doc/help/api-filter.html

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 354 <div class='body'>
 355 <!--
 356   Filter and backend programming header for CUPS.
 357
 358   Copyright 2008-2016 by Apple Inc.
 359
 360   These coded instructions, statements, and computer programs are the
 361   property of Apple Inc. and are protected by Federal copyright
 362   law.  Distribution and use rights are outlined in the file "LICENSE.txt"
 363   which should have been included with this file.  If this file is
 364   file is missing or damaged, see the license at "http://www.cups.org/".
 365 -->
 366
 367 <h1 class='title'>Filter and Backend Programming</h1>
 368
 369 <div class='summary'><table summary='General Information'>
 370 <thead>
 371 <tr>
 372         <th>Headers</th>
 373         <th>cups/backend.h<br>
 374         cups/ppd.h<br>
 375         cups/sidechannel.h</th>
 376 </tr>
 377 </thead>
 378 <tbody>
 379 <tr>
 380         <th>Library</th>
 381         <td>-lcups</td>
 382 </tr>
 383 <tr>
 384         <th>See Also</th>
 385         <td>Programming: <a href='api-overview.html' target='_top'>Introduction to CUPS Programming</a><br>
 386         Programming: <a href='api-cups.html' target='_top'>CUPS API</a><br>
 387         Programming: <a href='api-ppd.html' target='_top'>PPD API</a><br>
 388         Programming: <a href='api-raster.html' target='_top'>Raster API</a><br>
 389         Programming: <a href='postscript-driver.html' target='_top'>Developing PostScript Printer Drivers</a><br>
 390         Programming: <a href='raster-driver.html' target='_top'>Developing Raster Printer Drivers</a><br>
 391         Specifications: <a href='spec-design.html' target='_top'>CUPS Design Description</a></td>
 392 </tr>
 393 </tbody>
 394 </table></div>
 395 <h2 class="title">Contents</h2>
 396 <ul class="contents">
 397 <li><a href="#OVERVIEW">Overview</a><ul class="subcontents">
 398         <li><a href="#SECURITY">Security Considerations</a></li>
 399         <li><a href="#SIGNALS">Canceled Jobs and Signal Handling</a></li>
 400         <li><a href="#PERMISSIONS">File Permissions</a></li>
 401         <li><a href="#TEMPFILES">Temporary Files</a></li>
 402         <li><a href="#COPIES">Copy Generation</a></li>
 403         <li><a href="#EXITCODES">Exit Codes</a></li>
 404         <li><a href="#ENVIRONMENT">Environment Variables</a></li>
 405         <li><a href="#MESSAGES">Communicating with the Scheduler</a></li>
 406         <li><a href="#COMMUNICATING_BACKEND">Communicating with the Backend</a></li>
 407         <li><a href="#COMMUNICATING_FILTER">Communicating with Filters</a></li>
 408         <li><a href="#SNMP">Doing SNMP Queries with Network Printers</a></li>
 409 </ul></li>
 410 <li><a href="#SANDBOXING">Sandboxing on macOS</a></li>
 411 <li><a href="#FUNCTIONS">Functions</a><ul class="code">
 412         <li><a href="#cupsBackChannelRead" title="Read data from the backchannel.">cupsBackChannelRead</a></li>
 413         <li><a href="#cupsBackChannelWrite" title="Write data to the backchannel.">cupsBackChannelWrite</a></li>
 414         <li><a href="#cupsBackendDeviceURI" title="Get the device URI for a backend.">cupsBackendDeviceURI</a></li>
 415         <li><a href="#cupsBackendReport" title="Write a device line from a backend.">cupsBackendReport</a></li>
 416         <li><a href="#cupsSideChannelDoRequest" title="Send a side-channel command to a backend and wait for a response.">cupsSideChannelDoRequest</a></li>
 417         <li><a href="#cupsSideChannelRead" title="Read a side-channel message.">cupsSideChannelRead</a></li>
 418         <li><a href="#cupsSideChannelSNMPGet" title="Query a SNMP OID's value.">cupsSideChannelSNMPGet</a></li>
 419         <li><a href="#cupsSideChannelSNMPWalk" title="Query multiple SNMP OID values.">cupsSideChannelSNMPWalk</a></li>
 420         <li><a href="#cupsSideChannelWrite" title="Write a side-channel message.">cupsSideChannelWrite</a></li>
 421 </ul></li>
 422 <li><a href="#TYPES">Data Types</a><ul class="code">
 423         <li><a href="#cups_backend_t" title="Backend exit codes">cups_backend_t</a></li>
 424         <li><a href="#cups_sc_bidi_t" title="Bidirectional capabilities">cups_sc_bidi_t</a></li>
 425         <li><a href="#cups_sc_command_t" title="Request command codes">cups_sc_command_t</a></li>
 426         <li><a href="#cups_sc_connected_t" title="Connectivity values">cups_sc_connected_t</a></li>
 427         <li><a href="#cups_sc_state_t" title="Printer state bits">cups_sc_state_t</a></li>
 428         <li><a href="#cups_sc_status_t" title="Response status codes">cups_sc_status_t</a></li>
 429         <li><a href="#cups_sc_walk_func_t" title="SNMP walk callback">cups_sc_walk_func_t</a></li>
 430 </ul></li>
 431 <li><a href="#ENUMERATIONS">Constants</a><ul class="code">
 432         <li><a href="#cups_backend_e" title="Backend exit codes">cups_backend_e</a></li>
 433         <li><a href="#cups_sc_bidi_e" title="Bidirectional capability values">cups_sc_bidi_e</a></li>
 434         <li><a href="#cups_sc_command_e" title="Request command codes">cups_sc_command_e</a></li>
 435         <li><a href="#cups_sc_connected_e" title="Connectivity values">cups_sc_connected_e</a></li>
 436         <li><a href="#cups_sc_state_e" title="Printer state bits">cups_sc_state_e</a></li>
 437         <li><a href="#cups_sc_status_e" title="Response status codes">cups_sc_status_e</a></li>
 438 </ul></li>
 439 </ul>
 440 <!--
 441   Filter and backend programming introduction for CUPS.
 442
 443   Copyright 2007-2016 by Apple Inc.
 444   Copyright 1997-2006 by Easy Software Products, all rights reserved.
 445
 446   These coded instructions, statements, and computer programs are the
 447   property of Apple Inc. and are protected by Federal copyright
 448   law.  Distribution and use rights are outlined in the file "LICENSE.txt"
 449   which should have been included with this file.  If this file is
 450   file is missing or damaged, see the license at "http://www.cups.org/".
 451 -->
 452
 453 <h2 class='title'><a name="OVERVIEW">Overview</a></h2>
 454
 455 <p>Filters (which include printer drivers and port monitors) and backends
 456 are used to convert job files to a printable format and send that data to the
 457 printer itself. All of these programs use a common interface for processing
 458 print jobs and communicating status information to the scheduler. Each is run
 459 with a standard set of command-line arguments:<p>
 460
 461 <dl class="code">
 462
 463         <dt>argv[1]</dt>
 464         <dd>The job ID</dd>
 465
 466         <dt>argv[2]</dt>
 467         <dd>The user printing the job</dd>
 468
 469         <dt>argv[3]</dt>
 470         <dd>The job name/title</dd>
 471
 472         <dt>argv[4]</dt>
 473         <dd>The number of copies to print</dd>
 474
 475         <dt>argv[5]</dt>
 476         <dd>The options that were provided when the job was submitted</dd>
 477
 478         <dt>argv[6]</dt>
 479         <dd>The file to print (first program only)</dd>
 480 </dl>
 481
 482 <p>The scheduler runs one or more of these programs to print any given job. The
 483 first filter reads from the print file and writes to the standard output, while
 484 the remaining filters read from the standard input and write to the standard
 485 output. The backend is the last filter in the chain and writes to the
 486 device.</p>
 487
 488 <p>Filters are always run as a non-privileged user, typically "lp", with no
 489 connection to the user's desktop. Backends are run either as a non-privileged
 490 user or as root if the file permissions do not allow user or group execution.
 491 The <a href="#PERMISSIONS">file permissions</a> section talks about this in
 492 more detail.</p>
 493
 494 <h3><a name="SECURITY">Security Considerations</a></h3>
 495
 496 <p>It is always important to use security programming practices. Filters and
 497 most backends are run as a non-privileged user, so the major security
 498 consideration is resource utilization - filters should not depend on unlimited
 499 amounts of CPU, memory, or disk space, and should protect against conditions
 500 that could lead to excess usage of any resource like infinite loops and
 501 unbounded recursion. In addition, filters must <em>never</em> allow the user to
 502 specify an arbitrary file path to a separator page, template, or other file
 503 used by the filter since that can lead to an unauthorized disclosure of
 504 information. <em>Always</em> treat input as suspect and validate it!</p>
 505
 506 <p>If you are developing a backend that runs as root, make sure to check for
 507 potential buffer overflows, integer under/overflow conditions, and file
 508 accesses since these can lead to privilege escalations. When writing files,
 509 always validate the file path and <em>never</em> allow a user to determine
 510 where to store a file.</p>
 511
 512 <blockquote><b>Note:</b>
 513
 514 <p><em>Never</em> write files to a user's home directory. Aside from the
 515 security implications, CUPS is a network print service and as such the network
 516 user may not be the same as the local user and/or there may not be a local home
 517 directory to write to.</p>
 518
 519 <p>In addition, some operating systems provide additional security mechanisms
 520 that further limit file system access, even for backends running as root. On
 521 macOS, for example, no backend may write to a user's home directory. See the <a href="#SANDBOXING">Sandboxing on macOS</a> section for more information.</p>
 522 </blockquote>
 523
 524 <h3><a name="SIGNALS">Canceled Jobs and Signal Handling</a></h3>
 525
 526 <p>The scheduler sends <code>SIGTERM</code> when a printing job is canceled or
 527 held. Filters, backends, and port monitors <em>must</em> catch
 528 <code>SIGTERM</code> and perform any cleanup necessary to produce a valid output
 529 file or return the printer to a known good state. The recommended behavior is to
 530 end the output on the current page, preferably on the current line or object
 531 being printed.</p>
 532
 533 <p>Filters and backends may also receive <code>SIGPIPE</code> when an upstream or downstream filter/backend exits with a non-zero status. Developers should generally ignore <code>SIGPIPE</code> at the beginning of <code>main()</code> with the following function call:</p>
 534
 535 <pre class="example">
 536 #include &lt;signal.h&gt;>
 537
 538 ...
 539
 540 int
 541 main(int argc, char *argv[])
 542 {
 543   signal(SIGPIPE, SIG_IGN);
 544
 545   ...
 546 }
 547 </pre>
 548
 549 <h3><a name="PERMISSIONS">File Permissions</a></h3>
 550
 551 <p>For security reasons, CUPS will only run filters and backends that are owned
 552 by root and do not have world or group write permissions. The recommended
 553 permissions for filters and backends are 0555 - read and execute but no write.
 554 Backends that must run as root should use permissions of 0500 - read and execute
 555 by root, no access for other users. Write permissions can be enabled for the
 556 root user only.</p>
 557
 558 <p>To avoid a warning message, the directory containing your filter(s) must also
 559 be owned by root and have world and group write disabled - permissions of 0755
 560 or 0555 are strongly encouraged.</p>
 561
 562 <h3><a name="TEMPFILES">Temporary Files</a></h3>
 563
 564 <p>Temporary files should be created in the directory specified by the
 565 "TMPDIR" environment variable. The
 566 <a href="#cupsTempFile2"><code>cupsTempFile2</code></a> function can be
 567 used to safely create temporary files in this directory.</p>
 568
 569 <h3><a name="COPIES">Copy Generation</a></h3>
 570
 571 <p>The <code>argv[4]</code> argument specifies the number of copies to produce
 572 of the input file. In general, you should only generate copies if the
 573 <em>filename</em> argument is supplied. The only exception to this are
 574 filters that produce device-independent PostScript output, since the PostScript
 575 filter <var>pstops</var> is responsible for generating copies of PostScript
 576 files.</p>
 577
 578 <h3><a name="EXITCODES">Exit Codes</a></h3>
 579
 580 <p>Filters must exit with status 0 when they successfully generate print data
 581 or 1 when they encounter an error. Backends can return any of the
 582 <a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p>
 583
 584 <h3><a name="ENVIRONMENT">Environment Variables</a></h3>
 585
 586 <p>The following environment variables are defined by the printing system
 587 when running print filters and backends:</p>
 588
 589 <dl class="code">
 590
 591         <dt>APPLE_LANGUAGE</dt>
 592         <dd>The Apple language identifier associated with the job
 593         (macOS only).</dd>
 594
 595         <dt>CHARSET</dt>
 596         <dd>The job character set, typically "utf-8".</dd>
 597
 598         <dt>CLASS</dt>
 599         <dd>When a job is submitted to a printer class, contains the name of
 600         the destination printer class. Otherwise this environment
 601         variable will not be set.</dd>
 602
 603         <dt>CONTENT_TYPE</dt>
 604         <dd>The MIME type associated with the file (e.g.
 605         application/postscript).</dd>
 606
 607         <dt>CUPS_CACHEDIR</dt>
 608         <dd>The directory where cache files can be stored. Cache files can be
 609         used to retain information between jobs or files in a job.</dd>
 610
 611         <dt>CUPS_DATADIR</dt>
 612         <dd>The directory where (read-only) CUPS data files can be found.</dd>
 613
 614         <dt>CUPS_FILETYPE</dt>
 615         <dd>The type of file being printed: "job-sheet" for a banner page and
 616         "document" for a regular print file.</dd>
 617
 618         <dt>CUPS_SERVERROOT</dt>
 619         <dd>The root directory of the server.</dd>
 620
 621         <dt>DEVICE_URI</dt>
 622         <dd>The device-uri associated with the printer.</dd>
 623
 624         <dt>FINAL_CONTENT_TYPE</dt>
 625         <dd>The MIME type associated with the printer (e.g.
 626         application/vnd.cups-postscript).</dd>
 627
 628         <dt>LANG</dt>
 629         <dd>The language locale associated with the job.</dd>
 630
 631         <dt>PPD</dt>
 632         <dd>The full pathname of the PostScript Printer Description (PPD)
 633         file for this printer.</dd>
 634
 635         <dt>PRINTER</dt>
 636         <dd>The queue name of the class or printer.</dd>
 637
 638         <dt>RIP_CACHE</dt>
 639         <dd>The recommended amount of memory to use for Raster Image
 640         Processors (RIPs).</dd>
 641
 642         <dt>TMPDIR</dt>
 643         <dd>The directory where temporary files should be created.</dd>
 644
 645 </dl>
 646
 647 <h3><a name="MESSAGES">Communicating with the Scheduler</a></h3>
 648
 649 <p>Filters and backends communicate with the scheduler by writing messages
 650 to the standard error file. The scheduler reads messages from all filters in
 651 a job and processes the message based on its prefix. For example, the following
 652 code sets the current printer state message to "Printing page 5":</p>
 653
 654 <pre class="example">
 655 int page = 5;
 656
 657 fprintf(stderr, "INFO: Printing page %d\n", page);
 658 </pre>
 659
 660 <p>Each message is a single line of text starting with one of the following
 661 prefix strings:</p>
 662
 663 <dl class="code">
 664
 665         <dt>ALERT: message</dt>
 666         <dd>Sets the printer-state-message attribute and adds the specified
 667         message to the current error log file using the "alert" log level.</dd>
 668
 669         <dt>ATTR: attribute=value [attribute=value]</dt>
 670         <dd>Sets the named printer or job attribute(s). Typically this is used
 671         to set the <code>marker-colors</code>, <code>marker-high-levels</code>,
 672         <code>marker-levels</code>, <code>marker-low-levels</code>,
 673         <code>marker-message</code>, <code>marker-names</code>,
 674         <code>marker-types</code>, <code>printer-alert</code>, and
 675         <code>printer-alert-description</code> printer attributes. Standard
 676         <code>marker-types</code> values are listed in <a href='#TABLE1'>Table
 677         1</a>. String values need special handling - see <a href="#ATTR_STRINGS">Reporting Attribute String Values</a> below.</dd>
 678
 679         <dt>CRIT: message</dt>
 680         <dd>Sets the printer-state-message attribute and adds the specified
 681         message to the current error log file using the "critical" log
 682         level.</dd>
 683
 684         <dt>DEBUG: message</dt>
 685         <dd>Sets the printer-state-message attribute and adds the specified
 686         message to the current error log file using the "debug" log level.</dd>
 687
 688         <dt>DEBUG2: message</dt>
 689         <dd>Sets the printer-state-message attribute and adds the specified
 690         message to the current error log file using the "debug2" log level.</dd>
 691
 692         <dt>EMERG: message</dt>
 693         <dd>Sets the printer-state-message attribute and adds the specified
 694         message to the current error log file using the "emergency" log
 695         level.</dd>
 696
 697         <dt>ERROR: message</dt>
 698         <dd>Sets the printer-state-message attribute and adds the specified
 699         message to the current error log file using the "error" log level.
 700         Use "ERROR:" messages for non-persistent processing errors.</dd>
 701
 702         <dt>INFO: message</dt>
 703         <dd>Sets the printer-state-message attribute. If the current log level
 704         is set to "debug2", also adds the specified message to the current error
 705         log file using the "info" log level.</dd>
 706
 707         <dt>NOTICE: message</dt>
 708         <dd>Sets the printer-state-message attribute and adds the specified
 709         message to the current error log file using the "notice" log level.</dd>
 710
 711         <dt>PAGE: page-number #-copies</dt>
 712         <dt>PAGE: total #-pages</dt>
 713         <dd>Adds an entry to the current page log file. The first form adds
 714         #-copies to the job-media-sheets-completed attribute. The second
 715         form sets the job-media-sheets-completed attribute to #-pages.</dd>
 716
 717         <dt>PPD: keyword=value [keyword=value ...]</dt>
 718         <dd>Changes or adds keywords to the printer's PPD file. Typically
 719         this is used to update installable options or default media settings
 720         based on the printer configuration.</dd>
 721
 722         <dt>STATE: + printer-state-reason [printer-state-reason ...]</dt>
 723         <dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
 724         <dd>Sets or clears printer-state-reason keywords for the current queue.
 725         Typically this is used to indicate persistent media, ink, toner, and
 726         configuration conditions or errors on a printer.
 727         <a href='#TABLE2'>Table 2</a> lists some of the standard "printer-state-reasons" keywords from the <a href="http://www.iana.org/assignments/ipp-registrations/ipp-registrations.xhtml#ipp-registrations-4">IANA IPP Registry</a> -
 728         use vendor-prefixed ("com.example.foo") keywords for custom states. See
 729         <a href="#MANAGING_STATE">Managing Printer State in a Filter</a> for more
 730         information.
 731
 732         <dt>WARNING: message</dt>
 733         <dd>Sets the printer-state-message attribute and adds the specified
 734         message to the current error log file using the "warning" log
 735         level.</dd>
 736
 737 </dl>
 738
 739 <p>Messages without one of these prefixes are treated as if they began with
 740 the "DEBUG:" prefix string.</p>
 741
 742 <div class='table'><table width='80%' summary='Table 1: Standard marker-types Values'>
 743 <caption>Table 1: <a name='TABLE1'>Standard marker-types Values</a></caption>
 744 <thead>
 745 <tr>
 746         <th>marker-type</th>
 747         <th>Description</th>
 748 </tr>
 749 </thead>
 750 <tbody>
 751 <tr>
 752         <td>developer</td>
 753         <td>Developer unit</td>
 754 </tr>
 755 <tr>
 756         <td>fuser</td>
 757         <td>Fuser unit</td>
 758 </tr>
 759 <tr>
 760         <td>fuser-cleaning-pad</td>
 761         <td>Fuser cleaning pad</td>
 762 </tr>
 763 <tr>
 764         <td>fuser-oil</td>
 765         <td>Fuser oil</td>
 766 </tr>
 767 <tr>
 768         <td>ink</td>
 769         <td>Ink supply</td>
 770 </tr>
 771 <tr>
 772         <td>opc</td>
 773         <td>Photo conductor</td>
 774 </tr>
 775 <tr>
 776         <td>solid-wax</td>
 777         <td>Wax supply</td>
 778 </tr>
 779 <tr>
 780         <td>staples</td>
 781         <td>Staple supply</td>
 782 </tr>
 783 <tr>
 784         <td>toner</td>
 785         <td>Toner supply</td>
 786 </tr>
 787 <tr>
 788         <td>transfer-unit</td>
 789         <td>Transfer unit</td>
 790 </tr>
 791 <tr>
 792         <td>waste-ink</td>
 793         <td>Waste ink tank</td>
 794 </tr>
 795 <tr>
 796         <td>waste-toner</td>
 797         <td>Waste toner tank</td>
 798 </tr>
 799 <tr>
 800         <td>waste-wax</td>
 801         <td>Waste wax tank</td>
 802 </tr>
 803 </tbody>
 804 </table></div>
 805
 806 <br>
 807
 808 <div class='table'><table width='80%' summary='Table 2: Standard State Keywords'>
 809 <caption>Table 2: <a name='TABLE2'>Standard State Keywords</a></caption>
 810 <thead>
 811 <tr>
 812         <th>Keyword</th>
 813         <th>Description</th>
 814 </tr>
 815 </thead>
 816 <tbody>
 817 <tr>
 818         <td>connecting-to-device</td>
 819         <td>Connecting to printer but not printing yet.</td>
 820 </tr>
 821 <tr>
 822         <td>cover-open</td>
 823         <td>The printer's cover is open.</td>
 824 </tr>
 825 <tr>
 826         <td>input-tray-missing</td>
 827         <td>The paper tray is missing.</td>
 828 </tr>
 829 <tr>
 830         <td>marker-supply-empty</td>
 831         <td>The printer is out of ink.</td>
 832 </tr>
 833 <tr>
 834         <td>marker-supply-low</td>
 835         <td>The printer is almost out of ink.</td>
 836 </tr>
 837 <tr>
 838         <td>marker-waste-almost-full</td>
 839         <td>The printer's waste bin is almost full.</td>
 840 </tr>
 841 <tr>
 842         <td>marker-waste-full</td>
 843         <td>The printer's waste bin is full.</td>
 844 </tr>
 845 <tr>
 846         <td>media-empty</td>
 847         <td>The paper tray (any paper tray) is empty.</td>
 848 </tr>
 849 <tr>
 850         <td>media-jam</td>
 851         <td>There is a paper jam.</td>
 852 </tr>
 853 <tr>
 854         <td>media-low</td>
 855         <td>The paper tray (any paper tray) is almost empty.</td>
 856 </tr>
 857 <tr>
 858         <td>media-needed</td>
 859         <td>The paper tray needs to be filled (for a job that is printing).</td>
 860 </tr>
 861 <tr>
 862         <td>paused</td>
 863         <td>Stop the printer.</td>
 864 </tr>
 865 <tr>
 866         <td>timed-out</td>
 867         <td>Unable to connect to printer.</td>
 868 </tr>
 869 <tr>
 870         <td>toner-empty</td>
 871         <td>The printer is out of toner.</td>
 872 </tr>
 873 <tr>
 874         <td>toner-low</td>
 875         <td>The printer is low on toner.</td>
 876 </tr>
 877 </tbody>
 878 </table></div>
 879
 880
 881 <h4><a name="ATTR_STRINGS">Reporting Attribute String Values</a></h4>
 882
 883 <p>When reporting string values using "ATTR:" messages, a filter or backend must take special care to appropriately quote those values. The scheduler uses the CUPS option parsing code for attributes, so the general syntax is:</p>
 884
 885 <pre class="example">
 886 name=simple
 887 name=simple,simple,...
 888 name='complex value'
 889 name="complex value"
 890 name='"complex value"','"complex value"',...
 891 </pre>
 892
 893 <p>Simple values are strings that do not contain spaces, quotes, backslashes, or the comma and can be placed verbatim in the "ATTR:" message, for example:</p>
 894
 895 <pre class="example">
 896 int levels[4] = { 40, 50, 60, 70 }; /* CMYK */
 897
 898 fputs("ATTR: marker-colors=#00FFFF,#FF00FF,#FFFF00,#000000\n", stderr);
 899 fputs("ATTR: marker-high-levels=100,100,100,100\n", stderr);
 900 fprintf(stderr, "ATTR: marker-levels=%d,%d,%d,%d\n", levels[0], levels[1],
 901         levels[2], levels[3], levels[4]);
 902 fputs("ATTR: marker-low-levels=5,5,5,5\n", stderr);
 903 fputs("ATTR: marker-types=toner,toner,toner,toner\n", stderr);
 904 </pre>
 905
 906 <p>Complex values that contains spaces, quotes, backslashes, or the comma must be quoted. For a single value a single set of quotes is sufficient:</p>
 907
 908 <pre class="example">
 909 fputs("ATTR: marker-message='Levels shown are approximate.'\n", stderr);
 910 </pre>
 911
 912 <p>When multiple values are reported, each value must be enclosed by a set of single and double quotes:</p>
 913
 914 <pre class="example">
 915 fputs("ATTR: marker-names='\"Cyan Toner\"','\"Magenta Toner\"',"
 916       "'\"Yellow Toner\"','\"Black Toner\"'\n", stderr);
 917 </pre>
 918
 919 <p>The IPP backend includes a <var>quote_string</var> function that may be used to properly quote a complex value in an "ATTR:" message:</p>
 920
 921 <pre class="example">
 922 static const char *                     /* O - Quoted string */
 923 quote_string(const char *s,             /* I - String */
 924              char       *q,             /* I - Quoted string buffer */
 925              size_t     qsize)          /* I - Size of quoted string buffer */
 926 {
 927   char  *qptr,                          /* Pointer into string buffer */
 928         *qend;                          /* End of string buffer */
 929
 930
 931   qptr = q;
 932   qend = q + qsize - 5;
 933
 934   if (qend &lt; q)
 935   {
 936     *q = '\0';
 937     return (q);
 938   }
 939
 940   *qptr++ = '\'';
 941   *qptr++ = '\"';
 942
 943   while (*s && qptr &lt; qend)
 944   {
 945     if (*s == '\\' || *s == '\"' || *s == '\'')
 946     {
 947       if (qptr &lt; (qend - 4))
 948       {
 949         *qptr++ = '\\';
 950         *qptr++ = '\\';
 951         *qptr++ = '\\';
 952       }
 953       else
 954         break;
 955     }
 956
 957     *qptr++ = *s++;
 958   }
 959
 960   *qptr++ = '\"';
 961   *qptr++ = '\'';
 962   *qptr   = '\0';
 963
 964   return (q);
 965 }
 966 </pre>
 967
 968
 969 <h4><a name="MANAGING_STATE">Managing Printer State in a Filter</a></h4>
 970
 971 <p>Filters are responsible for managing the state keywords they set using
 972 "STATE:" messages. Typically you will update <em>all</em> of the keywords that
 973 are used by the filter at startup, for example:</p>
 974
 975 <pre class="example">
 976 if (foo_condition != 0)
 977   fputs("STATE: +com.example.foo\n", stderr);
 978 else
 979   fputs("STATE: -com.example.foo\n", stderr);
 980
 981 if (bar_condition != 0)
 982   fputs("STATE: +com.example.bar\n", stderr);
 983 else
 984   fputs("STATE: -com.example.bar\n", stderr);
 985 </pre>
 986
 987 <p>Then as conditions change, your filter sends "STATE: +keyword" or "STATE:
 988 -keyword" messages as necessary to set or clear the corresponding keyword,
 989 respectively.</p>
 990
 991 <p>State keywords are often used to notify the user of issues that span across
 992 jobs, for example "media-empty-warning" that indicates one or more paper trays
 993 are empty. These keywords should not be cleared unless the corresponding issue
 994 no longer exists.</p>
 995
 996 <p>Filters should clear job-related keywords on startup and exit so that they
 997 do not remain set between jobs.  For example, "connecting-to-device" is a job
 998 sub-state and not an issue that applies when a job is not printing.</p>
 999
1000 <blockquote><b>Note:</b>
1001
1002 <p>"STATE:" messages often provide visible alerts to the user. For example,
1003 on macOS setting a printer-state-reason value with an "-error" or
1004 "-warning" suffix will cause the printer's dock item to bounce if the
1005 corresponding reason is localized with a cupsIPPReason keyword in the
1006 printer's PPD file.</p>
1007
1008 <p>When providing a vendor-prefixed keyword, <em>always</em> provide the
1009 corresponding standard keyword (if any) to allow clients to respond to the
1010 condition correctly. For example, if you provide a vendor-prefixed keyword
1011 for a low cyan ink condition ("com.example.cyan-ink-low") you must also set the
1012 "marker-supply-low-warning" keyword. In such cases you should also refrain
1013 from localizing the vendor-prefixed keyword in the PPD file - otherwise both
1014 the generic and vendor-specific keyword will be shown in the user
1015 interface.</p>
1016
1017 </blockquote>
1018
1019 <h4><a name="REPORTING_SUPPLIES">Reporting Supply Levels</a></h4>
1020
1021 <p>CUPS tracks several "marker-*" attributes for ink/toner supply level
1022 reporting. These attributes allow applications to display the current supply
1023 levels for a printer without printer-specific software. <a href="#TABLE3">Table 3</a> lists the marker attributes and what they represent.</p>
1024
1025 <p>Filters set marker attributes by sending "ATTR:" messages to stderr. For
1026 example, a filter supporting an inkjet printer with black and tri-color ink
1027 cartridges would use the following to initialize the supply attributes:</p>
1028
1029 <pre class="example">
1030 fputs("ATTR: marker-colors=#000000,#00FFFF#FF00FF#FFFF00\n", stderr);
1031 fputs("ATTR: marker-low-levels=5,10\n", stderr);
1032 fputs("ATTR: marker-names=Black,Tri-Color\n", stderr);
1033 fputs("ATTR: marker-types=ink,ink\n", stderr);
1034 </pre>
1035
1036 <p>Then periodically the filter queries the printer for its current supply
1037 levels and updates them with a separate "ATTR:" message:</p>
1038
1039 <pre class="example">
1040 int black_level, tri_level;
1041 ...
1042 fprintf(stderr, "ATTR: marker-levels=%d,%d\n", black_level, tri_level);
1043 </pre>
1044
1045 <div class='table'><table width='80%' summary='Table 3: Supply Level Attributes'>
1046 <caption>Table 3: <a name='TABLE3'>Supply Level Attributes</a></caption>
1047 <thead>
1048 <tr>
1049         <th>Attribute</th>
1050         <th>Description</th>
1051 </tr>
1052 </thead>
1053 <tbody>
1054 <tr>
1055         <td>marker-colors</td>
1056         <td>A list of comma-separated colors; each color is either "none" or one or
1057         more hex-encoded sRGB colors of the form "#RRGGBB".</td>
1058 </tr>
1059 <tr>
1060         <td>marker-high-levels</td>
1061         <td>A list of comma-separated "almost full" level values from 0 to 100; a
1062         value of 100 should be used for supplies that are consumed/emptied like ink
1063         cartridges.</td>
1064 </tr>
1065 <tr>
1066         <td>marker-levels</td>
1067         <td>A list of comma-separated level values for each supply. A value of -1
1068         indicates the level is unavailable, -2 indicates unknown, and -3 indicates
1069         the level is unknown but has not yet reached capacity. Values from 0 to 100
1070         indicate the corresponding percentage.</td>
1071 </tr>
1072 <tr>
1073         <td>marker-low-levels</td>
1074         <td>A list of comma-separated "almost empty" level values from 0 to 100; a
1075         value of 0 should be used for supplies that are filled like waste ink
1076         tanks.</td>
1077 </tr>
1078 <tr>
1079         <td>marker-message</td>
1080         <td>A human-readable supply status message for the user like "12 pages of
1081         ink remaining."</td>
1082 </tr>
1083 <tr>
1084         <td>marker-names</td>
1085         <td>A list of comma-separated supply names like "Cyan Ink", "Fuser",
1086         etc.</td>
1087 </tr>
1088 <tr>
1089         <td>marker-types</td>
1090         <td>A list of comma-separated supply types; the types are listed in
1091         <a href="#TABLE1">Table 1</a>.</td>
1092 </tr>
1093 </tbody>
1094 </table></div>
1095
1096 <h3><a name="COMMUNICATING_BACKEND">Communicating with the Backend</a></h3>
1097
1098 <p>Filters can communicate with the backend via the
1099 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and
1100 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
1101 functions. The
1102 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function
1103 reads data that has been sent back from the device and is typically used to
1104 obtain status and configuration information. For example, the following code
1105 polls the backend for back-channel data:</p>
1106
1107 <pre class="example">
1108 #include &lt;cups/cups.h&gt;
1109
1110 char buffer[8192];
1111 ssize_t bytes;
1112
1113 /* Use a timeout of 0.0 seconds to poll for back-channel data */
1114 bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
1115 </pre>
1116
1117 <p>Filters can also use <code>select()</code> or <code>poll()</code> on the
1118 back-channel file descriptor (3 or <code>CUPS_BC_FD</code>) to read data only
1119 when it is available.</p>
1120
1121 <p>The
1122 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
1123 function allows you to get out-of-band status information and do synchronization
1124 with the device. For example, the following code gets the current IEEE-1284
1125 device ID string from the backend:</p>
1126
1127 <pre class="example">
1128 #include &lt;cups/sidechannel.h&gt;
1129
1130 char data[2049];
1131 int datalen;
1132 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
1133
1134 /* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for
1135    nul-termination... */
1136 datalen = sizeof(data) - 1;
1137
1138 /* Get the IEEE-1284 device ID, waiting for up to 1 second */
1139 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &amp;datalen, 1.0);
1140
1141 /* Use the returned value if OK was returned and the length is non-zero */
1142 if (status == CUPS_SC_STATUS_OK &amp;&amp; datalen > 0)
1143   data[datalen] = '\0';
1144 else
1145   data[0] = '\0';
1146 </pre>
1147
1148 <h4><a name="DRAIN_OUTPUT">Forcing All Output to a Printer</a></h4>
1149
1150 <p>The
1151 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
1152 function allows you to tell the backend to send all pending data to the printer.
1153 This is most often needed when sending query commands to the printer. For example:</p>
1154
1155 <pre class="example">
1156 #include &lt;cups/cups.h&gt;
1157 #include &lt;cups/sidechannel.h&gt;
1158
1159 char data[1024];
1160 int datalen = sizeof(data);
1161 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
1162
1163 /* Flush pending output to stdout */
1164 fflush(stdout);
1165
1166 /* Drain output to backend, waiting for up to 30 seconds */
1167 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_DRAIN_OUTPUT, data, &amp;datalen, 30.0);
1168
1169 /* Read the response if the output was sent */
1170 if (status == CUPS_SC_STATUS_OK)
1171 {
1172   ssize_t bytes;
1173
1174   /* Wait up to 10.0 seconds for back-channel data */
1175   bytes = cupsBackChannelRead(data, sizeof(data), 10.0);
1176   /* do something with the data from the printer */
1177 }
1178 </pre>
1179
1180 <h3><a name="COMMUNICATING_FILTER">Communicating with Filters</a></h3>
1181
1182 <p>Backends communicate with filters using the reciprocal functions
1183 <a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>,
1184 <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and
1185 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We
1186 recommend writing back-channel data using a timeout of 1.0 seconds:</p>
1187
1188 <pre class="example">
1189 #include &lt;cups/cups.h&gt;
1190
1191 char buffer[8192];
1192 ssize_t bytes;
1193
1194 /* Obtain data from printer/device */
1195 ...
1196
1197 /* Use a timeout of 1.0 seconds to give filters a chance to read */
1198 cupsBackChannelWrite(buffer, bytes, 1.0);
1199 </pre>
1200
1201 <p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>
1202 function reads a side-channel command from a filter, driver, or port monitor.
1203 Backends can either poll for commands using a <code>timeout</code> of 0.0, wait
1204 indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a
1205 separate thread for that purpose), or use <code>select</code> or
1206 <code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle
1207 input and output on several file descriptors at the same time.</p>
1208
1209 <p>Once a command is processed, the backend uses the
1210 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function
1211 to send its response. For example, the following code shows how to poll for a
1212 side-channel command and respond to it:</p>
1213
1214 <pre class="example">
1215 #include &lt;cups/sidechannel.h&gt;
1216
1217 <a href="#cups_sc_command_t">cups_sc_command_t</a> command;
1218 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
1219 char data[2048];
1220 int datalen = sizeof(data);
1221
1222 /* Poll for a command... */
1223 if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&amp;command, &amp;status, data, &amp;datalen, 0.0))
1224 {
1225   switch (command)
1226   {
1227     /* handle supported commands, fill data/datalen/status with values as needed */
1228
1229     default :
1230         status  = CUPS_SC_STATUS_NOT_IMPLEMENTED;
1231         datalen = 0;
1232         break;
1233   }
1234
1235   /* Send a response... */
1236   <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0);
1237 }
1238 </pre>
1239
1240 <h3><a name="SNMP">Doing SNMP Queries with Network Printers</a></h3>
1241
1242 <p>The Simple Network Management Protocol (SNMP) allows you to get the current
1243 status, page counter, and supply levels from most network printers. Every
1244 piece of information is associated with an Object Identifier (OID), and
1245 every printer has a <em>community</em> name associated with it. OIDs can be
1246 queried directly or by "walking" over a range of OIDs with a common prefix.</p>
1247
1248 <p>The two CUPS SNMP functions provide a simple API for querying network
1249 printers through the side-channel interface. Each accepts a string containing
1250 an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID)
1251 along with a timeout for the query.</p>
1252
1253 <p>The <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>
1254 function queries a single OID and returns the value as a string in a buffer
1255 you supply:</p>
1256
1257 <pre class="example">
1258 #include &lt;cups/sidechannel.h&gt;
1259
1260 char data[512];
1261 int datalen = sizeof(data);
1262
1263 if (<a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a>(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &amp;datalen, 5.0)
1264         == CUPS_SC_STATUS_OK)
1265 {
1266   /* Do something with the value */
1267   printf("Page counter is: %s\n", data);
1268 }
1269 </pre>
1270
1271 <p>The
1272 <a href="#cupsSideChannelSNMPWalk"><code>cupsSideChannelSNMPWalk</code></a>
1273 function allows you to query a whole group of OIDs, calling a function of your
1274 choice for each OID that is found:</p>
1275
1276 <pre class="example">
1277 #include &lt;cups/sidechannel.h&gt;
1278
1279 void
1280 my_callback(const char *oid, const char *data, int datalen, void *context)
1281 {
1282   /* Do something with the value */
1283   printf("%s=%s\n", oid, data);
1284 }
1285
1286 ...
1287
1288 void *my_data;
1289
1290 <a href="#cupsSideChannelSNMPWalk">cupsSNMPSideChannelWalk</a>(".1.3.6.1.2.1.43", 5.0, my_callback, my_data);
1291 </pre>
1292
1293 <h2><a name="SANDBOXING">Sandboxing on macOS</a></h2>
1294
1295 <p>Starting with macOS 10.6, filters and backends are run inside a security "sandbox" which further limits (beyond the normal UNIX user/group permissions) what a filter or backend can do. This helps to both secure the printing system from malicious software and enforce the functional separation of components in the CUPS filter chain. What follows is a list of actions that are explicitly allowed for all filters and backends:</p>
1296
1297 <ol>
1298
1299         <li>Reading of files: pursuant to normal UNIX file permissions, filters and backends can read files for the current job from the <var>/private/var/spool/cups</var> directory and other files on mounted filesystems <em>except</em> for user home directories under <var>/Users</var>.</li>
1300
1301         <li>Writing of files: pursuant to normal UNIX file permissions, filters and backends can read/write files to the cache directory specified by the <code>CUPS_CACHEDIR</code> environment variable, to the state directory specified by the <code>CUPS_STATEDIR</code> environment variable, to the temporary directory specified by the <code>TMPDIR</code> environment variable, and under the <var>/private/var/db</var>, <var>/private/var/folders</var>, <var>/private/var/lib</var>, <var>/private/var/mysql</var>, <var>/private/var/run</var>, <var>/private/var/spool</var> (except <var>/private/var/spool/cups</var>), <var>/Library/Application&nbsp;Support</var>, <var>/Library/Caches</var>, <var>/Library/Logs</var>, <var>/Library/Preferences</var>, <var>/Library/WebServer</var>, and <var>/Users/Shared</var> directories.</li>
1302
1303         <li>Execution of programs: pursuant to normal UNIX file permissions, filters and backends can execute any program not located under the <var>/Users</var> directory. Child processes inherit the sandbox and are subject to the same restrictions as the parent.</li>
1304
1305         <li>Bluetooth and USB: backends can access Bluetooth and USB printers through IOKit. <em>Filters cannot access Bluetooth and USB printers directly.</em></li>
1306
1307         <li>Network: filters and backends can access UNIX domain sockets under the <var>/private/tmp</var>, <var>/private/var/run</var>, and <var>/private/var/tmp</var> directories. Backends can also create IPv4 and IPv6 TCP (outgoing) and UDP (incoming and outgoing) socket, and bind to local source ports. <em>Filters cannot directly create IPv4 and IPv6 TCP or UDP sockets.</em></li>
1308
1309         <li>Notifications: filters and backends can send notifications via the Darwin <code>notify_post()</code> API.</li>
1310
1311 </ol>
1312
1313 <blockquote><b>Note:</b> The sandbox profile used in CUPS 2.0 still allows some actions that are not listed above - these privileges will be removed over time until the profile matches the list above.</blockquote>
1314 <h2 class="title"><a name="FUNCTIONS">Functions</a></h2>
1315 <h3 class="function"><span class="info">&nbsp;CUPS 1.2/macOS 10.5&nbsp;</span><a name="cupsBackChannelRead">cupsBackChannelRead</a></h3>
1316 <p class="description">Read data from the backchannel.</p>
1317 <p class="code">
1318 ssize_t cupsBackChannelRead (<br>
1319 &nbsp;&nbsp;&nbsp;&nbsp;char *buffer,<br>
1320 &nbsp;&nbsp;&nbsp;&nbsp;size_t bytes,<br>
1321 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1322 );</p>
1323 <h4 class="parameters">Parameters</h4>
1324 <dl>
1325 <dt>buffer</dt>
1326 <dd class="description">Buffer to read into</dd>
1327 <dt>bytes</dt>
1328 <dd class="description">Bytes to read</dd>
1329 <dt>timeout</dt>
1330 <dd class="description">Timeout in seconds, typically 0.0 to poll</dd>
1331 </dl>
1332 <h4 class="returnvalue">Return Value</h4>
1333 <p class="description">Bytes read or -1 on error</p>
1334 <h4 class="discussion">Discussion</h4>
1335 <p class="discussion">Reads up to &quot;bytes&quot; bytes from the backchannel/backend. The &quot;timeout&quot;
1336 parameter controls how many seconds to wait for the data - use 0.0 to
1337 return immediately if there is no data, -1.0 to wait for data indefinitely.
1338
1339 </p>
1340 <h3 class="function"><span class="info">&nbsp;CUPS 1.2/macOS 10.5&nbsp;</span><a name="cupsBackChannelWrite">cupsBackChannelWrite</a></h3>
1341 <p class="description">Write data to the backchannel.</p>
1342 <p class="code">
1343 ssize_t cupsBackChannelWrite (<br>
1344 &nbsp;&nbsp;&nbsp;&nbsp;const char *buffer,<br>
1345 &nbsp;&nbsp;&nbsp;&nbsp;size_t bytes,<br>
1346 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1347 );</p>
1348 <h4 class="parameters">Parameters</h4>
1349 <dl>
1350 <dt>buffer</dt>
1351 <dd class="description">Buffer to write</dd>
1352 <dt>bytes</dt>
1353 <dd class="description">Bytes to write</dd>
1354 <dt>timeout</dt>
1355 <dd class="description">Timeout in seconds, typically 1.0</dd>
1356 </dl>
1357 <h4 class="returnvalue">Return Value</h4>
1358 <p class="description">Bytes written or -1 on error</p>
1359 <h4 class="discussion">Discussion</h4>
1360 <p class="discussion">Writes &quot;bytes&quot; bytes to the backchannel/filter. The &quot;timeout&quot; parameter
1361 controls how many seconds to wait for the data to be written - use
1362 0.0 to return immediately if the data cannot be written, -1.0 to wait
1363 indefinitely.
1364
1365 </p>
1366 <h3 class="function"><span class="info">&nbsp;CUPS 1.2/macOS 10.5&nbsp;</span><a name="cupsBackendDeviceURI">cupsBackendDeviceURI</a></h3>
1367 <p class="description">Get the device URI for a backend.</p>
1368 <p class="code">
1369 const char *cupsBackendDeviceURI (<br>
1370 &nbsp;&nbsp;&nbsp;&nbsp;char **argv<br>
1371 );</p>
1372 <h4 class="parameters">Parameters</h4>
1373 <dl>
1374 <dt>argv</dt>
1375 <dd class="description">Command-line arguments</dd>
1376 </dl>
1377 <h4 class="returnvalue">Return Value</h4>
1378 <p class="description">Device URI or <code>NULL</code></p>
1379 <h4 class="discussion">Discussion</h4>
1380 <p class="discussion">The &quot;argv&quot; argument is the argv argument passed to main(). This
1381 function returns the device URI passed in the DEVICE_URI environment
1382 variable or the device URI passed in argv[0], whichever is found
1383 first.
1384
1385 </p>
1386 <h3 class="function"><span class="info">&nbsp;CUPS 1.4/macOS 10.6&nbsp;</span><a name="cupsBackendReport">cupsBackendReport</a></h3>
1387 <p class="description">Write a device line from a backend.</p>
1388 <p class="code">
1389 void cupsBackendReport (<br>
1390 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_scheme,<br>
1391 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_uri,<br>
1392 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_make_and_model,<br>
1393 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_info,<br>
1394 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_id,<br>
1395 &nbsp;&nbsp;&nbsp;&nbsp;const char *device_location<br>
1396 );</p>
1397 <h4 class="parameters">Parameters</h4>
1398 <dl>
1399 <dt>device_scheme</dt>
1400 <dd class="description">device-scheme string</dd>
1401 <dt>device_uri</dt>
1402 <dd class="description">device-uri string</dd>
1403 <dt>device_make_and_model</dt>
1404 <dd class="description">device-make-and-model string or <code>NULL</code></dd>
1405 <dt>device_info</dt>
1406 <dd class="description">device-info string or <code>NULL</code></dd>
1407 <dt>device_id</dt>
1408 <dd class="description">device-id string or <code>NULL</code></dd>
1409 <dt>device_location</dt>
1410 <dd class="description">device-location string or <code>NULL</code></dd>
1411 </dl>
1412 <h4 class="discussion">Discussion</h4>
1413 <p class="discussion">This function writes a single device line to stdout for a backend.
1414 It handles quoting of special characters in the device-make-and-model,
1415 device-info, device-id, and device-location strings.
1416
1417 </p>
1418 <h3 class="function"><span class="info">&nbsp;CUPS 1.3/macOS 10.5&nbsp;</span><a name="cupsSideChannelDoRequest">cupsSideChannelDoRequest</a></h3>
1419 <p class="description">Send a side-channel command to a backend and wait for a response.</p>
1420 <p class="code">
1421 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelDoRequest (<br>
1422 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> command,<br>
1423 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
1424 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
1425 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1426 );</p>
1427 <h4 class="parameters">Parameters</h4>
1428 <dl>
1429 <dt>command</dt>
1430 <dd class="description">Command to send</dd>
1431 <dt>data</dt>
1432 <dd class="description">Response data buffer pointer</dd>
1433 <dt>datalen</dt>
1434 <dd class="description">Size of data buffer on entry, number of bytes in buffer on return</dd>
1435 <dt>timeout</dt>
1436 <dd class="description">Timeout in seconds</dd>
1437 </dl>
1438 <h4 class="returnvalue">Return Value</h4>
1439 <p class="description">Status of command</p>
1440 <h4 class="discussion">Discussion</h4>
1441 <p class="discussion">This function is normally only called by filters, drivers, or port
1442 monitors in order to communicate with the backend used by the current
1443 printer.  Programs must be prepared to handle timeout or &quot;not
1444 implemented&quot; status codes, which indicate that the backend or device
1445 do not support the specified side-channel command.<br>
1446 <br>
1447 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
1448 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
1449 update the value to contain the number of data bytes in the buffer.
1450
1451 </p>
1452 <h3 class="function"><span class="info">&nbsp;CUPS 1.3/macOS 10.5&nbsp;</span><a name="cupsSideChannelRead">cupsSideChannelRead</a></h3>
1453 <p class="description">Read a side-channel message.</p>
1454 <p class="code">
1455 int cupsSideChannelRead (<br>
1456 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> *command,<br>
1457 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_status_t">cups_sc_status_t</a> *status,<br>
1458 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
1459 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
1460 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1461 );</p>
1462 <h4 class="parameters">Parameters</h4>
1463 <dl>
1464 <dt>command</dt>
1465 <dd class="description">Command code</dd>
1466 <dt>status</dt>
1467 <dd class="description">Status code</dd>
1468 <dt>data</dt>
1469 <dd class="description">Data buffer pointer</dd>
1470 <dt>datalen</dt>
1471 <dd class="description">Size of data buffer on entry, number of bytes in buffer on return</dd>
1472 <dt>timeout</dt>
1473 <dd class="description">Timeout in seconds</dd>
1474 </dl>
1475 <h4 class="returnvalue">Return Value</h4>
1476 <p class="description">0 on success, -1 on error</p>
1477 <h4 class="discussion">Discussion</h4>
1478 <p class="discussion">This function is normally only called by backend programs to read
1479 commands from a filter, driver, or port monitor program.  The
1480 caller must be prepared to handle incomplete or invalid messages
1481 and return the corresponding status codes.<br>
1482 <br>
1483 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
1484 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
1485 update the value to contain the number of data bytes in the buffer.
1486
1487 </p>
1488 <h3 class="function"><span class="info">&nbsp;CUPS 1.4/macOS 10.6&nbsp;</span><a name="cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a></h3>
1489 <p class="description">Query a SNMP OID's value.</p>
1490 <p class="code">
1491 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPGet (<br>
1492 &nbsp;&nbsp;&nbsp;&nbsp;const char *oid,<br>
1493 &nbsp;&nbsp;&nbsp;&nbsp;char *data,<br>
1494 &nbsp;&nbsp;&nbsp;&nbsp;int *datalen,<br>
1495 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1496 );</p>
1497 <h4 class="parameters">Parameters</h4>
1498 <dl>
1499 <dt>oid</dt>
1500 <dd class="description">OID to query</dd>
1501 <dt>data</dt>
1502 <dd class="description">Buffer for OID value</dd>
1503 <dt>datalen</dt>
1504 <dd class="description">Size of OID buffer on entry, size of value on return</dd>
1505 <dt>timeout</dt>
1506 <dd class="description">Timeout in seconds</dd>
1507 </dl>
1508 <h4 class="returnvalue">Return Value</h4>
1509 <p class="description">Query status</p>
1510 <h4 class="discussion">Discussion</h4>
1511 <p class="discussion">This function asks the backend to do a SNMP OID query on behalf of the
1512 filter, port monitor, or backend using the default community name.<br>
1513 <br>
1514 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
1515 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
1516 supported and must be converted to their numeric forms.<br>
1517 <br>
1518 On input, &quot;data&quot; and &quot;datalen&quot; provide the location and size of the
1519 buffer to hold the OID value as a string. HEX-String (binary) values are
1520 converted to hexadecimal strings representing the binary data, while
1521 NULL-Value and unknown OID types are returned as the empty string.
1522 The returned &quot;datalen&quot; does not include the trailing nul.
1523
1524 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
1525 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
1526 the printer does not respond to the SNMP query.
1527
1528 </p>
1529 <h3 class="function"><span class="info">&nbsp;CUPS 1.4/macOS 10.6&nbsp;</span><a name="cupsSideChannelSNMPWalk">cupsSideChannelSNMPWalk</a></h3>
1530 <p class="description">Query multiple SNMP OID values.</p>
1531 <p class="code">
1532 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPWalk (<br>
1533 &nbsp;&nbsp;&nbsp;&nbsp;const char *oid,<br>
1534 &nbsp;&nbsp;&nbsp;&nbsp;double timeout,<br>
1535 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_walk_func_t">cups_sc_walk_func_t</a> cb,<br>
1536 &nbsp;&nbsp;&nbsp;&nbsp;void *context<br>
1537 );</p>
1538 <h4 class="parameters">Parameters</h4>
1539 <dl>
1540 <dt>oid</dt>
1541 <dd class="description">First numeric OID to query</dd>
1542 <dt>timeout</dt>
1543 <dd class="description">Timeout for each query in seconds</dd>
1544 <dt>cb</dt>
1545 <dd class="description">Function to call with each value</dd>
1546 <dt>context</dt>
1547 <dd class="description">Application-defined pointer to send to callback</dd>
1548 </dl>
1549 <h4 class="returnvalue">Return Value</h4>
1550 <p class="description">Status of first query of <code>CUPS_SC_STATUS_OK</code> on success</p>
1551 <h4 class="discussion">Discussion</h4>
1552 <p class="discussion">This function asks the backend to do multiple SNMP OID queries on behalf
1553 of the filter, port monitor, or backend using the default community name.
1554 All OIDs under the &quot;parent&quot; OID are queried and the results are sent to
1555 the callback function you provide.<br>
1556 <br>
1557 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
1558 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
1559 supported and must be converted to their numeric forms.<br>
1560 <br>
1561 &quot;timeout&quot; specifies the timeout for each OID query. The total amount of
1562 time will depend on the number of OID values found and the time required
1563 for each query.<br>
1564 <br>
1565 &quot;cb&quot; provides a function to call for every value that is found. &quot;context&quot;
1566 is an application-defined pointer that is sent to the callback function
1567 along with the OID and current data. The data passed to the callback is the
1568 same as returned by <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>.
1569
1570 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
1571 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
1572 the printer does not respond to the first SNMP query.
1573
1574 </p>
1575 <h3 class="function"><span class="info">&nbsp;CUPS 1.3/macOS 10.5&nbsp;</span><a name="cupsSideChannelWrite">cupsSideChannelWrite</a></h3>
1576 <p class="description">Write a side-channel message.</p>
1577 <p class="code">
1578 int cupsSideChannelWrite (<br>
1579 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_command_t">cups_sc_command_t</a> command,<br>
1580 &nbsp;&nbsp;&nbsp;&nbsp;<a href="#cups_sc_status_t">cups_sc_status_t</a> status,<br>
1581 &nbsp;&nbsp;&nbsp;&nbsp;const char *data,<br>
1582 &nbsp;&nbsp;&nbsp;&nbsp;int datalen,<br>
1583 &nbsp;&nbsp;&nbsp;&nbsp;double timeout<br>
1584 );</p>
1585 <h4 class="parameters">Parameters</h4>
1586 <dl>
1587 <dt>command</dt>
1588 <dd class="description">Command code</dd>
1589 <dt>status</dt>
1590 <dd class="description">Status code</dd>
1591 <dt>data</dt>
1592 <dd class="description">Data buffer pointer</dd>
1593 <dt>datalen</dt>
1594 <dd class="description">Number of bytes of data</dd>
1595 <dt>timeout</dt>
1596 <dd class="description">Timeout in seconds</dd>
1597 </dl>
1598 <h4 class="returnvalue">Return Value</h4>
1599 <p class="description">0 on success, -1 on error</p>
1600 <h4 class="discussion">Discussion</h4>
1601 <p class="discussion">This function is normally only called by backend programs to send
1602 responses to a filter, driver, or port monitor program.
1603
1604 </p>
1605 <h2 class="title"><a name="TYPES">Data Types</a></h2>
1606 <h3 class="typedef"><a name="cups_backend_t">cups_backend_t</a></h3>
1607 <p class="description">Backend exit codes</p>
1608 <p class="code">
1609 typedef enum <a href="#cups_backend_e">cups_backend_e</a> cups_backend_t;
1610 </p>
1611 <h3 class="typedef"><a name="cups_sc_bidi_t">cups_sc_bidi_t</a></h3>
1612 <p class="description">Bidirectional capabilities</p>
1613 <p class="code">
1614 typedef enum <a href="#cups_sc_bidi_e">cups_sc_bidi_e</a> cups_sc_bidi_t;
1615 </p>
1616 <h3 class="typedef"><a name="cups_sc_command_t">cups_sc_command_t</a></h3>
1617 <p class="description">Request command codes</p>
1618 <p class="code">
1619 typedef enum <a href="#cups_sc_command_e">cups_sc_command_e</a> cups_sc_command_t;
1620 </p>
1621 <h3 class="typedef"><a name="cups_sc_connected_t">cups_sc_connected_t</a></h3>
1622 <p class="description">Connectivity values</p>
1623 <p class="code">
1624 typedef enum <a href="#cups_sc_connected_e">cups_sc_connected_e</a> cups_sc_connected_t;
1625 </p>
1626 <h3 class="typedef"><a name="cups_sc_state_t">cups_sc_state_t</a></h3>
1627 <p class="description">Printer state bits</p>
1628 <p class="code">
1629 typedef enum <a href="#cups_sc_state_e">cups_sc_state_e</a> cups_sc_state_t;
1630 </p>
1631 <h3 class="typedef"><a name="cups_sc_status_t">cups_sc_status_t</a></h3>
1632 <p class="description">Response status codes</p>
1633 <p class="code">
1634 typedef enum <a href="#cups_sc_status_e">cups_sc_status_e</a> cups_sc_status_t;
1635 </p>
1636 <h3 class="typedef"><a name="cups_sc_walk_func_t">cups_sc_walk_func_t</a></h3>
1637 <p class="description">SNMP walk callback</p>
1638 <p class="code">
1639 typedef void (*cups_sc_walk_func_t)(const char *oid, const char *data, int datalen, void *context);
1640 </p>
1641 <h2 class="title"><a name="ENUMERATIONS">Constants</a></h2>
1642 <h3 class="enumeration"><a name="cups_backend_e">cups_backend_e</a></h3>
1643 <p class="description">Backend exit codes</p>
1644 <h4 class="constants">Constants</h4>
1645 <dl>
1646 <dt>CUPS_BACKEND_AUTH_REQUIRED </dt>
1647 <dd class="description">Job failed, authentication required</dd>
1648 <dt>CUPS_BACKEND_CANCEL </dt>
1649 <dd class="description">Job failed, cancel job</dd>
1650 <dt>CUPS_BACKEND_FAILED </dt>
1651 <dd class="description">Job failed, use error-policy</dd>
1652 <dt>CUPS_BACKEND_HOLD </dt>
1653 <dd class="description">Job failed, hold job</dd>
1654 <dt>CUPS_BACKEND_OK </dt>
1655 <dd class="description">Job completed successfully</dd>
1656 <dt>CUPS_BACKEND_RETRY </dt>
1657 <dd class="description">Job failed, retry this job later</dd>
1658 <dt>CUPS_BACKEND_RETRY_CURRENT </dt>
1659 <dd class="description">Job failed, retry this job immediately</dd>
1660 <dt>CUPS_BACKEND_STOP </dt>
1661 <dd class="description">Job failed, stop queue</dd>
1662 </dl>
1663 <h3 class="enumeration"><a name="cups_sc_bidi_e">cups_sc_bidi_e</a></h3>
1664 <p class="description">Bidirectional capability values</p>
1665 <h4 class="constants">Constants</h4>
1666 <dl>
1667 <dt>CUPS_SC_BIDI_NOT_SUPPORTED </dt>
1668 <dd class="description">Bidirectional I/O is not supported</dd>
1669 <dt>CUPS_SC_BIDI_SUPPORTED </dt>
1670 <dd class="description">Bidirectional I/O is supported</dd>
1671 </dl>
1672 <h3 class="enumeration"><a name="cups_sc_command_e">cups_sc_command_e</a></h3>
1673 <p class="description">Request command codes</p>
1674 <h4 class="constants">Constants</h4>
1675 <dl>
1676 <dt>CUPS_SC_CMD_DRAIN_OUTPUT </dt>
1677 <dd class="description">Drain all pending output</dd>
1678 <dt>CUPS_SC_CMD_GET_BIDI </dt>
1679 <dd class="description">Return bidirectional capabilities</dd>
1680 <dt>CUPS_SC_CMD_GET_CONNECTED <span class="info">&nbsp;CUPS 1.5/macOS 10.7&nbsp;</span></dt>
1681 <dd class="description">Return whether the backend is &quot;connected&quot; to the printer </dd>
1682 <dt>CUPS_SC_CMD_GET_DEVICE_ID </dt>
1683 <dd class="description">Return the IEEE-1284 device ID</dd>
1684 <dt>CUPS_SC_CMD_GET_STATE </dt>
1685 <dd class="description">Return the device state</dd>
1686 <dt>CUPS_SC_CMD_SNMP_GET <span class="info">&nbsp;CUPS 1.4/macOS 10.6&nbsp;</span></dt>
1687 <dd class="description">Query an SNMP OID </dd>
1688 <dt>CUPS_SC_CMD_SNMP_GET_NEXT <span class="info">&nbsp;CUPS 1.4/macOS 10.6&nbsp;</span></dt>
1689 <dd class="description">Query the next SNMP OID </dd>
1690 <dt>CUPS_SC_CMD_SOFT_RESET </dt>
1691 <dd class="description">Do a soft reset</dd>
1692 </dl>
1693 <h3 class="enumeration"><a name="cups_sc_connected_e">cups_sc_connected_e</a></h3>
1694 <p class="description">Connectivity values</p>
1695 <h4 class="constants">Constants</h4>
1696 <dl>
1697 <dt>CUPS_SC_CONNECTED </dt>
1698 <dd class="description">Backend is &quot;connected&quot; to printer</dd>
1699 <dt>CUPS_SC_NOT_CONNECTED </dt>
1700 <dd class="description">Backend is not &quot;connected&quot; to printer</dd>
1701 </dl>
1702 <h3 class="enumeration"><a name="cups_sc_state_e">cups_sc_state_e</a></h3>
1703 <p class="description">Printer state bits</p>
1704 <h4 class="constants">Constants</h4>
1705 <dl>
1706 <dt>CUPS_SC_STATE_BUSY </dt>
1707 <dd class="description">Device is busy</dd>
1708 <dt>CUPS_SC_STATE_ERROR </dt>
1709 <dd class="description">Other error condition</dd>
1710 <dt>CUPS_SC_STATE_MARKER_EMPTY </dt>
1711 <dd class="description">Toner/ink out condition</dd>
1712 <dt>CUPS_SC_STATE_MARKER_LOW </dt>
1713 <dd class="description">Toner/ink low condition</dd>
1714 <dt>CUPS_SC_STATE_MEDIA_EMPTY </dt>
1715 <dd class="description">Paper out condition</dd>
1716 <dt>CUPS_SC_STATE_MEDIA_LOW </dt>
1717 <dd class="description">Paper low condition</dd>
1718 <dt>CUPS_SC_STATE_OFFLINE </dt>
1719 <dd class="description">Device is offline</dd>
1720 <dt>CUPS_SC_STATE_ONLINE </dt>
1721 <dd class="description">Device is online</dd>
1722 </dl>
1723 <h3 class="enumeration"><a name="cups_sc_status_e">cups_sc_status_e</a></h3>
1724 <p class="description">Response status codes</p>
1725 <h4 class="constants">Constants</h4>
1726 <dl>
1727 <dt>CUPS_SC_STATUS_BAD_MESSAGE </dt>
1728 <dd class="description">The command/response message was invalid</dd>
1729 <dt>CUPS_SC_STATUS_IO_ERROR </dt>
1730 <dd class="description">An I/O error occurred</dd>
1731 <dt>CUPS_SC_STATUS_NONE </dt>
1732 <dd class="description">No status</dd>
1733 <dt>CUPS_SC_STATUS_NOT_IMPLEMENTED </dt>
1734 <dd class="description">Command not implemented</dd>
1735 <dt>CUPS_SC_STATUS_NO_RESPONSE </dt>
1736 <dd class="description">The device did not respond</dd>
1737 <dt>CUPS_SC_STATUS_OK </dt>
1738 <dd class="description">Operation succeeded</dd>
1739 <dt>CUPS_SC_STATUS_TIMEOUT </dt>
1740 <dd class="description">The backend did not respond</dd>
1741 <dt>CUPS_SC_STATUS_TOO_BIG </dt>
1742 <dd class="description">Response too big</dd>
1743 </dl>
1744 </div>
1745 </body>
1746 </html>