>
 749         <td>Developer unit</td>
 750 </tr>
 751 <tr>
 752         <td>fuser</td>
 753         <td>Fuser unit</td>
 754 </tr>
 755 <tr>
 756         <td>fuserCleaningPad</td>
 757         <td>Fuser cleaning pad</td>
 758 </tr>
 759 <tr>
 760         <td>fuserOil</td>
 761         <td>Fuser oil</td>
 762 </tr>
 763 <tr>
 764         <td>ink</td>
 765         <td>Ink supply</td>
 766 </tr>
 767 <tr>
 768         <td>opc</td>
 769         <td>Photo conductor</td>
 770 </tr>
 771 <tr>
 772         <td>solidWax</td>
 773         <td>Wax supply</td>
 774 </tr>
 775 <tr>
 776         <td>staples</td>
 777         <td>Staple supply</td>
 778 </tr>
 779 <tr>
 780         <td>toner</td>
 781         <td>Toner supply</td>
 782 </tr>
 783 <tr>
 784         <td>transferUnit</td>
 785         <td>Transfer unit</td>
 786 </tr>
 787 <tr>
 788         <td>wasteInk</td>
 789         <td>Waste ink tank</td>
 790 </tr>
 791 <tr>
 792         <td>wasteToner</td>
 793         <td>Waste toner tank</td>
 794 </tr>
 795 <tr>
 796         <td>wasteWax</td>
 797         <td>Waste wax tank</td>
 798 </tr>
 799 </tbody>
 800 </table></div>
 801
 802 <br>
 803
 804 <div class='table'><table width='80%' summary='Table 2: Standard State Keywords'>
 805 <caption>Table 2: <a name='TABLE2'>Standard State Keywords</a></caption>
 806 <thead>
 807 <tr>
 808         <th>Keyword</th>
 809         <th>Description</th>
 810 </tr>
 811 </thead>
 812 <tbody>
 813 <tr>
 814         <td>connecting-to-device</td>
 815         <td>Connecting to printer but not printing yet</td>
 816 </tr>
 817 <tr>
 818         <td>cover-open</td>
 819         <td>A cover is open on the printer</td>
 820 </tr>
 821 <tr>
 822         <td>input-tray-missing</td>
 823         <td>An input tray is missing from the printer</td>
 824 </tr>
 825 <tr>
 826         <td>marker-supply-empty</td>
 827         <td>Out of ink</td>
 828 </tr>
 829 <tr>
 830         <td>marker-supply-low</td>
 831         <td>Low on ink</td>
 832 </tr>
 833 <tr>
 834         <td>marker-waste-almost-full</td>
 835         <td>Waste tank almost full</td>
 836 </tr>
 837 <tr>
 838         <td>marker-waste-full</td>
 839         <td>Waste tank full</td>
 840 </tr>
 841 <tr>
 842         <td>media-empty</td>
 843         <td>Out of media</td>
 844 </tr>
 845 <tr>
 846         <td>media-jam</td>
 847         <td>Media is jammed in the printer</td>
 848 </tr>
 849 <tr>
 850         <td>media-low</td>
 851         <td>Low on media</td>
 852 </tr>
 853 <tr>
 854         <td>paused</td>
 855         <td>Stop the printer</td>
 856 </tr>
 857 <tr>
 858         <td>timed-out</td>
 859         <td>Unable to connect to printer</td>
 860 </tr>
 861 <tr>
 862         <td>toner-empty</td>
 863         <td>Out of toner</td>
 864 </tr>
 865 <tr>
 866         <td>toner-low</td>
 867         <td>Low on toner</td>
 868 </tr>
 869 </tbody>
 870 </table></div>
 871
 872 <h3><a name="COMMUNICATING_BACKEND">Communicating with the Backend</a></h3>
 873
 874 <p>Filters can communicate with the backend via the
 875 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and
 876 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
 877 functions. The
 878 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function
 879 reads data that has been sent back from the device and is typically used to
 880 obtain status and configuration information. For example, the following code
 881 polls the backend for back-channel data:</p>
 882
 883 <pre class="example">
 884 #include &lt;cups/cups.h&gt;
 885
 886 char buffer[8192];
 887 ssize_t bytes;
 888
 889 /* Use a timeout of 0.0 seconds to poll for back-channel data */
 890 bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
 891 </pre>
 892
 893 <p>Filters can also use <code>select()</code> or <code>poll()</code> on the
 894 back-channel file descriptor (3 or <code>CUPS_BC_FD</code>) to read data only
 895 when it is available.</p>
 896
 897 <p>The
 898 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
 899 function allows you to get out-of-band status information and do synchronization
 900 with the device. For example, the following code gets the current IEEE-1284
 901 device ID string from the backend:</p>
 902
 903 <pre class="example">
 904 #include &lt;cups/sidechannel.h&gt;
 905
 906 char data[2049];
 907 int datalen;
 908 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
 909
 910 /* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for
 911    nul-termination... */
 912 datalen = sizeof(data) - 1;
 913
 914 /* Get the IEEE-1284 device ID, waiting for up to 1 second */
 915 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &amp;datalen, 1.0);
 916
 917 /* Use the returned value if OK was returned and the length is non-zero */
 918 if (status == CUPS_SC_STATUS_OK && datalen > 0)
 919   data[datalen] = '\0';
 920 else
 921   data[0] = '\0';
 922 </pre>
 923
 924 <h3><a name="COMMUNICATING_FILTER">Communicating with Filters</a></h3>
 925
 926 <p>Backends communicate with filters using the reciprocal functions
 927 <a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>,
 928 <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and
 929 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We
 930 recommend writing back-channel data using a timeout of 1.0 seconds:</p>
 931
 932 <pre class="example">
 933 #include &lt;cups/cups.h&gt;
 934
 935 char buffer[8192];
 936 ssize_t bytes;
 937
 938 /* Obtain data from printer/device */
 939 ...
 940
 941 /* Use a timeout of 1.0 seconds to give filters a chance to read */
 942 cupsBackChannelWrite(buffer, bytes, 1.0);
 943 </pre>
 944
 945 <p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>
 946 function reads a side-channel command from a filter, driver, or port monitor.
 947 Backends can either poll for commands using a <code>timeout</code> of 0.0, wait
 948 indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a
 949 separate thread for that purpose), or use <code>select</code> or
 950 <code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle
 951 input and output on several file descriptors at the same time.</p>
 952
 953 <p>Once a command is processed, the backend uses the
 954 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function
 955 to send its response. For example, the following code shows how to poll for a
 956 side-channel command and respond to it:</p>
 957
 958 <pre class="example">
 959 #include &lt;cups/sidechannel.h&gt;
 960
 961 <a href="#cups_sc_command_t">cups_sc_command_t</a> command;
 962 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
 963 char data[2048];
 964 int datalen = sizeof(data);
 965
 966 /* Poll for a command... */
 967 if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&amp;command, &amp;status, data, &amp;datalen, 0.0))
 968 {
 969   switch (command)
 970   {
 971     /* handle supported commands, fill data/datalen/status with values as needed */
 972
 973     default :
 974         status  = CUPS_SC_STATUS_NOT_IMPLEMENTED;
 975         datalen = 0;
 976         break;
 977   }
 978
 979   /* Send a response... */
 980   <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0);
 981 }
 982 </pre>
 983
 984 <h3><a name="SNMP">Doing SNMP Queries with Network Printers</a></h3>
 985
 986 <p>The Simple Network Management Protocol (SNMP) allows you to get the current
 987 status, page counter, and supply levels from most network printers. Every
 988 piece of information is associated with an Object Identifier (OID), and
 989 every printer has a <em>community</em> name associated with it. OIDs can be
 990 queried directly or by "walking" over a range of OIDs with a common prefix.</p>
 991
 992 <p>The two CUPS SNMP functions provide a simple API for querying network
 993 printers through the side-channel interface. Each accepts a string containing
 994 an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID)
 995 along with a timeout for the query.</p>
 996
 997 <p>The <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>
 998 function queries a single OID and returns the value as a string in a buffer
 999 you supply:</p>
1000
1001 <pre class="example">
1002 #include &lt;cups/sidechannel.h&gt;
1003
1004 char data[512];
1005 int datalen = sizeof(data);
1006
1007 if (<a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a>(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &amp;datalen, 5.0)
1008         == CUPS_SC_STATUS_OK)
1009 {
1010   /* Do something with the value */
1011   printf("Page counter is: %s\n", data);
1012 }
1013 </pre>
1014
1015 <p>The
1016 <a href="#cupsSideChannelSNMPWalk"><code>cupsSideChannelSNMPWalk</code></a>
1017 function allows you to query a whole group of OIDs, calling a function of your
1018 choice for each OID that is found:</p>
1019
1020 <pre class="example">
1021 #include &lt;cups/sidechannel.h&gt;
1022
1023 void
1024 my_callback(const char *oid, const char *data, int datalen, void *context)
1025 {
1026   /* Do something with the value */
1027   printf("%s=%s\n", oid, data);
1028 }
1029
1030 ...
1031
1032 void *my_data;
1033
1034 <a href="#cupsSideChannelSNMPWalk">cupsSNMPSideChannelWalk</a>(".1.3.6.1.2.1.43", 5.0, my_callback, my_data);
1035 </pre>
1036 <h2 class="title"><a name="FUNCTIONS">Functions</a></h2>
1037 <h3 class="function"><span class="info">&nbsp;CUPS 1.2/Mac OS X 10.5&nbsp;</span><a name="cupsBackChannelRead">cupsBackChannelRead</a></h3>
1038 <p class="description">Read data from the backchannel.</p>
1039 <p class="code">
1040 ssize_t cupsBackChannelRead (<br>
1041 &nbsp;&nbsp;&nbsp;&nbsp;char *buffer,<br>
1042 &nbsp;&nbsp;&nbsp;&nbsp;size_t bytes,<br>
1043 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1044 );</p>
1045 <h4 class="parameters">Parameters</h4>
1046 <dl>
1047 <dt>buffer</dt>
1048 <dd class="description">Buffer to read into</dd>
1049 <dt>bytes</dt>
1050 <dd class="description">Bytes to read</dd>
1051 <dt>timeout</dt>
1052 <dd class="description">Timeout in seconds, typically 0.0 to poll</dd>
1053 </dl>
1054 <h4 class="returnvalue">Return Value</h4>
1055 <p class="description">Bytes read or -1 on error</p>
1056 <h4 class="discussion">Discussion</h4>
1057 <p class="discussion">Reads up to &quot;bytes&quot; bytes from the backchannel/backend. The &quot;timeout&quot;
1058 parameter controls how many seconds to wait for the data - use 0.0 to
1059 return immediately if there is no data, -1.0 to wait for data indefinitely.
1060
1061 </p>
1062 <h3 class="function"><span class="info">&nbsp;CUPS 1.2/Mac OS X 10.5&nbsp;</span><a name="cupsBackChannelWrite">cupsBackChannelWrite</a></h3>
1063 <p class="description">Write data to the backchannel.</p>
1064 <p class="code">
1065 ssize_t cupsBackChannelWrite (<br>
1066 &nbsp;&nbsp;&nbsp;&nbsp;const char *buffer,<br>
1067 &nbsp;&nbsp;&nbsp;&nbsp;size_t bytes,<br>
1068 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1069 );</p>
1070 <h4 class="parameters">Parameters</h4>
1071 <dl>
1072 <dt>buffer</dt>
1073 <dd class="description">Buffer to write</dd>
1074 <dt>bytes</dt>
1075 <dd class="description">Bytes to write</dd>
1076 <dt>timeout</dt>
1077 <dd class="description">Timeout in seconds, typically 1.0</dd>
1078 </dl>
1079 <h4 class="returnvalue">Return Value</h4>
1080 <p class="description">Bytes written or -1 on error</p>
1081 <h4 class="discussion">Discussion</h4>
1082 <p class="discussion">Writes &quot;bytes&quot; bytes to the backchannel/filter. The &quot;timeout&quot; parameter
1083 controls how many seconds to wait for the data to be written - use
1084 0.0 to return immediately if the data cannot be written, -1.0 to wait
1085 indefinitely.
1086
1087 </p>
1088 <h3 class="function"><span class="info">&nbsp;CUPS 1.2/Mac OS X 10.5&nbsp;</span><a name="cupsBackendDeviceURI">cupsBackendDeviceURI</a></h3>
1089 <p class="description">Get the device URI for a backend.</p>
1090 <p class="code">
1091 const char *cupsBackendDeviceURI (<br>
1092 &nbsp;&nbsp;&nbsp;&nbsp;char **argv<br>
1093 );</p>
1094 <h4 class="parameters">Parameters</h4>
1095 <dl>
1096 <dt>argv</dt>
1097 <dd class="description">Command-line arguments</dd>
1098 </dl>
1099 <h4 class="returnvalue">Return Value</h4>
1100 <p class="description">Device URI or <code>NULL</code></p>
1101 <h4 class="discussion">Discussion</h4>
1102 <p class="discussion">The &quot;argv&quot; argument is the argv argument passed to main(). This
1103 function returns the device URI passed in the DEVICE_URI environment
1104 variable or the device URI passed in argv[0], whichever is found
1105 first.
1106
1107 </p>
1108 <h3 class="function"><span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span><a name="cupsBackendReport">cupsBackendReport</a></h3>
1109 <p class="description">Write a device line from a backend.</p>
1110 <p class="code">
1111 void cupsBackendReport (<br>
1112 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_scheme,<br>
1113 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_uri,<br>
1114 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_make_and_model,<br>
1115 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_info,<br>
1116 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_id,<br>
1117 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_location<br>
1118 );</p>
1119 <h4 class="parameters">Parameters</h4>
1120 <dl>
1121 <dt>device_scheme</dt>
1122 <dd class="description">device-scheme string</dd>
1123 <dt>device_uri</dt>
1124 <dd class="description">device-uri string</dd>
1125 <dt>device_make_and_model</dt>
1126 <dd class="description">device-make-and-model string or <code>NULL</code></dd>
1127 <dt>device_info</dt>
1128 <dd class="description">device-info string or <code>NULL</code></dd>
1129 <dt>device_id</dt>
1130 <dd class="description">device-id string or <code>NULL</code></dd>
1131 <dt>device_location</dt>
1132 <dd class="description">device-location string or <code>NULL</code></dd>
1133 </dl>
1134 <h4 class="discussion">Discussion</h4>
1135 <p class="discussion">This function writes a single device line to stdout for a backend.
1136 It handles quoting of special characters in the device-make-and-model,
1137 device-info, device-id, and device-location strings.
1138
1139 </p>
1140 <h3 class="function"><span class="info">&nbsp;CUPS 1.3/Mac OS X 10.5&nbsp;</span><a name="cupsSideChannelDoRequest">cupsSideChannelDoRequest</a></h3>
1141 <p class="description">Send a side-channel command to a backend and wait for a response.</p>
1142 <p class="code">
1143 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelDoRequest (<br>
1144 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> command,<br>
1145 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
1146 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
1147 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1148 );</p>
1149 <h4 class="parameters">Parameters</h4>
1150 <dl>
1151 <dt>command</dt>
1152 <dd class="description">Command to send</dd>
1153 <dt>data</dt>
1154 <dd class="description">Response data buffer pointer</dd>
1155 <dt>datalen</dt>
1156 <dd class="description">Size of data buffer on entry, number of bytes in buffer on return</dd>
1157 <dt>timeout</dt>
1158 <dd class="description">Timeout in seconds</dd>
1159 </dl>
1160 <h4 class="returnvalue">Return Value</h4>
1161 <p class="description">Status of command</p>
1162 <h4 class="discussion">Discussion</h4>
1163 <p class="discussion">This function is normally only called by filters, drivers, or port
1164 monitors in order to communicate with the backend used by the current
1165 printer.  Programs must be prepared to handle timeout or &quot;not
1166 implemented&quot; status codes, which indicate that the backend or device
1167 do not support the specified side-channel command.<br>
1168 <br>
1169 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
1170 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
1171 update the value to contain the number of data bytes in the buffer.
1172
1173 </p>
1174 <h3 class="function"><span class="info">&nbsp;CUPS 1.3/Mac OS X 10.5&nbsp;</span><a name="cupsSideChannelRead">cupsSideChannelRead</a></h3>
1175 <p class="description">Read a side-channel message.</p>
1176 <p class="code">
1177 int cupsSideChannelRead (<br>
1178 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> *command,<br>
1179 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_status_t">cups_sc_status_t</a> *status,<br>
1180 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
1181 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
1182 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1183 );</p>
1184 <h4 class="parameters">Parameters</h4>
1185 <dl>
1186 <dt>command</dt>
1187 <dd class="description">Command code</dd>
1188 <dt>status</dt>
1189 <dd class="description">Status code</dd>
1190 <dt>data</dt>
1191 <dd class="description">Data buffer pointer</dd>
1192 <dt>datalen</dt>
1193 <dd class="description">Size of data buffer on entry, number of bytes in buffer on return</dd>
1194 <dt>timeout</dt>
1195 <dd class="description">Timeout in seconds</dd>
1196 </dl>
1197 <h4 class="returnvalue">Return Value</h4>
1198 <p class="description">0 on success, -1 on error</p>
1199 <h4 class="discussion">Discussion</h4>
1200 <p class="discussion">This function is normally only called by backend programs to read
1201 commands from a filter, driver, or port monitor program.  The
1202 caller must be prepared to handle incomplete or invalid messages
1203 and return the corresponding status codes.<br>
1204 <br>
1205 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
1206 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
1207 update the value to contain the number of data bytes in the buffer.
1208
1209 </p>
1210 <h3 class="function"><span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span><a name="cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a></h3>
1211 <p class="description">Query a SNMP OID's value.</p>
1212 <p class="code">
1213 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPGet (<br>
1214 &nbsp;&nbsp;&nbsp;&nbsp;const char *oid,<br>
1215 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
1216 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
1217 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1218 );</p>
1219 <h4 class="parameters">Parameters</h4>
1220 <dl>
1221 <dt>oid</dt>
1222 <dd class="description">OID to query</dd>
1223 <dt>data</dt>
1224 <dd class="description">Buffer for OID value</dd>
1225 <dt>datalen</dt>
1226 <dd class="description">Size of OID buffer on entry, size of value on return</dd>
1227 <dt>timeout</dt>
1228 <dd class="description">Timeout in seconds</dd>
1229 </dl>
1230 <h4 class="returnvalue">Return Value</h4>
1231 <p class="description">Query status</p>
1232 <h4 class="discussion">Discussion</h4>
1233 <p class="discussion">This function asks the backend to do a SNMP OID query on behalf of the
1234 filter, port monitor, or backend using the default community name.<br>
1235 <br>
1236 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
1237 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
1238 supported and must be converted to their numeric forms.<br>
1239 <br>
1240 On input, &quot;data&quot; and &quot;datalen&quot; provide the location and size of the
1241 buffer to hold the OID value as a string. HEX-String (binary) values are
1242 converted to hexadecimal strings representing the binary data, while
1243 NULL-Value and unknown OID types are returned as the empty string.
1244 The returned &quot;datalen&quot; does not include the trailing nul.
1245
1246 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
1247 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
1248 the printer does not respond to the SNMP query.
1249
1250 </p>
1251 <h3 class="function"><span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span><a name="cupsSideChannelSNMPWalk">cupsSideChannelSNMPWalk</a></h3>
1252 <p class="description">Query multiple SNMP OID values.</p>
1253 <p class="code">
1254 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPWalk (<br>
1255 &nbsp;&nbsp;&nbsp;&nbsp;const char *oid,<br>
1256 &nbsp;&nbsp;&nbsp;&nbsp;double timeout,<br>
1257 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_walk_func_t">cups_sc_walk_func_t</a> cb,<br>
1258 &nbsp;&nbsp;&nbsp;&nbsp;void *context<br>
1259 );</p>
1260 <h4 class="parameters">Parameters</h4>
1261 <dl>
1262 <dt>oid</dt>
1263 <dd class="description">First numeric OID to query</dd>
1264 <dt>timeout</dt>
1265 <dd class="description">Timeout for each query in seconds</dd>
1266 <dt>cb</dt>
1267 <dd class="description">Function to call with each value</dd>
1268 <dt>context</dt>
1269 <dd class="description">Application-defined pointer to send to callback</dd>
1270 </dl>
1271 <h4 class="returnvalue">Return Value</h4>
1272 <p class="description">Status of first query of <code>CUPS_SC_STATUS_OK</code> on success</p>
1273 <h4 class="discussion">Discussion</h4>
1274 <p class="discussion">This function asks the backend to do multiple SNMP OID queries on behalf
1275 of the filter, port monitor, or backend using the default community name.
1276 All OIDs under the &quot;parent&quot; OID are queried and the results are sent to
1277 the callback function you provide.<br>
1278 <br>
1279 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
1280 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
1281 supported and must be converted to their numeric forms.<br>
1282 <br>
1283 &quot;timeout&quot; specifies the timeout for each OID query. The total amount of
1284 time will depend on the number of OID values found and the time required
1285 for each query.<br>
1286 <br>
1287 &quot;cb&quot; provides a function to call for every value that is found. &quot;context&quot;
1288 is an application-defined pointer that is sent to the callback function
1289 along with the OID and current data. The data passed to the callback is the
1290 same as returned by <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>.
1291
1292 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
1293 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
1294 the printer does not respond to the first SNMP query.
1295
1296 </p>
1297 <h3 class="function"><span class="info">&nbsp;CUPS 1.3/Mac OS X 10.5&nbsp;</span><a name="cupsSideChannelWrite">cupsSideChannelWrite</a></h3>
1298 <p class="description">Write a side-channel message.</p>
1299 <p class="code">
1300 int cupsSideChannelWrite (<br>
1301 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> command,<br>
1302 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_status_t">cups_sc_status_t</a> status,<br>
1303 &nbsp;&nbsp;&nbsp;&nbsp;const char *data,<br>
1304 &nbsp;&nbsp;&nbsp;&nbsp;int datalen,<br>
1305 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1306 );</p>
1307 <h4 class="parameters">Parameters</h4>
1308 <dl>
1309 <dt>command</dt>
1310 <dd class="description">Command code</dd>
1311 <dt>status</dt>
1312 <dd class="description">Status code</dd>
1313 <dt>data</dt>
1314 <dd class="description">Data buffer pointer</dd>
1315 <dt>datalen</dt>
1316 <dd class="description">Number of bytes of data</dd>
1317 <dt>timeout</dt>
1318 <dd class="description">Timeout in seconds</dd>
1319 </dl>
1320 <h4 class="returnvalue">Return Value</h4>
1321 <p class="description">0 on success, -1 on error</p>
1322 <h4 class="discussion">Discussion</h4>
1323 <p class="discussion">This function is normally only called by backend programs to send
1324 responses to a filter, driver, or port monitor program.
1325
1326 </p>
1327 <h2 class="title"><a name="TYPES">Data Types</a></h2>
1328 <h3 class="typedef"><a name="cups_backend_t">cups_backend_t</a></h3>
1329 <p class="description">Backend exit codes</p>
1330 <p class="code">
1331 typedef enum <a href="#cups_backend_e">cups_backend_e</a> cups_backend_t;
1332 </p>
1333 <h3 class="typedef"><a name="cups_sc_bidi_t">cups_sc_bidi_t</a></h3>
1334 <p class="description">Bidirectional capabilities</p>
1335 <p class="code">
1336 typedef enum <a href="#cups_sc_bidi_e">cups_sc_bidi_e</a> cups_sc_bidi_t;
1337 </p>
1338 <h3 class="typedef"><a name="cups_sc_command_t">cups_sc_command_t</a></h3>
1339 <p class="description">Request command codes</p>
1340 <p class="code">
1341 typedef enum <a href="#cups_sc_command_e">cups_sc_command_e</a> cups_sc_command_t;
1342 </p>
1343 <h3 class="typedef"><a name="cups_sc_connected_t">cups_sc_connected_t</a></h3>
1344 <p class="description">Connectivity values</p>
1345 <p class="code">
1346 typedef enum <a href="#cups_sc_connected_e">cups_sc_connected_e</a> cups_sc_connected_t;
1347 </p>
1348 <h3 class="typedef"><a name="cups_sc_state_t">cups_sc_state_t</a></h3>
1349 <p class="description">Printer state bits</p>
1350 <p class="code">
1351 typedef enum <a href="#cups_sc_state_e">cups_sc_state_e</a> cups_sc_state_t;
1352 </p>
1353 <h3 class="typedef"><a name="cups_sc_status_t">cups_sc_status_t</a></h3>
1354 <p class="description">Response status codes</p>
1355 <p class="code">
1356 typedef enum <a href="#cups_sc_status_e">cups_sc_status_e</a> cups_sc_status_t;
1357 </p>
1358 <h3 class="typedef"><a name="cups_sc_walk_func_t">cups_sc_walk_func_t</a></h3>
1359 <p class="description">SNMP walk callback</p>
1360 <p class="code">
1361 typedef void (*cups_sc_walk_func_t)(const char *oid, const char *data, int datalen, void *context);
1362 </p>
1363 <h2 class="title"><a name="ENUMERATIONS">Constants</a></h2>
1364 <h3 class="enumeration"><a name="cups_backend_e">cups_backend_e</a></h3>
1365 <p class="description">Backend exit codes</p>
1366 <h4 class="constants">Constants</h4>
1367 <dl>
1368 <dt>CUPS_BACKEND_AUTH_REQUIRED </dt>
1369 <dd class="description">Job failed, authentication required</dd>
1370 <dt>CUPS_BACKEND_CANCEL </dt>
1371 <dd class="description">Job failed, cancel job</dd>
1372 <dt>CUPS_BACKEND_FAILED </dt>
1373 <dd class="description">Job failed, use error-policy</dd>
1374 <dt>CUPS_BACKEND_HOLD </dt>
1375 <dd class="description">Job failed, hold job</dd>
1376 <dt>CUPS_BACKEND_OK </dt>
1377 <dd class="description">Job completed successfully</dd>
1378 <dt>CUPS_BACKEND_RETRY </dt>
1379 <dd class="description">Job failed, retry this job later</dd>
1380 <dt>CUPS_BACKEND_RETRY_CURRENT </dt>
1381 <dd class="description">Job failed, retry this job immediately</dd>
1382 <dt>CUPS_BACKEND_STOP </dt>
1383 <dd class="description">Job failed, stop queue</dd>
1384 </dl>
1385 <h3 class="enumeration"><a name="cups_sc_bidi_e">cups_sc_bidi_e</a></h3>
1386 <p class="description">Bidirectional capability values</p>
1387 <h4 class="constants">Constants</h4>
1388 <dl>
1389 <dt>CUPS_SC_BIDI_NOT_SUPPORTED </dt>
1390 <dd class="description">Bidirectional I/O is not supported</dd>
1391 <dt>CUPS_SC_BIDI_SUPPORTED </dt>
1392 <dd class="description">Bidirectional I/O is supported</dd>
1393 </dl>
1394 <h3 class="enumeration"><a name="cups_sc_command_e">cups_sc_command_e</a></h3>
1395 <p class="description">Request command codes</p>
1396 <h4 class="constants">Constants</h4>
1397 <dl>
1398 <dt>CUPS_SC_CMD_DRAIN_OUTPUT </dt>
1399 <dd class="description">Drain all pending output</dd>
1400 <dt>CUPS_SC_CMD_GET_BIDI </dt>
1401 <dd class="description">Return bidirectional capabilities</dd>
1402 <dt>CUPS_SC_CMD_GET_CONNECTED <span class="info">&nbsp;CUPS 1.5&nbsp;</span></dt>
1403 <dd class="description">Return whether the backend is &quot;connected&quot; to the printer </dd>
1404 <dt>CUPS_SC_CMD_GET_DEVICE_ID </dt>
1405 <dd class="description">Return the IEEE-1284 device ID</dd>
1406 <dt>CUPS_SC_CMD_GET_STATE </dt>
1407 <dd class="description">Return the device state</dd>
1408 <dt>CUPS_SC_CMD_SNMP_GET <span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span></dt>
1409 <dd class="description">Query an SNMP OID </dd>
1410 <dt>CUPS_SC_CMD_SNMP_GET_NEXT <span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span></dt>
1411 <dd class="description">Query the next SNMP OID </dd>
1412 <dt>CUPS_SC_CMD_SOFT_RESET </dt>
1413 <dd class="description">Do a soft reset</dd>
1414 </dl>
1415 <h3 class="enumeration"><a name="cups_sc_connected_e">cups_sc_connected_e</a></h3>
1416 <p class="description">Connectivity values</p>
1417 <h4 class="constants">Constants</h4>
1418 <dl>
1419 <dt>CUPS_SC_CONNECTED </dt>
1420 <dd class="description">Backend is &quot;connected&quot; to printer</dd>
1421 <dt>CUPS_SC_NOT_CONNECTED </dt>
1422 <dd class="description">Backend is not &quot;connected&quot; to printer</dd>
1423 </dl>
1424 <h3 class="enumeration"><a name="cups_sc_state_e">cups_sc_state_e</a></h3>
1425 <p class="description">Printer state bits</p>
1426 <h4 class="constants">Constants</h4>
1427 <dl>
1428 <dt>CUPS_SC_STATE_BUSY </dt>
1429 <dd class="description">Device is busy</dd>
1430 <dt>CUPS_SC_STATE_ERROR </dt>
1431 <dd class="description">Other error condition</dd>
1432 <dt>CUPS_SC_STATE_MARKER_EMPTY </dt>
1433 <dd class="description">Toner/ink out condition</dd>
1434 <dt>CUPS_SC_STATE_MARKER_LOW </dt>
1435 <dd class="description">Toner/ink low condition</dd>
1436 <dt>CUPS_SC_STATE_MEDIA_EMPTY </dt>
1437 <dd class="description">Paper out condition</dd>
1438 <dt>CUPS_SC_STATE_MEDIA_LOW </dt>
1439 <dd class="description">Paper low condition</dd>
1440 <dt>CUPS_SC_STATE_OFFLINE </dt>
1441 <dd class="description">Device is offline</dd>
1442 <dt>CUPS_SC_STATE_ONLINE </dt>
1443 <dd class="description">Device is online</dd>
1444 </dl>
1445 <h3 class="enumeration"><a name="cups_sc_status_e">cups_sc_status_e</a></h3>
1446 <p class="description">Response status codes</p>
1447 <h4 class="constants">Constants</h4>
1448 <dl>
1449 <dt>CUPS_SC_STATUS_BAD_MESSAGE </dt>
1450 <dd class="description">The command/response message was invalid</dd>
1451 <dt>CUPS_SC_STATUS_IO_ERROR </dt>
1452 <dd class="description">An I/O error occurred</dd>
1453 <dt>CUPS_SC_STATUS_NONE </dt>
1454 <dd class="description">No status</dd>
1455 <dt>CUPS_SC_STATUS_NOT_IMPLEMENTED </dt>
1456 <dd class="description">Command not implemented</dd>
1457 <dt>CUPS_SC_STATUS_NO_RESPONSE </dt>
1458 <dd class="description">The device did not respond</dd>
1459 <dt>CUPS_SC_STATUS_OK </dt>
1460 <dd class="description">Operation succeeded</dd>
1461 <dt>CUPS_SC_STATUS_TIMEOUT </dt>
1462 <dd class="description">The backend did not respond</dd>
1463 <dt>CUPS_SC_STATUS_TOO_BIG </dt>
1464 <dd class="description">Response too big</dd>
1465 </dl>
1466 </div>
1467 </body>
1468 </html>