doc/help/api-filter.html

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 342 <div class='body'>
 343 <!--
 344   "$Id$"
 345
 346   Filter and backend programming header for CUPS.
 347
 348   Copyright 2008-2011 by Apple Inc.
 349
 350   These coded instructions, statements, and computer programs are the
 351   property of Apple Inc. and are protected by Federal copyright
 352   law.  Distribution and use rights are outlined in the file "LICENSE.txt"
 353   which should have been included with this file.  If this file is
 354   file is missing or damaged, see the license at "http://www.cups.org/".
 355 -->
 356
 357 <h1 class='title'>Filter and Backend Programming</h1>
 358
 359 <div class='summary'><table summary='General Information'>
 360 <thead>
 361 <tr>
 362         <th>Headers</th>
 363         <th>cups/backend.h<br>
 364         cups/sidechannel.h</th>
 365 </tr>
 366 </thead>
 367 <tbody>
 368 <tr>
 369         <th>Library</th>
 370         <td>-lcups</td>
 371 </tr>
 372 <tr>
 373         <th>See Also</th>
 374         <td>Programming: <a href='api-overview.html' target='_top'>Introduction to CUPS Programming</a><br>
 375         Programming: <a href='api-cups.html' target='_top'>CUPS API</a><br>
 376         Programming: <a href='api-ppd.html' target='_top'>PPD API</a><br>
 377         Programming: <a href='api-raster.html' target='_top'>Raster API</a><br>
 378         Programming: <a href='postscript-driver.html' target='_top'>Developing PostScript Printer Drivers</a><br>
 379         Programming: <a href='raster-driver.html' target='_top'>Developing Raster Printer Drivers</a><br>
 380         Specifications: <a href='spec-design.html' target='_top'>CUPS Design Description</a></td>
 381 </tr>
 382 </tbody>
 383 </table></div>
 384 <h2 class="title">Contents</h2>
 385 <ul class="contents">
 386 <li><a href="#OVERVIEW">Overview</a><ul class="subcontents">
 387         <li><a href="#SECURITY">Security Considerations</a></li>
 388         <li><a href="#SIGNALS">Signal Handling</a></li>
 389         <li><a href="#TEMPFILES">Temporary Files</a></li>
 390         <li><a href="#COPIES">Copy Generation</a></li>
 391         <li><a href="#EXITCODES">Exit Codes</a></li>
 392         <li><a href="#ENVIRONMENT">Environment Variables</a></li>
 393         <li><a href="#MESSAGES">Communicating with the Scheduler</a></li>
 394         <li><a href="#COMMUNICATING_BACKEND">Communicating with the Backend</a></li>
 395         <li><a href="#COMMUNICATING_FILTER">Communicating with Filters</a></li>
 396         <li><a href="#SNMP">Doing SNMP Queries with Network Printers</a></li>
 397 </ul></li>
 398 <li><a href="#FUNCTIONS">Functions</a><ul class="code">
 399         <li><a href="#cupsBackChannelRead" title="Read data from the backchannel.">cupsBackChannelRead</a></li>
 400         <li><a href="#cupsBackChannelWrite" title="Write data to the backchannel.">cupsBackChannelWrite</a></li>
 401         <li><a href="#cupsBackendDeviceURI" title="Get the device URI for a backend.">cupsBackendDeviceURI</a></li>
 402         <li><a href="#cupsBackendReport" title="Write a device line from a backend.">cupsBackendReport</a></li>
 403         <li><a href="#cupsSideChannelDoRequest" title="Send a side-channel command to a backend and wait for a response.">cupsSideChannelDoRequest</a></li>
 404         <li><a href="#cupsSideChannelRead" title="Read a side-channel message.">cupsSideChannelRead</a></li>
 405         <li><a href="#cupsSideChannelSNMPGet" title="Query a SNMP OID's value.">cupsSideChannelSNMPGet</a></li>
 406         <li><a href="#cupsSideChannelSNMPWalk" title="Query multiple SNMP OID values.">cupsSideChannelSNMPWalk</a></li>
 407         <li><a href="#cupsSideChannelWrite" title="Write a side-channel message.">cupsSideChannelWrite</a></li>
 408 </ul></li>
 409 <li><a href="#TYPES">Data Types</a><ul class="code">
 410         <li><a href="#cups_backend_t" title="Backend exit codes">cups_backend_t</a></li>
 411         <li><a href="#cups_sc_bidi_t" title="Bidirectional capabilities">cups_sc_bidi_t</a></li>
 412         <li><a href="#cups_sc_command_t" title="Request command codes">cups_sc_command_t</a></li>
 413         <li><a href="#cups_sc_connected_t" title="Connectivity values">cups_sc_connected_t</a></li>
 414         <li><a href="#cups_sc_state_t" title="Printer state bits">cups_sc_state_t</a></li>
 415         <li><a href="#cups_sc_status_t" title="Response status codes">cups_sc_status_t</a></li>
 416         <li><a href="#cups_sc_walk_func_t" title="SNMP walk callback">cups_sc_walk_func_t</a></li>
 417 </ul></li>
 418 <li><a href="#ENUMERATIONS">Constants</a><ul class="code">
 419         <li><a href="#cups_backend_e" title="Backend exit codes">cups_backend_e</a></li>
 420         <li><a href="#cups_sc_bidi_e" title="Bidirectional capability values">cups_sc_bidi_e</a></li>
 421         <li><a href="#cups_sc_command_e" title="Request command codes">cups_sc_command_e</a></li>
 422         <li><a href="#cups_sc_connected_e" title="Connectivity values">cups_sc_connected_e</a></li>
 423         <li><a href="#cups_sc_state_e" title="Printer state bits">cups_sc_state_e</a></li>
 424         <li><a href="#cups_sc_status_e" title="Response status codes">cups_sc_status_e</a></li>
 425 </ul></li>
 426 </ul>
 427 <!--
 428   "$Id$"
 429
 430   Filter and backend programming introduction for CUPS.
 431
 432   Copyright 2007-2011 by Apple Inc.
 433   Copyright 1997-2006 by Easy Software Products, all rights reserved.
 434
 435   These coded instructions, statements, and computer programs are the
 436   property of Apple Inc. and are protected by Federal copyright
 437   law.  Distribution and use rights are outlined in the file "LICENSE.txt"
 438   which should have been included with this file.  If this file is
 439   file is missing or damaged, see the license at "http://www.cups.org/".
 440 -->
 441
 442 <h2 class='title'><a name="OVERVIEW">Overview</a></h2>
 443
 444 <p>Filters (which include printer drivers and port monitors) and backends
 445 are used to convert job files to a printable format and send that data to the
 446 printer itself. All of these programs use a common interface for processing
 447 print jobs and communicating status information to the scheduler. Each is run
 448 with a standard set of command-line arguments:<p>
 449
 450 <dl class="code">
 451
 452         <dt>argv[1]</dt>
 453         <dd>The job ID</dd>
 454
 455         <dt>argv[2]</dt>
 456         <dd>The user printing the job</dd>
 457
 458         <dt>argv[3]</dt>
 459         <dd>The job name/title</dd>
 460
 461         <dt>argv[4]</dt>
 462         <dd>The number of copies to print</dd>
 463
 464         <dt>argv[5]</dt>
 465         <dd>The options that were provided when the job was submitted</dd>
 466
 467         <dt>argv[6]</dt>
 468         <dd>The file to print (first program only)</dd>
 469 </dl>
 470
 471 <p>The scheduler runs one or more of these programs to print any given job. The
 472 first filter reads from the print file and writes to the standard output, while
 473 the remaining filters read from the standard input and write to the standard
 474 output. The backend is the last filter in the chain and writes to the
 475 device.</p>
 476
 477 <p>Filters are always run as a non-privileged user, typically "lp", with no
 478 connection to the user's desktop. Backends are run either as a non-privileged
 479 user or as root if the file permissions do not allow user or group execution.
 480 The <a href="#PERMISSIONS">file permissions</a> section talks about this in
 481 more detail.</p>
 482
 483 <h3><a name="SECURITY">Security Considerations</a></h3>
 484
 485 <p>It is always important to use security programming practices. Filters and
 486 most backends are run as a non-privileged user, so the major security
 487 consideration is resource utilization - filters should not depend on unlimited
 488 amounts of CPU, memory, or disk space, and should protect against conditions
 489 that could lead to excess usage of any resource like infinite loops and
 490 unbounded recursion. In addition, filters must <em>never</em> allow the user to
 491 specify an arbitrary file path to a separator page, template, or other file
 492 used by the filter since that can lead to an unauthorized disclosure of
 493 information. <em>Always</em> treat input as suspect and validate it!</p>
 494
 495 <p>If you are developing a backend that runs as root, make sure to check for
 496 potential buffer overflows, integer under/overflow conditions, and file
 497 accesses since these can lead to privilege escalations. When writing files,
 498 always validate the file path and <em>never</em> allow a user to determine
 499 where to store a file.</p>
 500
 501 <blockquote><b>Note:</b>
 502
 503 <p><em>Never</em> write files to a user's home directory. Aside from the
 504 security implications, CUPS is a network print service and as such the network
 505 user may not be the same as the local user and/or there may not be a local home
 506 directory to write to.</p>
 507
 508 <p>In addition, some operating systems provide additional security mechanisms
 509 that further limit file system access, even for backends running as root. On
 510 Mac OS X, for example, no backend may write to a user's home directory.</p>
 511 </blockquote>
 512
 513 <h3><a name="SIGNALS">Signal Handling</a><h3>
 514
 515 <p>The scheduler sends <code>SIGTERM</code> when a printing job is canceled or
 516 held. Filters, backends, and port monitors <em>must</em> catch
 517 <code>SIGTERM</code> and perform any cleanup necessary to produce a valid output
 518 file or return the printer to a known good state. The recommended behavior is to
 519 end the output on the current page.</p>
 520
 521 <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>
 522
 523 <pre class="example">
 524 #include &lt;signal.h&gt;>
 525
 526 ...
 527
 528 int
 529 main(int argc, char *argv[])
 530 {
 531   signal(SIGPIPE, SIG_IGN);
 532
 533   ...
 534 }
 535 </pre>
 536
 537 <h3><a name="PERMISSIONS">File Permissions</a></h3>
 538
 539 <p>For security reasons, CUPS will only run filters and backends that are owned
 540 by root and do not have world or group write permissions. The recommended
 541 permissions for filters and backends are 0555 - read and execute but no write.
 542 Backends that must run as root should use permissions of 0500 - read and execute
 543 by root, no access for other users. Write permissions can be enabled for the
 544 root user only.</p>
 545
 546 <p>To avoid a warning message, the directory containing your filter(s) must also
 547 be owned by root and have world and group write disabled - permissions of 0755
 548 or 0555 are strongly encouraged.</p>
 549
 550 <h3><a name="TEMPFILES">Temporary Files</a></h3>
 551
 552 <p>Temporary files should be created in the directory specified by the
 553 "TMPDIR" environment variable. The
 554 <a href="#cupsTempFile2"><code>cupsTempFile2</code></a> function can be
 555 used to safely create temporary files in this directory.</p>
 556
 557 <h3><a name="COPIES">Copy Generation</a></h3>
 558
 559 <p>The <code>argv[4]</code> argument specifies the number of copies to produce
 560 of the input file. In general, you should only generate copies if the
 561 <em>filename</em> argument is supplied. The only exception to this are
 562 filters that produce device-independent PostScript output, since the PostScript
 563 filter <var>pstops</var> is responsible for generating copies of PostScript
 564 files.</p>
 565
 566 <h3><a name="EXITCODES">Exit Codes</a></h3>
 567
 568 <p>Filters must exit with status 0 when they successfully generate print data
 569 or 1 when they encounter an error. Backends can return any of the
 570 <a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p>
 571
 572 <h3><a name="ENVIRONMENT">Environment Variables</a></h3>
 573
 574 <p>The following environment variables are defined by the printing system
 575 when running print filters and backends:</p>
 576
 577 <dl class="code">
 578
 579         <dt>APPLE_LANGUAGE</dt>
 580         <dd>The Apple language identifier associated with the job
 581         (Mac OS X only).</dd>
 582
 583         <dt>CHARSET</dt>
 584         <dd>The job character set, typically "utf-8".</dd>
 585
 586         <dt>CLASS</dt>
 587         <dd>When a job is submitted to a printer class, contains the name of
 588         the destination printer class. Otherwise this environment
 589         variable will not be set.</dd>
 590
 591         <dt>CONTENT_TYPE</dt>
 592         <dd>The MIME type associated with the file (e.g.
 593         application/postscript).</dd>
 594
 595         <dt>CUPS_CACHEDIR</dt>
 596         <dd>The directory where cache files can be stored. Cache files can be
 597         used to retain information between jobs or files in a job.</dd>
 598
 599         <dt>CUPS_DATADIR</dt>
 600         <dd>The directory where (read-only) CUPS data files can be found.</dd>
 601
 602         <dt>CUPS_FILETYPE</dt>
 603         <dd>The type of file being printed: "job-sheet" for a banner page and
 604         "document" for a regular print file.</dd>
 605
 606         <dt>CUPS_SERVERROOT</dt>
 607         <dd>The root directory of the server.</dd>
 608
 609         <dt>DEVICE_URI</dt>
 610         <dd>The device-uri associated with the printer.</dd>
 611
 612         <dt>FINAL_CONTENT_TYPE</dt>
 613         <dd>The MIME type associated with the printer (e.g.
 614         application/vnd.cups-postscript).</dd>
 615
 616         <dt>LANG</dt>
 617         <dd>The language locale associated with the job.</dd>
 618
 619         <dt>PPD</dt>
 620         <dd>The full pathname of the PostScript Printer Description (PPD)
 621         file for this printer.</dd>
 622
 623         <dt>PRINTER</dt>
 624         <dd>The queue name of the class or printer.</dd>
 625
 626         <dt>RIP_CACHE</dt>
 627         <dd>The recommended amount of memory to use for Raster Image
 628         Processors (RIPs).</dd>
 629
 630         <dt>TMPDIR</dt>
 631         <dd>The directory where temporary files should be created.</dd>
 632
 633 </dl>
 634
 635 <h3><a name="MESSAGES">Communicating with the Scheduler</a></h3>
 636
 637 <p>Filters and backends communicate with the scheduler by writing messages
 638 to the standard error file. The scheduler reads messages from all filters in
 639 a job and processes the message based on its prefix. For example, the following
 640 code sets the current printer state message to "Printing page 5":</p>
 641
 642 <pre class="example">
 643 int page = 5;
 644
 645 fprintf(stderr, "INFO: Printing page %d\n", page);
 646 </pre>
 647
 648 <p>Each message is a single line of text starting with one of the following
 649 prefix strings:</p>
 650
 651 <dl class="code">
 652
 653         <dt>ALERT: message</dt>
 654         <dd>Sets the printer-state-message attribute and adds the specified
 655         message to the current error log file using the "alert" log level.</dd>
 656
 657         <dt>ATTR: attribute=value [attribute=value]</dt>
 658         <dd>Sets the named printer or job attribute(s). Typically this is used
 659         to set the <code>marker-colors</code>, <code>marker-levels</code>,
 660         <code>marker-message</code>, <code>marker-names</code>,
 661         <code>marker-types</code>, <code>printer-alert</code>, and
 662         <code>printer-alert-description</code> printer attributes. Standard
 663         <code>marker-types</code> values are listed in <a href='#TABLE1'>Table
 664         1</a>.</dd>
 665
 666         <dt>CRIT: message</dt>
 667         <dd>Sets the printer-state-message attribute and adds the specified
 668         message to the current error log file using the "critical" log
 669         level.</dd>
 670
 671         <dt>DEBUG: message</dt>
 672         <dd>Sets the printer-state-message attribute and adds the specified
 673         message to the current error log file using the "debug" log level.</dd>
 674
 675         <dt>DEBUG2: message</dt>
 676         <dd>Sets the printer-state-message attribute and adds the specified
 677         message to the current error log file using the "debug2" log level.</dd>
 678
 679         <dt>EMERG: message</dt>
 680         <dd>Sets the printer-state-message attribute and adds the specified
 681         message to the current error log file using the "emergency" log
 682         level.</dd>
 683
 684         <dt>ERROR: message</dt>
 685         <dd>Sets the printer-state-message attribute and adds the specified
 686         message to the current error log file using the "error" log level.
 687         Use "ERROR:" messages for non-persistent processing errors.</dd>
 688
 689         <dt>INFO: message</dt>
 690         <dd>Sets the printer-state-message attribute. If the current log level
 691         is set to "debug2", also adds the specified message to the current error
 692         log file using the "info" log level.</dd>
 693
 694         <dt>NOTICE: message</dt>
 695         <dd>Sets the printer-state-message attribute and adds the specified
 696         message to the current error log file using the "notice" log level.</dd>
 697
 698         <dt>PAGE: page-number #-copies</dt>
 699         <dt>PAGE: total #-pages</dt>
 700         <dd>Adds an entry to the current page log file. The first form adds
 701         #-copies to the job-media-sheets-completed attribute. The second
 702         form sets the job-media-sheets-completed attribute to #-pages.</dd>
 703
 704         <dt>PPD: keyword=value [keyword=value ...]</dt>
 705         <dd>Changes or adds keywords to the printer's PPD file. Typically
 706         this is used to update installable options or default media settings
 707         based on the printer configuration.</dd>
 708
 709         <dt>STATE: printer-state-reason [printer-state-reason ...]</dt>
 710         <dt>STATE: + printer-state-reason [printer-state-reason ...]</dt>
 711         <dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
 712         <dd>Sets, adds, or removes printer-state-reason keywords to the
 713         current queue. Typically this is used to indicate persistent media,
 714         ink, toner, and configuration conditions or errors on a printer.
 715         <a href='#TABLE2'>Table 2</a> lists the standard state keywords -
 716         use vendor-prefixed ("com.acme.foo") keywords for custom states.
 717
 718         <blockquote><b>Note:</b>
 719
 720         <p>"STATE:" messages often provide visible alerts to the user. For example, on
 721         Mac OS X setting a printer-state-reason value with an "-error" or "-warning"
 722         suffix will cause the printer's dock item to bounce if the corresponding reason
 723         is localized with a cupsIPPReason keyword in the printer's PPD file.</p>
 724
 725         </blockquote></dd>
 726
 727         <dt>WARNING: message</dt>
 728         <dd>Sets the printer-state-message attribute and adds the specified
 729         message to the current error log file using the "warning" log
 730         level.</dd>
 731
 732 </dl>
 733
 734 <p>Messages without one of these prefixes are treated as if they began with
 735 the "DEBUG:" prefix string.</p>
 736
 737 <div class='table'><table width='80%' summary='Table 1: Standard marker-types Values'>
 738 <caption>Table 1: <a name='TABLE1'>Standard marker-types Values</a></caption>
 739 <thead>
 740 <tr>
 741         <th>marker-type</th>
 742         <th>Description</th>
 743 </tr>
 744 </thead>
 745 <tbody>
 746 <tr>
 747         <td>developer</td>
 748         <td>Developer unit</td>
 749 </tr>
 750 <tr>
 751         <td>fuser</td>
 752         <td>Fuser unit</td>
 753 </tr>
 754 <tr>
 755         <td>fuserCleaningPad</td>
 756         <td>Fuser cleaning pad</td>
 757 </tr>
 758 <tr>
 759         <td>fuserOil</td>
 760         <td>Fuser oil</td>
 761 </tr>
 762 <tr>
 763         <td>ink</td>
 764         <td>Ink supply</td>
 765 </tr>
 766 <tr>
 767         <td>opc</td>
 768         <td>Photo conductor</td>
 769 </tr>
 770 <tr>
 771         <td>solidWax</td>
 772         <td>Wax supply</td>
 773 </tr>
 774 <tr>
 775         <td>staples</td>
 776         <td>Staple supply</td>
 777 </tr>
 778 <tr>
 779         <td>toner</td>
 780         <td>Toner supply</td>
 781 </tr>
 782 <tr>
 783         <td>transferUnit</td>
 784         <td>Transfer unit</td>
 785 </tr>
 786 <tr>
 787         <td>wasteInk</td>
 788         <td>Waste ink tank</td>
 789 </tr>
 790 <tr>
 791         <td>wasteToner</td>
 792         <td>Waste toner tank</td>
 793 </tr>
 794 <tr>
 795         <td>wasteWax</td>
 796         <td>Waste wax tank</td>
 797 </tr>
 798 </tbody>
 799 </table></div>
 800
 801 <br>
 802
 803 <div class='table'><table width='80%' summary='Table 2: Standard State Keywords'>
 804 <caption>Table 2: <a name='TABLE2'>Standard State Keywords</a></caption>
 805 <thead>
 806 <tr>
 807         <th>Keyword</th>
 808         <th>Description</th>
 809 </tr>
 810 </thead>
 811 <tbody>
 812 <tr>
 813         <td>connecting-to-device</td>
 814         <td>Connecting to printer but not printing yet</td>
 815 </tr>
 816 <tr>
 817         <td>cover-open</td>
 818         <td>A cover is open on the printer</td>
 819 </tr>
 820 <tr>
 821         <td>input-tray-missing</td>
 822         <td>An input tray is missing from the printer</td>
 823 </tr>
 824 <tr>
 825         <td>marker-supply-empty</td>
 826         <td>Out of ink</td>
 827 </tr>
 828 <tr>
 829         <td>marker-supply-low</td>
 830         <td>Low on ink</td>
 831 </tr>
 832 <tr>
 833         <td>marker-waste-almost-full</td>
 834         <td>Waste tank almost full</td>
 835 </tr>
 836 <tr>
 837         <td>marker-waste-full</td>
 838         <td>Waste tank full</td>
 839 </tr>
 840 <tr>
 841         <td>media-empty</td>
 842         <td>Out of media</td>
 843 </tr>
 844 <tr>
 845         <td>media-jam</td>
 846         <td>Media is jammed in the printer</td>
 847 </tr>
 848 <tr>
 849         <td>media-low</td>
 850         <td>Low on media</td>
 851 </tr>
 852 <tr>
 853         <td>paused</td>
 854         <td>Stop the printer</td>
 855 </tr>
 856 <tr>
 857         <td>timed-out</td>
 858         <td>Unable to connect to printer</td>
 859 </tr>
 860 <tr>
 861         <td>toner-empty</td>
 862         <td>Out of toner</td>
 863 </tr>
 864 <tr>
 865         <td>toner-low</td>
 866         <td>Low on toner</td>
 867 </tr>
 868 </tbody>
 869 </table></div>
 870
 871 <h3><a name="COMMUNICATING_BACKEND">Communicating with the Backend</a></h3>
 872
 873 <p>Filters can communicate with the backend via the
 874 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and
 875 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
 876 functions. The
 877 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function
 878 reads data that has been sent back from the device and is typically used to
 879 obtain status and configuration information. For example, the following code
 880 polls the backend for back-channel data:</p>
 881
 882 <pre class="example">
 883 #include &lt;cups/cups.h&gt;
 884
 885 char buffer[8192];
 886 ssize_t bytes;
 887
 888 /* Use a timeout of 0.0 seconds to poll for back-channel data */
 889 bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
 890 </pre>
 891
 892 <p>Filters can also use <code>select()</code> or <code>poll()</code> on the
 893 back-channel file descriptor (3 or <code>CUPS_BC_FD</code>) to read data only
 894 when it is available.</p>
 895
 896 <p>The
 897 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
 898 function allows you to get out-of-band status information and do synchronization
 899 with the device. For example, the following code gets the current IEEE-1284
 900 device ID string from the backend:</p>
 901
 902 <pre class="example">
 903 #include &lt;cups/sidechannel.h&gt;
 904
 905 char data[2049];
 906 int datalen;
 907 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
 908
 909 /* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for
 910    nul-termination... */
 911 datalen = sizeof(data) - 1;
 912
 913 /* Get the IEEE-1284 device ID, waiting for up to 1 second */
 914 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &amp;datalen, 1.0);
 915
 916 /* Use the returned value if OK was returned and the length is non-zero */
 917 if (status == CUPS_SC_STATUS_OK && datalen > 0)
 918   data[datalen] = '\0';
 919 else
 920   data[0] = '\0';
 921 </pre>
 922
 923 <h3><a name="COMMUNICATING_FILTER">Communicating with Filters</a></h3>
 924
 925 <p>Backends communicate with filters using the reciprocal functions
 926 <a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>,
 927 <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and
 928 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We
 929 recommend writing back-channel data using a timeout of 1.0 seconds:</p>
 930
 931 <pre class="example">
 932 #include &lt;cups/cups.h&gt;
 933
 934 char buffer[8192];
 935 ssize_t bytes;
 936
 937 /* Obtain data from printer/device */
 938 ...
 939
 940 /* Use a timeout of 1.0 seconds to give filters a chance to read */
 941 cupsBackChannelWrite(buffer, bytes, 1.0);
 942 </pre>
 943
 944 <p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>
 945 function reads a side-channel command from a filter, driver, or port monitor.
 946 Backends can either poll for commands using a <code>timeout</code> of 0.0, wait
 947 indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a
 948 separate thread for that purpose), or use <code>select</code> or
 949 <code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle
 950 input and output on several file descriptors at the same time.</p>
 951
 952 <p>Once a command is processed, the backend uses the
 953 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function
 954 to send its response. For example, the following code shows how to poll for a
 955 side-channel command and respond to it:</p>
 956
 957 <pre class="example">
 958 #include &lt;cups/sidechannel.h&gt;
 959
 960 <a href="#cups_sc_command_t">cups_sc_command_t</a> command;
 961 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
 962 char data[2048];
 963 int datalen = sizeof(data);
 964
 965 /* Poll for a command... */
 966 if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&amp;command, &amp;status, data, &amp;datalen, 0.0))
 967 {
 968   switch (command)
 969   {
 970     /* handle supported commands, fill data/datalen/status with values as needed */
 971
 972     default :
 973         status  = CUPS_SC_STATUS_NOT_IMPLEMENTED;
 974         datalen = 0;
 975         break;
 976   }
 977
 978   /* Send a response... */
 979   <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0);
 980 }
 981 </pre>
 982
 983 <h3><a name="SNMP">Doing SNMP Queries with Network Printers</a></h3>
 984
 985 <p>The Simple Network Management Protocol (SNMP) allows you to get the current
 986 status, page counter, and supply levels from most network printers. Every
 987 piece of information is associated with an Object Identifier (OID), and
 988 every printer has a <em>community</em> name associated with it. OIDs can be
 989 queried directly or by "walking" over a range of OIDs with a common prefix.</p>
 990
 991 <p>The two CUPS SNMP functions provide a simple API for querying network
 992 printers through the side-channel interface. Each accepts a string containing
 993 an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID)
 994 along with a timeout for the query.</p>
 995
 996 <p>The <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>
 997 function queries a single OID and returns the value as a string in a buffer
 998 you supply:</p>
 999
1000 <pre class="example">
1001 #include &lt;cups/sidechannel.h&gt;
1002
1003 char data[512];
1004 int datalen = sizeof(data);
1005
1006 if (<a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a>(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &amp;datalen, 5.0)
1007         == CUPS_SC_STATUS_OK)
1008 {
1009   /* Do something with the value */
1010   printf("Page counter is: %s\n", data);
1011 }
1012 </pre>
1013
1014 <p>The
1015 <a href="#cupsSideChannelSNMPWalk"><code>cupsSideChannelSNMPWalk</code></a>
1016 function allows you to query a whole group of OIDs, calling a function of your
1017 choice for each OID that is found:</p>
1018
1019 <pre class="example">
1020 #include &lt;cups/sidechannel.h&gt;
1021
1022 void
1023 my_callback(const char *oid, const char *data, int datalen, void *context)
1024 {
1025   /* Do something with the value */
1026   printf("%s=%s\n", oid, data);
1027 }
1028
1029 ...
1030
1031 void *my_data;
1032
1033 <a href="#cupsSideChannelSNMPWalk">cupsSNMPSideChannelWalk</a>(".1.3.6.1.2.1.43", 5.0, my_callback, my_data);
1034 </pre>
1035 <h2 class="title"><a name="FUNCTIONS">Functions</a></h2>
1036 <h3 class="function"><span class="info">&nbsp;CUPS 1.2/Mac OS X 10.5&nbsp;</span><a name="cupsBackChannelRead">cupsBackChannelRead</a></h3>
1037 <p class="description">Read data from the backchannel.</p>
1038 <p class="code">
1039 ssize_t cupsBackChannelRead (<br>
1040 &nbsp;&nbsp;&nbsp;&nbsp;char *buffer,<br>
1041 &nbsp;&nbsp;&nbsp;&nbsp;size_t bytes,<br>
1042 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1043 );</p>
1044 <h4 class="parameters">Parameters</h4>
1045 <dl>
1046 <dt>buffer</dt>
1047 <dd class="description">Buffer to read into</dd>
1048 <dt>bytes</dt>
1049 <dd class="description">Bytes to read</dd>
1050 <dt>timeout</dt>
1051 <dd class="description">Timeout in seconds, typically 0.0 to poll</dd>
1052 </dl>
1053 <h4 class="returnvalue">Return Value</h4>
1054 <p class="description">Bytes read or -1 on error</p>
1055 <h4 class="discussion">Discussion</h4>
1056 <p class="discussion">Reads up to &quot;bytes&quot; bytes from the backchannel/backend. The &quot;timeout&quot;
1057 parameter controls how many seconds to wait for the data - use 0.0 to
1058 return immediately if there is no data, -1.0 to wait for data indefinitely.
1059
1060 </p>
1061 <h3 class="function"><span class="info">&nbsp;CUPS 1.2/Mac OS X 10.5&nbsp;</span><a name="cupsBackChannelWrite">cupsBackChannelWrite</a></h3>
1062 <p class="description">Write data to the backchannel.</p>
1063 <p class="code">
1064 ssize_t cupsBackChannelWrite (<br>
1065 &nbsp;&nbsp;&nbsp;&nbsp;const char *buffer,<br>
1066 &nbsp;&nbsp;&nbsp;&nbsp;size_t bytes,<br>
1067 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1068 );</p>
1069 <h4 class="parameters">Parameters</h4>
1070 <dl>
1071 <dt>buffer</dt>
1072 <dd class="description">Buffer to write</dd>
1073 <dt>bytes</dt>
1074 <dd class="description">Bytes to write</dd>
1075 <dt>timeout</dt>
1076 <dd class="description">Timeout in seconds, typically 1.0</dd>
1077 </dl>
1078 <h4 class="returnvalue">Return Value</h4>
1079 <p class="description">Bytes written or -1 on error</p>
1080 <h4 class="discussion">Discussion</h4>
1081 <p class="discussion">Writes &quot;bytes&quot; bytes to the backchannel/filter. The &quot;timeout&quot; parameter
1082 controls how many seconds to wait for the data to be written - use
1083 0.0 to return immediately if the data cannot be written, -1.0 to wait
1084 indefinitely.
1085
1086 </p>
1087 <h3 class="function"><span class="info">&nbsp;CUPS 1.2/Mac OS X 10.5&nbsp;</span><a name="cupsBackendDeviceURI">cupsBackendDeviceURI</a></h3>
1088 <p class="description">Get the device URI for a backend.</p>
1089 <p class="code">
1090 const char *cupsBackendDeviceURI (<br>
1091 &nbsp;&nbsp;&nbsp;&nbsp;char **argv<br>
1092 );</p>
1093 <h4 class="parameters">Parameters</h4>
1094 <dl>
1095 <dt>argv</dt>
1096 <dd class="description">Command-line arguments</dd>
1097 </dl>
1098 <h4 class="returnvalue">Return Value</h4>
1099 <p class="description">Device URI or <code>NULL</code></p>
1100 <h4 class="discussion">Discussion</h4>
1101 <p class="discussion">The &quot;argv&quot; argument is the argv argument passed to main(). This
1102 function returns the device URI passed in the DEVICE_URI environment
1103 variable or the device URI passed in argv[0], whichever is found
1104 first.
1105
1106 </p>
1107 <h3 class="function"><span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span><a name="cupsBackendReport">cupsBackendReport</a></h3>
1108 <p class="description">Write a device line from a backend.</p>
1109 <p class="code">
1110 void cupsBackendReport (<br>
1111 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_scheme,<br>
1112 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_uri,<br>
1113 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_make_and_model,<br>
1114 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_info,<br>
1115 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_id,<br>
1116 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_location<br>
1117 );</p>
1118 <h4 class="parameters">Parameters</h4>
1119 <dl>
1120 <dt>device_scheme</dt>
1121 <dd class="description">device-scheme string</dd>
1122 <dt>device_uri</dt>
1123 <dd class="description">device-uri string</dd>
1124 <dt>device_make_and_model</dt>
1125 <dd class="description">device-make-and-model string or <code>NULL</code></dd>
1126 <dt>device_info</dt>
1127 <dd class="description">device-info string or <code>NULL</code></dd>
1128 <dt>device_id</dt>
1129 <dd class="description">device-id string or <code>NULL</code></dd>
1130 <dt>device_location</dt>
1131 <dd class="description">device-location string or <code>NULL</code></dd>
1132 </dl>
1133 <h4 class="discussion">Discussion</h4>
1134 <p class="discussion">This function writes a single device line to stdout for a backend.
1135 It handles quoting of special characters in the device-make-and-model,
1136 device-info, device-id, and device-location strings.
1137
1138 </p>
1139 <h3 class="function"><span class="info">&nbsp;CUPS 1.3/Mac OS X 10.5&nbsp;</span><a name="cupsSideChannelDoRequest">cupsSideChannelDoRequest</a></h3>
1140 <p class="description">Send a side-channel command to a backend and wait for a response.</p>
1141 <p class="code">
1142 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelDoRequest (<br>
1143 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> command,<br>
1144 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
1145 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
1146 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1147 );</p>
1148 <h4 class="parameters">Parameters</h4>
1149 <dl>
1150 <dt>command</dt>
1151 <dd class="description">Command to send</dd>
1152 <dt>data</dt>
1153 <dd class="description">Response data buffer pointer</dd>
1154 <dt>datalen</dt>
1155 <dd class="description">Size of data buffer on entry, number of bytes in buffer on return</dd>
1156 <dt>timeout</dt>
1157 <dd class="description">Timeout in seconds</dd>
1158 </dl>
1159 <h4 class="returnvalue">Return Value</h4>
1160 <p class="description">Status of command</p>
1161 <h4 class="discussion">Discussion</h4>
1162 <p class="discussion">This function is normally only called by filters, drivers, or port
1163 monitors in order to communicate with the backend used by the current
1164 printer.  Programs must be prepared to handle timeout or &quot;not
1165 implemented&quot; status codes, which indicate that the backend or device
1166 do not support the specified side-channel command.<br>
1167 <br>
1168 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
1169 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
1170 update the value to contain the number of data bytes in the buffer.
1171
1172 </p>
1173 <h3 class="function"><span class="info">&nbsp;CUPS 1.3/Mac OS X 10.5&nbsp;</span><a name="cupsSideChannelRead">cupsSideChannelRead</a></h3>
1174 <p class="description">Read a side-channel message.</p>
1175 <p class="code">
1176 int cupsSideChannelRead (<br>
1177 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> *command,<br>
1178 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_status_t">cups_sc_status_t</a> *status,<br>
1179 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
1180 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
1181 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1182 );</p>
1183 <h4 class="parameters">Parameters</h4>
1184 <dl>
1185 <dt>command</dt>
1186 <dd class="description">Command code</dd>
1187 <dt>status</dt>
1188 <dd class="description">Status code</dd>
1189 <dt>data</dt>
1190 <dd class="description">Data buffer pointer</dd>
1191 <dt>datalen</dt>
1192 <dd class="description">Size of data buffer on entry, number of bytes in buffer on return</dd>
1193 <dt>timeout</dt>
1194 <dd class="description">Timeout in seconds</dd>
1195 </dl>
1196 <h4 class="returnvalue">Return Value</h4>
1197 <p class="description">0 on success, -1 on error</p>
1198 <h4 class="discussion">Discussion</h4>
1199 <p class="discussion">This function is normally only called by backend programs to read
1200 commands from a filter, driver, or port monitor program.  The
1201 caller must be prepared to handle incomplete or invalid messages
1202 and return the corresponding status codes.<br>
1203 <br>
1204 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
1205 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
1206 update the value to contain the number of data bytes in the buffer.
1207
1208 </p>
1209 <h3 class="function"><span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span><a name="cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a></h3>
1210 <p class="description">Query a SNMP OID's value.</p>
1211 <p class="code">
1212 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPGet (<br>
1213 &nbsp;&nbsp;&nbsp;&nbsp;const char *oid,<br>
1214 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
1215 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
1216 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1217 );</p>
1218 <h4 class="parameters">Parameters</h4>
1219 <dl>
1220 <dt>oid</dt>
1221 <dd class="description">OID to query</dd>
1222 <dt>data</dt>
1223 <dd class="description">Buffer for OID value</dd>
1224 <dt>datalen</dt>
1225 <dd class="description">Size of OID buffer on entry, size of value on return</dd>
1226 <dt>timeout</dt>
1227 <dd class="description">Timeout in seconds</dd>
1228 </dl>
1229 <h4 class="returnvalue">Return Value</h4>
1230 <p class="description">Query status</p>
1231 <h4 class="discussion">Discussion</h4>
1232 <p class="discussion">This function asks the backend to do a SNMP OID query on behalf of the
1233 filter, port monitor, or backend using the default community name.<br>
1234 <br>
1235 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
1236 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
1237 supported and must be converted to their numeric forms.<br>
1238 <br>
1239 On input, &quot;data&quot; and &quot;datalen&quot; provide the location and size of the
1240 buffer to hold the OID value as a string. HEX-String (binary) values are
1241 converted to hexadecimal strings representing the binary data, while
1242 NULL-Value and unknown OID types are returned as the empty string.
1243 The returned &quot;datalen&quot; does not include the trailing nul.
1244
1245 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
1246 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
1247 the printer does not respond to the SNMP query.
1248
1249 </p>
1250 <h3 class="function"><span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span><a name="cupsSideChannelSNMPWalk">cupsSideChannelSNMPWalk</a></h3>
1251 <p class="description">Query multiple SNMP OID values.</p>
1252 <p class="code">
1253 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPWalk (<br>
1254 &nbsp;&nbsp;&nbsp;&nbsp;const char *oid,<br>
1255 &nbsp;&nbsp;&nbsp;&nbsp;double timeout,<br>
1256 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_walk_func_t">cups_sc_walk_func_t</a> cb,<br>
1257 &nbsp;&nbsp;&nbsp;&nbsp;void *context<br>
1258 );</p>
1259 <h4 class="parameters">Parameters</h4>
1260 <dl>
1261 <dt>oid</dt>
1262 <dd class="description">First numeric OID to query</dd>
1263 <dt>timeout</dt>
1264 <dd class="description">Timeout for each query in seconds</dd>
1265 <dt>cb</dt>
1266 <dd class="description">Function to call with each value</dd>
1267 <dt>context</dt>
1268 <dd class="description">Application-defined pointer to send to callback</dd>
1269 </dl>
1270 <h4 class="returnvalue">Return Value</h4>
1271 <p class="description">Status of first query of <code>CUPS_SC_STATUS_OK</code> on success</p>
1272 <h4 class="discussion">Discussion</h4>
1273 <p class="discussion">This function asks the backend to do multiple SNMP OID queries on behalf
1274 of the filter, port monitor, or backend using the default community name.
1275 All OIDs under the &quot;parent&quot; OID are queried and the results are sent to
1276 the callback function you provide.<br>
1277 <br>
1278 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
1279 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
1280 supported and must be converted to their numeric forms.<br>
1281 <br>
1282 &quot;timeout&quot; specifies the timeout for each OID query. The total amount of
1283 time will depend on the number of OID values found and the time required
1284 for each query.<br>
1285 <br>
1286 &quot;cb&quot; provides a function to call for every value that is found. &quot;context&quot;
1287 is an application-defined pointer that is sent to the callback function
1288 along with the OID and current data. The data passed to the callback is the
1289 same as returned by <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>.
1290
1291 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
1292 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
1293 the printer does not respond to the first SNMP query.
1294
1295 </p>
1296 <h3 class="function"><span class="info">&nbsp;CUPS 1.3/Mac OS X 10.5&nbsp;</span><a name="cupsSideChannelWrite">cupsSideChannelWrite</a></h3>
1297 <p class="description">Write a side-channel message.</p>
1298 <p class="code">
1299 int cupsSideChannelWrite (<br>
1300 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> command,<br>
1301 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_status_t">cups_sc_status_t</a> status,<br>
1302 &nbsp;&nbsp;&nbsp;&nbsp;const char *data,<br>
1303 &nbsp;&nbsp;&nbsp;&nbsp;int datalen,<br>
1304 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1305 );</p>
1306 <h4 class="parameters">Parameters</h4>
1307 <dl>
1308 <dt>command</dt>
1309 <dd class="description">Command code</dd>
1310 <dt>status</dt>
1311 <dd class="description">Status code</dd>
1312 <dt>data</dt>
1313 <dd class="description">Data buffer pointer</dd>
1314 <dt>datalen</dt>
1315 <dd class="description">Number of bytes of data</dd>
1316 <dt>timeout</dt>
1317 <dd class="description">Timeout in seconds</dd>
1318 </dl>
1319 <h4 class="returnvalue">Return Value</h4>
1320 <p class="description">0 on success, -1 on error</p>
1321 <h4 class="discussion">Discussion</h4>
1322 <p class="discussion">This function is normally only called by backend programs to send
1323 responses to a filter, driver, or port monitor program.
1324
1325 </p>
1326 <h2 class="title"><a name="TYPES">Data Types</a></h2>
1327 <h3 class="typedef"><a name="cups_backend_t">cups_backend_t</a></h3>
1328 <p class="description">Backend exit codes</p>
1329 <p class="code">
1330 typedef enum <a href="#cups_backend_e">cups_backend_e</a> cups_backend_t;
1331 </p>
1332 <h3 class="typedef"><a name="cups_sc_bidi_t">cups_sc_bidi_t</a></h3>
1333 <p class="description">Bidirectional capabilities</p>
1334 <p class="code">
1335 typedef enum <a href="#cups_sc_bidi_e">cups_sc_bidi_e</a> cups_sc_bidi_t;
1336 </p>
1337 <h3 class="typedef"><a name="cups_sc_command_t">cups_sc_command_t</a></h3>
1338 <p class="description">Request command codes</p>
1339 <p class="code">
1340 typedef enum <a href="#cups_sc_command_e">cups_sc_command_e</a> cups_sc_command_t;
1341 </p>
1342 <h3 class="typedef"><a name="cups_sc_connected_t">cups_sc_connected_t</a></h3>
1343 <p class="description">Connectivity values</p>
1344 <p class="code">
1345 typedef enum <a href="#cups_sc_connected_e">cups_sc_connected_e</a> cups_sc_connected_t;
1346 </p>
1347 <h3 class="typedef"><a name="cups_sc_state_t">cups_sc_state_t</a></h3>
1348 <p class="description">Printer state bits</p>
1349 <p class="code">
1350 typedef enum <a href="#cups_sc_state_e">cups_sc_state_e</a> cups_sc_state_t;
1351 </p>
1352 <h3 class="typedef"><a name="cups_sc_status_t">cups_sc_status_t</a></h3>
1353 <p class="description">Response status codes</p>
1354 <p class="code">
1355 typedef enum <a href="#cups_sc_status_e">cups_sc_status_e</a> cups_sc_status_t;
1356 </p>
1357 <h3 class="typedef"><a name="cups_sc_walk_func_t">cups_sc_walk_func_t</a></h3>
1358 <p class="description">SNMP walk callback</p>
1359 <p class="code">
1360 typedef void (*cups_sc_walk_func_t)(const char *oid, const char *data, int datalen, void *context);
1361 </p>
1362 <h2 class="title"><a name="ENUMERATIONS">Constants</a></h2>
1363 <h3 class="enumeration"><a name="cups_backend_e">cups_backend_e</a></h3>
1364 <p class="description">Backend exit codes</p>
1365 <h4 class="constants">Constants</h4>
1366 <dl>
1367 <dt>CUPS_BACKEND_AUTH_REQUIRED </dt>
1368 <dd class="description">Job failed, authentication required</dd>
1369 <dt>CUPS_BACKEND_CANCEL </dt>
1370 <dd class="description">Job failed, cancel job</dd>
1371 <dt>CUPS_BACKEND_FAILED </dt>
1372 <dd class="description">Job failed, use error-policy</dd>
1373 <dt>CUPS_BACKEND_HOLD </dt>
1374 <dd class="description">Job failed, hold job</dd>
1375 <dt>CUPS_BACKEND_OK </dt>
1376 <dd class="description">Job completed successfully</dd>
1377 <dt>CUPS_BACKEND_RETRY </dt>
1378 <dd class="description">Job failed, retry this job later</dd>
1379 <dt>CUPS_BACKEND_RETRY_CURRENT </dt>
1380 <dd class="description">Job failed, retry this job immediately</dd>
1381 <dt>CUPS_BACKEND_STOP </dt>
1382 <dd class="description">Job failed, stop queue</dd>
1383 </dl>
1384 <h3 class="enumeration"><a name="cups_sc_bidi_e">cups_sc_bidi_e</a></h3>
1385 <p class="description">Bidirectional capability values</p>
1386 <h4 class="constants">Constants</h4>
1387 <dl>
1388 <dt>CUPS_SC_BIDI_NOT_SUPPORTED </dt>
1389 <dd class="description">Bidirectional I/O is not supported</dd>
1390 <dt>CUPS_SC_BIDI_SUPPORTED </dt>
1391 <dd class="description">Bidirectional I/O is supported</dd>
1392 </dl>
1393 <h3 class="enumeration"><a name="cups_sc_command_e">cups_sc_command_e</a></h3>
1394 <p class="description">Request command codes</p>
1395 <h4 class="constants">Constants</h4>
1396 <dl>
1397 <dt>CUPS_SC_CMD_DRAIN_OUTPUT </dt>
1398 <dd class="description">Drain all pending output</dd>
1399 <dt>CUPS_SC_CMD_GET_BIDI </dt>
1400 <dd class="description">Return bidirectional capabilities</dd>
1401 <dt>CUPS_SC_CMD_GET_CONNECTED <span class="info">&nbsp;CUPS 1.5/Mac OS X 10.7&nbsp;</span></dt>
1402 <dd class="description">Return whether the backend is &quot;connected&quot; to the printer </dd>
1403 <dt>CUPS_SC_CMD_GET_DEVICE_ID </dt>
1404 <dd class="description">Return the IEEE-1284 device ID</dd>
1405 <dt>CUPS_SC_CMD_GET_STATE </dt>
1406 <dd class="description">Return the device state</dd>
1407 <dt>CUPS_SC_CMD_SNMP_GET <span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span></dt>
1408 <dd class="description">Query an SNMP OID </dd>
1409 <dt>CUPS_SC_CMD_SNMP_GET_NEXT <span class="info">&nbsp;CUPS 1.4/Mac OS X 10.6&nbsp;</span></dt>
1410 <dd class="description">Query the next SNMP OID </dd>
1411 <dt>CUPS_SC_CMD_SOFT_RESET </dt>
1412 <dd class="description">Do a soft reset</dd>
1413 </dl>
1414 <h3 class="enumeration"><a name="cups_sc_connected_e">cups_sc_connected_e</a></h3>
1415 <p class="description">Connectivity values</p>
1416 <h4 class="constants">Constants</h4>
1417 <dl>
1418 <dt>CUPS_SC_CONNECTED </dt>
1419 <dd class="description">Backend is &quot;connected&quot; to printer</dd>
1420 <dt>CUPS_SC_NOT_CONNECTED </dt>
1421 <dd class="description">Backend is not &quot;connected&quot; to printer</dd>
1422 </dl>
1423 <h3 class="enumeration"><a name="cups_sc_state_e">cups_sc_state_e</a></h3>
1424 <p class="description">Printer state bits</p>
1425 <h4 class="constants">Constants</h4>
1426 <dl>
1427 <dt>CUPS_SC_STATE_BUSY </dt>
1428 <dd class="description">Device is busy</dd>
1429 <dt>CUPS_SC_STATE_ERROR </dt>
1430 <dd class="description">Other error condition</dd>
1431 <dt>CUPS_SC_STATE_MARKER_EMPTY </dt>
1432 <dd class="description">Toner/ink out condition</dd>
1433 <dt>CUPS_SC_STATE_MARKER_LOW </dt>
1434 <dd class="description">Toner/ink low condition</dd>
1435 <dt>CUPS_SC_STATE_MEDIA_EMPTY </dt>
1436 <dd class="description">Paper out condition</dd>
1437 <dt>CUPS_SC_STATE_MEDIA_LOW </dt>
1438 <dd class="description">Paper low condition</dd>
1439 <dt>CUPS_SC_STATE_OFFLINE </dt>
1440 <dd class="description">Device is offline</dd>
1441 <dt>CUPS_SC_STATE_ONLINE </dt>
1442 <dd class="description">Device is online</dd>
1443 </dl>
1444 <h3 class="enumeration"><a name="cups_sc_status_e">cups_sc_status_e</a></h3>
1445 <p class="description">Response status codes</p>
1446 <h4 class="constants">Constants</h4>
1447 <dl>
1448 <dt>CUPS_SC_STATUS_BAD_MESSAGE </dt>
1449 <dd class="description">The command/response message was invalid</dd>
1450 <dt>CUPS_SC_STATUS_IO_ERROR </dt>
1451 <dd class="description">An I/O error occurred</dd>
1452 <dt>CUPS_SC_STATUS_NONE </dt>
1453 <dd class="description">No status</dd>
1454 <dt>CUPS_SC_STATUS_NOT_IMPLEMENTED </dt>
1455 <dd class="description">Command not implemented</dd>
1456 <dt>CUPS_SC_STATUS_NO_RESPONSE </dt>
1457 <dd class="description">The device did not respond</dd>
1458 <dt>CUPS_SC_STATUS_OK </dt>
1459 <dd class="description">Operation succeeded</dd>
1460 <dt>CUPS_SC_STATUS_TIMEOUT </dt>
1461 <dd class="description">The backend did not respond</dd>
1462 <dt>CUPS_SC_STATUS_TOO_BIG </dt>
1463 <dd class="description">Response too big</dd>
1464 </dl>
1465 </div>
1466 </body>
1467 </html>