doc/help/api-filter.html

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 441   <body>
 442 <!--
 443   Filter and backend programming header for CUPS.
 444
 445   Copyright © 2008-2016 by Apple Inc.
 446
 447   Licensed under Apache License v2.0.  See the file "LICENSE" for more
 448   information.
 449 -->
 450
 451 <h1 class='title'>Filter and Backend Programming</h1>
 452
 453 <div class='summary'><table summary='General Information'>
 454 <thead>
 455 <tr>
 456         <th>Headers</th>
 457         <th>cups/backend.h<br>
 458         cups/ppd.h<br>
 459         cups/sidechannel.h</th>
 460 </tr>
 461 </thead>
 462 <tbody>
 463 <tr>
 464         <th>Library</th>
 465         <td>-lcups</td>
 466 </tr>
 467 <tr>
 468         <th>See Also</th>
 469         <td>Programming: <a href='api-overview.html' target='_top'>Introduction to CUPS Programming</a><br>
 470         Programming: <a href='api-cups.html' target='_top'>CUPS API</a><br>
 471         Programming: <a href='api-ppd.html' target='_top'>PPD API</a><br>
 472         Programming: <a href='api-raster.html' target='_top'>Raster API</a><br>
 473         Programming: <a href='postscript-driver.html' target='_top'>Developing PostScript Printer Drivers</a><br>
 474         Programming: <a href='raster-driver.html' target='_top'>Developing Raster Printer Drivers</a><br>
 475         Specifications: <a href='spec-design.html' target='_top'>CUPS Design Description</a></td>
 476 </tr>
 477 </tbody>
 478 </table></div>
 479     <div class="contents">
 480       <h2 class="title">Contents</h2>
 481       <ul class="contents">
 482         <li><a href="#OVERVIEW">Overview</a><ul class="subcontents">
 483           <li><a href="#SECURITY">Security Considerations</a></li>
 484           <li><a href="#SIGNALS">Canceled Jobs and Signal Handling</a></li>
 485           <li><a href="#PERMISSIONS">File Permissions</a></li>
 486           <li><a href="#TEMPFILES">Temporary Files</a></li>
 487           <li><a href="#COPIES">Copy Generation</a></li>
 488           <li><a href="#EXITCODES">Exit Codes</a></li>
 489           <li><a href="#ENVIRONMENT">Environment Variables</a></li>
 490           <li><a href="#MESSAGES">Communicating with the Scheduler</a></li>
 491           <li><a href="#COMMUNICATING_BACKEND">Communicating with the Backend</a></li>
 492           <li><a href="#COMMUNICATING_FILTER">Communicating with Filters</a></li>
 493           <li><a href="#SNMP">Doing SNMP Queries with Network Printers</a></li>
 494         </ul></li>
 495         <li><a href="#SANDBOXING">Sandboxing on macOS</a></li>
 496         <li><a href="#FUNCTIONS">Functions</a><ul class="subcontents">
 497           <li><a href="#cupsBackChannelRead">cupsBackChannelRead</a></li>
 498           <li><a href="#cupsBackChannelWrite">cupsBackChannelWrite</a></li>
 499           <li><a href="#cupsBackendDeviceURI">cupsBackendDeviceURI</a></li>
 500           <li><a href="#cupsBackendReport">cupsBackendReport</a></li>
 501           <li><a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a></li>
 502           <li><a href="#cupsSideChannelRead">cupsSideChannelRead</a></li>
 503           <li><a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a></li>
 504           <li><a href="#cupsSideChannelSNMPWalk">cupsSideChannelSNMPWalk</a></li>
 505           <li><a href="#cupsSideChannelWrite">cupsSideChannelWrite</a></li>
 506         </ul></li>
 507         <li><a href="#TYPES">Data Types</a><ul class="subcontents">
 508           <li><a href="#cups_backend_t">cups_backend_t</a></li>
 509           <li><a href="#cups_sc_bidi_t">cups_sc_bidi_t</a></li>
 510           <li><a href="#cups_sc_command_t">cups_sc_command_t</a></li>
 511           <li><a href="#cups_sc_connected_t">cups_sc_connected_t</a></li>
 512           <li><a href="#cups_sc_state_t">cups_sc_state_t</a></li>
 513           <li><a href="#cups_sc_status_t">cups_sc_status_t</a></li>
 514           <li><a href="#cups_sc_walk_func_t">cups_sc_walk_func_t</a></li>
 515         </ul></li>
 516         <li><a href="#ENUMERATIONS">Enumerations</a><ul class="subcontents">
 517           <li><a href="#cups_backend_e">cups_backend_e</a></li>
 518           <li><a href="#cups_sc_bidi_e">cups_sc_bidi_e</a></li>
 519           <li><a href="#cups_sc_command_e">cups_sc_command_e</a></li>
 520           <li><a href="#cups_sc_connected_e">cups_sc_connected_e</a></li>
 521           <li><a href="#cups_sc_state_e">cups_sc_state_e</a></li>
 522           <li><a href="#cups_sc_status_e">cups_sc_status_e</a></li>
 523         </ul></li>
 524       </ul>
 525     </div>
 526     <div class="body">
 527 <!--
 528   Filter and backend programming introduction for CUPS.
 529
 530   Copyright © 2007-2016 by Apple Inc.
 531   Copyright © 1997-2006 by Easy Software Products, all rights reserved.
 532
 533   Licensed under Apache License v2.0.  See the file "LICENSE" for more
 534   information.
 535 -->
 536
 537 <h2 class='title'><a name="OVERVIEW">Overview</a></h2>
 538
 539 <p>Filters (which include printer drivers and port monitors) and backends
 540 are used to convert job files to a printable format and send that data to the
 541 printer itself. All of these programs use a common interface for processing
 542 print jobs and communicating status information to the scheduler. Each is run
 543 with a standard set of command-line arguments:<p>
 544
 545 <dl class="code">
 546
 547         <dt>argv[1]</dt>
 548         <dd>The job ID</dd>
 549
 550         <dt>argv[2]</dt>
 551         <dd>The user printing the job</dd>
 552
 553         <dt>argv[3]</dt>
 554         <dd>The job name/title</dd>
 555
 556         <dt>argv[4]</dt>
 557         <dd>The number of copies to print</dd>
 558
 559         <dt>argv[5]</dt>
 560         <dd>The options that were provided when the job was submitted</dd>
 561
 562         <dt>argv[6]</dt>
 563         <dd>The file to print (first program only)</dd>
 564 </dl>
 565
 566 <p>The scheduler runs one or more of these programs to print any given job. The
 567 first filter reads from the print file and writes to the standard output, while
 568 the remaining filters read from the standard input and write to the standard
 569 output. The backend is the last filter in the chain and writes to the
 570 device.</p>
 571
 572 <p>Filters are always run as a non-privileged user, typically "lp", with no
 573 connection to the user's desktop. Backends are run either as a non-privileged
 574 user or as root if the file permissions do not allow user or group execution.
 575 The <a href="#PERMISSIONS">file permissions</a> section talks about this in
 576 more detail.</p>
 577
 578 <h3><a name="SECURITY">Security Considerations</a></h3>
 579
 580 <p>It is always important to use security programming practices. Filters and
 581 most backends are run as a non-privileged user, so the major security
 582 consideration is resource utilization - filters should not depend on unlimited
 583 amounts of CPU, memory, or disk space, and should protect against conditions
 584 that could lead to excess usage of any resource like infinite loops and
 585 unbounded recursion. In addition, filters must <em>never</em> allow the user to
 586 specify an arbitrary file path to a separator page, template, or other file
 587 used by the filter since that can lead to an unauthorized disclosure of
 588 information. <em>Always</em> treat input as suspect and validate it!</p>
 589
 590 <p>If you are developing a backend that runs as root, make sure to check for
 591 potential buffer overflows, integer under/overflow conditions, and file
 592 accesses since these can lead to privilege escalations. When writing files,
 593 always validate the file path and <em>never</em> allow a user to determine
 594 where to store a file.</p>
 595
 596 <blockquote><b>Note:</b>
 597
 598 <p><em>Never</em> write files to a user's home directory. Aside from the
 599 security implications, CUPS is a network print service and as such the network
 600 user may not be the same as the local user and/or there may not be a local home
 601 directory to write to.</p>
 602
 603 <p>In addition, some operating systems provide additional security mechanisms
 604 that further limit file system access, even for backends running as root. On
 605 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>
 606 </blockquote>
 607
 608 <h3><a name="SIGNALS">Canceled Jobs and Signal Handling</a></h3>
 609
 610 <p>The scheduler sends <code>SIGTERM</code> when a printing job is canceled or
 611 held. Filters, backends, and port monitors <em>must</em> catch
 612 <code>SIGTERM</code> and perform any cleanup necessary to produce a valid output
 613 file or return the printer to a known good state. The recommended behavior is to
 614 end the output on the current page, preferably on the current line or object
 615 being printed.</p>
 616
 617 <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>
 618
 619 <pre class="example">
 620 #include &lt;signal.h&gt;
 621
 622 ...
 623
 624 int
 625 main(int argc, char *argv[])
 626 {
 627   signal(SIGPIPE, SIG_IGN);
 628
 629   ...
 630 }
 631 </pre>
 632
 633 <h3><a name="PERMISSIONS">File Permissions</a></h3>
 634
 635 <p>For security reasons, CUPS will only run filters and backends that are owned
 636 by root and do not have world or group write permissions. The recommended
 637 permissions for filters and backends are 0555 - read and execute but no write.
 638 Backends that must run as root should use permissions of 0500 - read and execute
 639 by root, no access for other users. Write permissions can be enabled for the
 640 root user only.</p>
 641
 642 <p>To avoid a warning message, the directory containing your filter(s) must also
 643 be owned by root and have world and group write disabled - permissions of 0755
 644 or 0555 are strongly encouraged.</p>
 645
 646 <h3><a name="TEMPFILES">Temporary Files</a></h3>
 647
 648 <p>Temporary files should be created in the directory specified by the
 649 "TMPDIR" environment variable. The
 650 <a href="#cupsTempFile2"><code>cupsTempFile2</code></a> function can be
 651 used to safely create temporary files in this directory.</p>
 652
 653 <h3><a name="COPIES">Copy Generation</a></h3>
 654
 655 <p>The <code>argv[4]</code> argument specifies the number of copies to produce
 656 of the input file. In general, you should only generate copies if the
 657 <em>filename</em> argument is supplied. The only exception to this are
 658 filters that produce device-independent PostScript output, since the PostScript
 659 filter <var>pstops</var> is responsible for generating copies of PostScript
 660 files.</p>
 661
 662 <h3><a name="EXITCODES">Exit Codes</a></h3>
 663
 664 <p>Filters must exit with status 0 when they successfully generate print data
 665 or 1 when they encounter an error. Backends can return any of the
 666 <a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p>
 667
 668 <h3><a name="ENVIRONMENT">Environment Variables</a></h3>
 669
 670 <p>The following environment variables are defined by the printing system
 671 when running print filters and backends:</p>
 672
 673 <dl class="code">
 674
 675         <dt>APPLE_LANGUAGE</dt>
 676         <dd>The Apple language identifier associated with the job
 677         (macOS only).</dd>
 678
 679         <dt>CHARSET</dt>
 680         <dd>The job character set, typically "utf-8".</dd>
 681
 682         <dt>CLASS</dt>
 683         <dd>When a job is submitted to a printer class, contains the name of
 684         the destination printer class. Otherwise this environment
 685         variable will not be set.</dd>
 686
 687         <dt>CONTENT_TYPE</dt>
 688         <dd>The MIME type associated with the file (e.g.
 689         application/postscript).</dd>
 690
 691         <dt>CUPS_CACHEDIR</dt>
 692         <dd>The directory where cache files can be stored. Cache files can be
 693         used to retain information between jobs or files in a job.</dd>
 694
 695         <dt>CUPS_DATADIR</dt>
 696         <dd>The directory where (read-only) CUPS data files can be found.</dd>
 697
 698         <dt>CUPS_FILETYPE</dt>
 699         <dd>The type of file being printed: "job-sheet" for a banner page and
 700         "document" for a regular print file.</dd>
 701
 702         <dt>CUPS_SERVERROOT</dt>
 703         <dd>The root directory of the server.</dd>
 704
 705         <dt>DEVICE_URI</dt>
 706         <dd>The device-uri associated with the printer.</dd>
 707
 708         <dt>FINAL_CONTENT_TYPE</dt>
 709         <dd>The MIME type associated with the printer (e.g.
 710         application/vnd.cups-postscript).</dd>
 711
 712         <dt>LANG</dt>
 713         <dd>The language locale associated with the job.</dd>
 714
 715         <dt>PPD</dt>
 716         <dd>The full pathname of the PostScript Printer Description (PPD)
 717         file for this printer.</dd>
 718
 719         <dt>PRINTER</dt>
 720         <dd>The queue name of the class or printer.</dd>
 721
 722         <dt>RIP_CACHE</dt>
 723         <dd>The recommended amount of memory to use for Raster Image
 724         Processors (RIPs).</dd>
 725
 726         <dt>TMPDIR</dt>
 727         <dd>The directory where temporary files should be created.</dd>
 728
 729 </dl>
 730
 731 <h3><a name="MESSAGES">Communicating with the Scheduler</a></h3>
 732
 733 <p>Filters and backends communicate with the scheduler by writing messages
 734 to the standard error file. The scheduler reads messages from all filters in
 735 a job and processes the message based on its prefix. For example, the following
 736 code sets the current printer state message to "Printing page 5":</p>
 737
 738 <pre class="example">
 739 int page = 5;
 740
 741 fprintf(stderr, "INFO: Printing page %d\n", page);
 742 </pre>
 743
 744 <p>Each message is a single line of text starting with one of the following
 745 prefix strings:</p>
 746
 747 <dl class="code">
 748
 749         <dt>ALERT: message</dt>
 750         <dd>Sets the printer-state-message attribute and adds the specified
 751         message to the current error log file using the "alert" log level.</dd>
 752
 753         <dt>ATTR: attribute=value [attribute=value]</dt>
 754         <dd>Sets the named printer or job attribute(s). Typically this is used
 755         to set the <code>marker-colors</code>, <code>marker-high-levels</code>,
 756         <code>marker-levels</code>, <code>marker-low-levels</code>,
 757         <code>marker-message</code>, <code>marker-names</code>,
 758         <code>marker-types</code>, <code>printer-alert</code>, and
 759         <code>printer-alert-description</code> printer attributes. Standard
 760         <code>marker-types</code> values are listed in <a href='#TABLE1'>Table
 761         1</a>. String values need special handling - see <a href="#ATTR_STRINGS">Reporting Attribute String Values</a> below.</dd>
 762
 763         <dt>CRIT: message</dt>
 764         <dd>Sets the printer-state-message attribute and adds the specified
 765         message to the current error log file using the "critical" log
 766         level.</dd>
 767
 768         <dt>DEBUG: message</dt>
 769         <dd>Sets the printer-state-message attribute and adds the specified
 770         message to the current error log file using the "debug" log level.</dd>
 771
 772         <dt>DEBUG2: message</dt>
 773         <dd>Sets the printer-state-message attribute and adds the specified
 774         message to the current error log file using the "debug2" log level.</dd>
 775
 776         <dt>EMERG: message</dt>
 777         <dd>Sets the printer-state-message attribute and adds the specified
 778         message to the current error log file using the "emergency" log
 779         level.</dd>
 780
 781         <dt>ERROR: message</dt>
 782         <dd>Sets the printer-state-message attribute and adds the specified
 783         message to the current error log file using the "error" log level.
 784         Use "ERROR:" messages for non-persistent processing errors.</dd>
 785
 786         <dt>INFO: message</dt>
 787         <dd>Sets the printer-state-message attribute. If the current log level
 788         is set to "debug2", also adds the specified message to the current error
 789         log file using the "info" log level.</dd>
 790
 791         <dt>NOTICE: message</dt>
 792         <dd>Sets the printer-state-message attribute and adds the specified
 793         message to the current error log file using the "notice" log level.</dd>
 794
 795         <dt>PAGE: page-number #-copies</dt>
 796         <dt>PAGE: total #-pages</dt>
 797         <dd>Adds an entry to the current page log file. The first form adds
 798         #-copies to the job-media-sheets-completed attribute. The second
 799         form sets the job-media-sheets-completed attribute to #-pages.</dd>
 800
 801         <dt>PPD: keyword=value [keyword=value ...]</dt>
 802         <dd>Changes or adds keywords to the printer's PPD file. Typically
 803         this is used to update installable options or default media settings
 804         based on the printer configuration.</dd>
 805
 806         <dt>STATE: + printer-state-reason [printer-state-reason ...]</dt>
 807         <dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
 808         <dd>Sets or clears printer-state-reason keywords for the current queue.
 809         Typically this is used to indicate persistent media, ink, toner, and
 810         configuration conditions or errors on a printer.
 811         <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> -
 812         use vendor-prefixed ("com.example.foo") keywords for custom states. See
 813         <a href="#MANAGING_STATE">Managing Printer State in a Filter</a> for more
 814         information.
 815
 816         <dt>WARNING: message</dt>
 817         <dd>Sets the printer-state-message attribute and adds the specified
 818         message to the current error log file using the "warning" log
 819         level.</dd>
 820
 821 </dl>
 822
 823 <p>Messages without one of these prefixes are treated as if they began with
 824 the "DEBUG:" prefix string.</p>
 825
 826 <div class='table'><table width='80%' summary='Table 1: Standard marker-types Values'>
 827 <caption>Table 1: <a name='TABLE1'>Standard marker-types Values</a></caption>
 828 <thead>
 829 <tr>
 830         <th>marker-type</th>
 831         <th>Description</th>
 832 </tr>
 833 </thead>
 834 <tbody>
 835 <tr>
 836         <td>developer</td>
 837         <td>Developer unit</td>
 838 </tr>
 839 <tr>
 840         <td>fuser</td>
 841         <td>Fuser unit</td>
 842 </tr>
 843 <tr>
 844         <td>fuser-cleaning-pad</td>
 845         <td>Fuser cleaning pad</td>
 846 </tr>
 847 <tr>
 848         <td>fuser-oil</td>
 849         <td>Fuser oil</td>
 850 </tr>
 851 <tr>
 852         <td>ink</td>
 853         <td>Ink supply</td>
 854 </tr>
 855 <tr>
 856         <td>opc</td>
 857         <td>Photo conductor</td>
 858 </tr>
 859 <tr>
 860         <td>solid-wax</td>
 861         <td>Wax supply</td>
 862 </tr>
 863 <tr>
 864         <td>staples</td>
 865         <td>Staple supply</td>
 866 </tr>
 867 <tr>
 868         <td>toner</td>
 869         <td>Toner supply</td>
 870 </tr>
 871 <tr>
 872         <td>transfer-unit</td>
 873         <td>Transfer unit</td>
 874 </tr>
 875 <tr>
 876         <td>waste-ink</td>
 877         <td>Waste ink tank</td>
 878 </tr>
 879 <tr>
 880         <td>waste-toner</td>
 881         <td>Waste toner tank</td>
 882 </tr>
 883 <tr>
 884         <td>waste-wax</td>
 885         <td>Waste wax tank</td>
 886 </tr>
 887 </tbody>
 888 </table></div>
 889
 890 <br>
 891
 892 <div class='table'><table width='80%' summary='Table 2: Standard State Keywords'>
 893 <caption>Table 2: <a name='TABLE2'>Standard State Keywords</a></caption>
 894 <thead>
 895 <tr>
 896         <th>Keyword</th>
 897         <th>Description</th>
 898 </tr>
 899 </thead>
 900 <tbody>
 901 <tr>
 902         <td>connecting-to-device</td>
 903         <td>Connecting to printer but not printing yet.</td>
 904 </tr>
 905 <tr>
 906         <td>cover-open</td>
 907         <td>The printer's cover is open.</td>
 908 </tr>
 909 <tr>
 910         <td>input-tray-missing</td>
 911         <td>The paper tray is missing.</td>
 912 </tr>
 913 <tr>
 914         <td>marker-supply-empty</td>
 915         <td>The printer is out of ink.</td>
 916 </tr>
 917 <tr>
 918         <td>marker-supply-low</td>
 919         <td>The printer is almost out of ink.</td>
 920 </tr>
 921 <tr>
 922         <td>marker-waste-almost-full</td>
 923         <td>The printer's waste bin is almost full.</td>
 924 </tr>
 925 <tr>
 926         <td>marker-waste-full</td>
 927         <td>The printer's waste bin is full.</td>
 928 </tr>
 929 <tr>
 930         <td>media-empty</td>
 931         <td>The paper tray (any paper tray) is empty.</td>
 932 </tr>
 933 <tr>
 934         <td>media-jam</td>
 935         <td>There is a paper jam.</td>
 936 </tr>
 937 <tr>
 938         <td>media-low</td>
 939         <td>The paper tray (any paper tray) is almost empty.</td>
 940 </tr>
 941 <tr>
 942         <td>media-needed</td>
 943         <td>The paper tray needs to be filled (for a job that is printing).</td>
 944 </tr>
 945 <tr>
 946         <td>paused</td>
 947         <td>Stop the printer.</td>
 948 </tr>
 949 <tr>
 950         <td>timed-out</td>
 951         <td>Unable to connect to printer.</td>
 952 </tr>
 953 <tr>
 954         <td>toner-empty</td>
 955         <td>The printer is out of toner.</td>
 956 </tr>
 957 <tr>
 958         <td>toner-low</td>
 959         <td>The printer is low on toner.</td>
 960 </tr>
 961 </tbody>
 962 </table></div>
 963
 964
 965 <h4><a name="ATTR_STRINGS">Reporting Attribute String Values</a></h4>
 966
 967 <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>
 968
 969 <pre class="example">
 970 name=simple
 971 name=simple,simple,...
 972 name='complex value'
 973 name="complex value"
 974 name='"complex value"','"complex value"',...
 975 </pre>
 976
 977 <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>
 978
 979 <pre class="example">
 980 int levels[4] = { 40, 50, 60, 70 }; /* CMYK */
 981
 982 fputs("ATTR: marker-colors=#00FFFF,#FF00FF,#FFFF00,#000000\n", stderr);
 983 fputs("ATTR: marker-high-levels=100,100,100,100\n", stderr);
 984 fprintf(stderr, "ATTR: marker-levels=%d,%d,%d,%d\n", levels[0], levels[1],
 985         levels[2], levels[3], levels[4]);
 986 fputs("ATTR: marker-low-levels=5,5,5,5\n", stderr);
 987 fputs("ATTR: marker-types=toner,toner,toner,toner\n", stderr);
 988 </pre>
 989
 990 <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>
 991
 992 <pre class="example">
 993 fputs("ATTR: marker-message='Levels shown are approximate.'\n", stderr);
 994 </pre>
 995
 996 <p>When multiple values are reported, each value must be enclosed by a set of single and double quotes:</p>
 997
 998 <pre class="example">
 999 fputs("ATTR: marker-names='\"Cyan Toner\"','\"Magenta Toner\"',"
1000       "'\"Yellow Toner\"','\"Black Toner\"'\n", stderr);
1001 </pre>
1002
1003 <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>
1004
1005 <pre class="example">
1006 static const char *                     /* O - Quoted string */
1007 quote_string(const char *s,             /* I - String */
1008              char       *q,             /* I - Quoted string buffer */
1009              size_t     qsize)          /* I - Size of quoted string buffer */
1010 {
1011   char  *qptr,                          /* Pointer into string buffer */
1012         *qend;                          /* End of string buffer */
1013
1014
1015   qptr = q;
1016   qend = q + qsize - 5;
1017
1018   if (qend &lt; q)
1019   {
1020     *q = '\0';
1021     return (q);
1022   }
1023
1024   *qptr++ = '\'';
1025   *qptr++ = '\"';
1026
1027   while (*s && qptr &lt; qend)
1028   {
1029     if (*s == '\\' || *s == '\"' || *s == '\'')
1030     {
1031       if (qptr &lt; (qend - 4))
1032       {
1033         *qptr++ = '\\';
1034         *qptr++ = '\\';
1035         *qptr++ = '\\';
1036       }
1037       else
1038         break;
1039     }
1040
1041     *qptr++ = *s++;
1042   }
1043
1044   *qptr++ = '\"';
1045   *qptr++ = '\'';
1046   *qptr   = '\0';
1047
1048   return (q);
1049 }
1050 </pre>
1051
1052
1053 <h4><a name="MANAGING_STATE">Managing Printer State in a Filter</a></h4>
1054
1055 <p>Filters are responsible for managing the state keywords they set using
1056 "STATE:" messages. Typically you will update <em>all</em> of the keywords that
1057 are used by the filter at startup, for example:</p>
1058
1059 <pre class="example">
1060 if (foo_condition != 0)
1061   fputs("STATE: +com.example.foo\n", stderr);
1062 else
1063   fputs("STATE: -com.example.foo\n", stderr);
1064
1065 if (bar_condition != 0)
1066   fputs("STATE: +com.example.bar\n", stderr);
1067 else
1068   fputs("STATE: -com.example.bar\n", stderr);
1069 </pre>
1070
1071 <p>Then as conditions change, your filter sends "STATE: +keyword" or "STATE:
1072 -keyword" messages as necessary to set or clear the corresponding keyword,
1073 respectively.</p>
1074
1075 <p>State keywords are often used to notify the user of issues that span across
1076 jobs, for example "media-empty-warning" that indicates one or more paper trays
1077 are empty. These keywords should not be cleared unless the corresponding issue
1078 no longer exists.</p>
1079
1080 <p>Filters should clear job-related keywords on startup and exit so that they
1081 do not remain set between jobs.  For example, "connecting-to-device" is a job
1082 sub-state and not an issue that applies when a job is not printing.</p>
1083
1084 <blockquote><b>Note:</b>
1085
1086 <p>"STATE:" messages often provide visible alerts to the user. For example,
1087 on macOS setting a printer-state-reason value with an "-error" or
1088 "-warning" suffix will cause the printer's dock item to bounce if the
1089 corresponding reason is localized with a cupsIPPReason keyword in the
1090 printer's PPD file.</p>
1091
1092 <p>When providing a vendor-prefixed keyword, <em>always</em> provide the
1093 corresponding standard keyword (if any) to allow clients to respond to the
1094 condition correctly. For example, if you provide a vendor-prefixed keyword
1095 for a low cyan ink condition ("com.example.cyan-ink-low") you must also set the
1096 "marker-supply-low-warning" keyword. In such cases you should also refrain
1097 from localizing the vendor-prefixed keyword in the PPD file - otherwise both
1098 the generic and vendor-specific keyword will be shown in the user
1099 interface.</p>
1100
1101 </blockquote>
1102
1103 <h4><a name="REPORTING_SUPPLIES">Reporting Supply Levels</a></h4>
1104
1105 <p>CUPS tracks several "marker-*" attributes for ink/toner supply level
1106 reporting. These attributes allow applications to display the current supply
1107 levels for a printer without printer-specific software. <a href="#TABLE3">Table 3</a> lists the marker attributes and what they represent.</p>
1108
1109 <p>Filters set marker attributes by sending "ATTR:" messages to stderr. For
1110 example, a filter supporting an inkjet printer with black and tri-color ink
1111 cartridges would use the following to initialize the supply attributes:</p>
1112
1113 <pre class="example">
1114 fputs("ATTR: marker-colors=#000000,#00FFFF#FF00FF#FFFF00\n", stderr);
1115 fputs("ATTR: marker-low-levels=5,10\n", stderr);
1116 fputs("ATTR: marker-names=Black,Tri-Color\n", stderr);
1117 fputs("ATTR: marker-types=ink,ink\n", stderr);
1118 </pre>
1119
1120 <p>Then periodically the filter queries the printer for its current supply
1121 levels and updates them with a separate "ATTR:" message:</p>
1122
1123 <pre class="example">
1124 int black_level, tri_level;
1125 ...
1126 fprintf(stderr, "ATTR: marker-levels=%d,%d\n", black_level, tri_level);
1127 </pre>
1128
1129 <div class='table'><table width='80%' summary='Table 3: Supply Level Attributes'>
1130 <caption>Table 3: <a name='TABLE3'>Supply Level Attributes</a></caption>
1131 <thead>
1132 <tr>
1133         <th>Attribute</th>
1134         <th>Description</th>
1135 </tr>
1136 </thead>
1137 <tbody>
1138 <tr>
1139         <td>marker-colors</td>
1140         <td>A list of comma-separated colors; each color is either "none" or one or
1141         more hex-encoded sRGB colors of the form "#RRGGBB".</td>
1142 </tr>
1143 <tr>
1144         <td>marker-high-levels</td>
1145         <td>A list of comma-separated "almost full" level values from 0 to 100; a
1146         value of 100 should be used for supplies that are consumed/emptied like ink
1147         cartridges.</td>
1148 </tr>
1149 <tr>
1150         <td>marker-levels</td>
1151         <td>A list of comma-separated level values for each supply. A value of -1
1152         indicates the level is unavailable, -2 indicates unknown, and -3 indicates
1153         the level is unknown but has not yet reached capacity. Values from 0 to 100
1154         indicate the corresponding percentage.</td>
1155 </tr>
1156 <tr>
1157         <td>marker-low-levels</td>
1158         <td>A list of comma-separated "almost empty" level values from 0 to 100; a
1159         value of 0 should be used for supplies that are filled like waste ink
1160         tanks.</td>
1161 </tr>
1162 <tr>
1163         <td>marker-message</td>
1164         <td>A human-readable supply status message for the user like "12 pages of
1165         ink remaining."</td>
1166 </tr>
1167 <tr>
1168         <td>marker-names</td>
1169         <td>A list of comma-separated supply names like "Cyan Ink", "Fuser",
1170         etc.</td>
1171 </tr>
1172 <tr>
1173         <td>marker-types</td>
1174         <td>A list of comma-separated supply types; the types are listed in
1175         <a href="#TABLE1">Table 1</a>.</td>
1176 </tr>
1177 </tbody>
1178 </table></div>
1179
1180 <h3><a name="COMMUNICATING_BACKEND">Communicating with the Backend</a></h3>
1181
1182 <p>Filters can communicate with the backend via the
1183 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and
1184 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
1185 functions. The
1186 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function
1187 reads data that has been sent back from the device and is typically used to
1188 obtain status and configuration information. For example, the following code
1189 polls the backend for back-channel data:</p>
1190
1191 <pre class="example">
1192 #include &lt;cups/cups.h&gt;
1193
1194 char buffer[8192];
1195 ssize_t bytes;
1196
1197 /* Use a timeout of 0.0 seconds to poll for back-channel data */
1198 bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
1199 </pre>
1200
1201 <p>Filters can also use <code>select()</code> or <code>poll()</code> on the
1202 back-channel file descriptor (3 or <code>CUPS_BC_FD</code>) to read data only
1203 when it is available.</p>
1204
1205 <p>The
1206 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
1207 function allows you to get out-of-band status information and do synchronization
1208 with the device. For example, the following code gets the current IEEE-1284
1209 device ID string from the backend:</p>
1210
1211 <pre class="example">
1212 #include &lt;cups/sidechannel.h&gt;
1213
1214 char data[2049];
1215 int datalen;
1216 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
1217
1218 /* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for
1219    nul-termination... */
1220 datalen = sizeof(data) - 1;
1221
1222 /* Get the IEEE-1284 device ID, waiting for up to 1 second */
1223 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &amp;datalen, 1.0);
1224
1225 /* Use the returned value if OK was returned and the length is non-zero */
1226 if (status == CUPS_SC_STATUS_OK &amp;&amp; datalen > 0)
1227   data[datalen] = '\0';
1228 else
1229   data[0] = '\0';
1230 </pre>
1231
1232 <h4><a name="DRAIN_OUTPUT">Forcing All Output to a Printer</a></h4>
1233
1234 <p>The
1235 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
1236 function allows you to tell the backend to send all pending data to the printer.
1237 This is most often needed when sending query commands to the printer. For example:</p>
1238
1239 <pre class="example">
1240 #include &lt;cups/cups.h&gt;
1241 #include &lt;cups/sidechannel.h&gt;
1242
1243 char data[1024];
1244 int datalen = sizeof(data);
1245 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
1246
1247 /* Flush pending output to stdout */
1248 fflush(stdout);
1249
1250 /* Drain output to backend, waiting for up to 30 seconds */
1251 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_DRAIN_OUTPUT, data, &amp;datalen, 30.0);
1252
1253 /* Read the response if the output was sent */
1254 if (status == CUPS_SC_STATUS_OK)
1255 {
1256   ssize_t bytes;
1257
1258   /* Wait up to 10.0 seconds for back-channel data */
1259   bytes = cupsBackChannelRead(data, sizeof(data), 10.0);
1260   /* do something with the data from the printer */
1261 }
1262 </pre>
1263
1264 <h3><a name="COMMUNICATING_FILTER">Communicating with Filters</a></h3>
1265
1266 <p>Backends communicate with filters using the reciprocal functions
1267 <a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>,
1268 <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and
1269 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We
1270 recommend writing back-channel data using a timeout of 1.0 seconds:</p>
1271
1272 <pre class="example">
1273 #include &lt;cups/cups.h&gt;
1274
1275 char buffer[8192];
1276 ssize_t bytes;
1277
1278 /* Obtain data from printer/device */
1279 ...
1280
1281 /* Use a timeout of 1.0 seconds to give filters a chance to read */
1282 cupsBackChannelWrite(buffer, bytes, 1.0);
1283 </pre>
1284
1285 <p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>
1286 function reads a side-channel command from a filter, driver, or port monitor.
1287 Backends can either poll for commands using a <code>timeout</code> of 0.0, wait
1288 indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a
1289 separate thread for that purpose), or use <code>select</code> or
1290 <code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle
1291 input and output on several file descriptors at the same time.</p>
1292
1293 <p>Once a command is processed, the backend uses the
1294 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function
1295 to send its response. For example, the following code shows how to poll for a
1296 side-channel command and respond to it:</p>
1297
1298 <pre class="example">
1299 #include &lt;cups/sidechannel.h&gt;
1300
1301 <a href="#cups_sc_command_t">cups_sc_command_t</a> command;
1302 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
1303 char data[2048];
1304 int datalen = sizeof(data);
1305
1306 /* Poll for a command... */
1307 if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&amp;command, &amp;status, data, &amp;datalen, 0.0))
1308 {
1309   switch (command)
1310   {
1311     /* handle supported commands, fill data/datalen/status with values as needed */
1312
1313     default :
1314         status  = CUPS_SC_STATUS_NOT_IMPLEMENTED;
1315         datalen = 0;
1316         break;
1317   }
1318
1319   /* Send a response... */
1320   <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0);
1321 }
1322 </pre>
1323
1324 <h3><a name="SNMP">Doing SNMP Queries with Network Printers</a></h3>
1325
1326 <p>The Simple Network Management Protocol (SNMP) allows you to get the current
1327 status, page counter, and supply levels from most network printers. Every
1328 piece of information is associated with an Object Identifier (OID), and
1329 every printer has a <em>community</em> name associated with it. OIDs can be
1330 queried directly or by "walking" over a range of OIDs with a common prefix.</p>
1331
1332 <p>The two CUPS SNMP functions provide a simple API for querying network
1333 printers through the side-channel interface. Each accepts a string containing
1334 an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID)
1335 along with a timeout for the query.</p>
1336
1337 <p>The <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>
1338 function queries a single OID and returns the value as a string in a buffer
1339 you supply:</p>
1340
1341 <pre class="example">
1342 #include &lt;cups/sidechannel.h&gt;
1343
1344 char data[512];
1345 int datalen = sizeof(data);
1346
1347 if (<a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a>(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &amp;datalen, 5.0)
1348         == CUPS_SC_STATUS_OK)
1349 {
1350   /* Do something with the value */
1351   printf("Page counter is: %s\n", data);
1352 }
1353 </pre>
1354
1355 <p>The
1356 <a href="#cupsSideChannelSNMPWalk"><code>cupsSideChannelSNMPWalk</code></a>
1357 function allows you to query a whole group of OIDs, calling a function of your
1358 choice for each OID that is found:</p>
1359
1360 <pre class="example">
1361 #include &lt;cups/sidechannel.h&gt;
1362
1363 void
1364 my_callback(const char *oid, const char *data, int datalen, void *context)
1365 {
1366   /* Do something with the value */
1367   printf("%s=%s\n", oid, data);
1368 }
1369
1370 ...
1371
1372 void *my_data;
1373
1374 <a href="#cupsSideChannelSNMPWalk">cupsSNMPSideChannelWalk</a>(".1.3.6.1.2.1.43", 5.0, my_callback, my_data);
1375 </pre>
1376
1377 <h2><a name="SANDBOXING">Sandboxing on macOS</a></h2>
1378
1379 <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>
1380
1381 <ol>
1382
1383         <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>
1384
1385         <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>
1386
1387         <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>
1388
1389         <li>Bluetooth and USB: backends can access Bluetooth and USB printers through IOKit. <em>Filters cannot access Bluetooth and USB printers directly.</em></li>
1390
1391         <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>
1392
1393         <li>Notifications: filters and backends can send notifications via the Darwin <code>notify_post()</code> API.</li>
1394
1395 </ol>
1396
1397 <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>
1398       <h2 class="title"><a id="FUNCTIONS">Functions</a></h2>
1399 <h3 class="function"><span class="info">&#160;CUPS 1.2/macOS 10.5&#160;</span><a id="cupsBackChannelRead">cupsBackChannelRead</a></h3>
1400         <p class="description">Read data from the backchannel.</p>
1401 <p class="code">
1402 ssize_t cupsBackChannelRead(char *buffer, size_t bytes, double timeout);</p>
1403 <h4 class="parameters">Parameters</h4>
1404 <table class="list"><tbody>
1405 <tr><th>buffer</th>
1406         <td class="description">Buffer to read into</td></tr>
1407 <tr><th>bytes</th>
1408         <td class="description">Bytes to read</td></tr>
1409 <tr><th>timeout</th>
1410         <td class="description">Timeout in seconds, typically 0.0 to poll</td></tr>
1411 </tbody></table>
1412 <h4 class="returnvalue">Return Value</h4>
1413         <p class="description">Bytes read or -1 on error</p>
1414 <h4 class="discussion">Discussion</h4>
1415         <p class="discussion">Reads up to &quot;bytes&quot; bytes from the backchannel/backend. The &quot;timeout&quot;
1416 parameter controls how many seconds to wait for the data - use 0.0 to
1417 return immediately if there is no data, -1.0 to wait for data indefinitely.
1418
1419 </p>
1420 <h3 class="function"><span class="info">&#160;CUPS 1.2/macOS 10.5&#160;</span><a id="cupsBackChannelWrite">cupsBackChannelWrite</a></h3>
1421         <p class="description">Write data to the backchannel.</p>
1422 <p class="code">
1423 ssize_t cupsBackChannelWrite(const char *buffer, size_t bytes, double timeout);</p>
1424 <h4 class="parameters">Parameters</h4>
1425 <table class="list"><tbody>
1426 <tr><th>buffer</th>
1427         <td class="description">Buffer to write</td></tr>
1428 <tr><th>bytes</th>
1429         <td class="description">Bytes to write</td></tr>
1430 <tr><th>timeout</th>
1431         <td class="description">Timeout in seconds, typically 1.0</td></tr>
1432 </tbody></table>
1433 <h4 class="returnvalue">Return Value</h4>
1434         <p class="description">Bytes written or -1 on error</p>
1435 <h4 class="discussion">Discussion</h4>
1436         <p class="discussion">Writes &quot;bytes&quot; bytes to the backchannel/filter. The &quot;timeout&quot; parameter
1437 controls how many seconds to wait for the data to be written - use
1438 0.0 to return immediately if the data cannot be written, -1.0 to wait
1439 indefinitely.
1440
1441 </p>
1442 <h3 class="function"><span class="info">&#160;CUPS 1.2/macOS 10.5&#160;</span><a id="cupsBackendDeviceURI">cupsBackendDeviceURI</a></h3>
1443         <p class="description">Get the device URI for a backend.</p>
1444 <p class="code">
1445 const char *cupsBackendDeviceURI(char **argv);</p>
1446 <h4 class="parameters">Parameters</h4>
1447 <table class="list"><tbody>
1448 <tr><th>argv</th>
1449         <td class="description">Command-line arguments</td></tr>
1450 </tbody></table>
1451 <h4 class="returnvalue">Return Value</h4>
1452         <p class="description">Device URI or <code>NULL</code></p>
1453 <h4 class="discussion">Discussion</h4>
1454         <p class="discussion">The &quot;argv&quot; argument is the argv argument passed to main(). This
1455 function returns the device URI passed in the DEVICE_URI environment
1456 variable or the device URI passed in argv[0], whichever is found
1457 first.
1458
1459 </p>
1460 <h3 class="function"><span class="info">&#160;CUPS 1.4/macOS 10.6&#160;</span><a id="cupsBackendReport">cupsBackendReport</a></h3>
1461         <p class="description">Write a device line from a backend.</p>
1462 <p class="code">
1463 void cupsBackendReport(const char *device_scheme, const char *device_uri, const char *device_make_and_model, const char *device_info, const char *device_id, const char *device_location);</p>
1464 <h4 class="parameters">Parameters</h4>
1465 <table class="list"><tbody>
1466 <tr><th>device_scheme</th>
1467         <td class="description">device-scheme string</td></tr>
1468 <tr><th>device_uri</th>
1469         <td class="description">device-uri string</td></tr>
1470 <tr><th>device_make_and_model</th>
1471         <td class="description">device-make-and-model string or <code>NULL</code></td></tr>
1472 <tr><th>device_info</th>
1473         <td class="description">device-info string or <code>NULL</code></td></tr>
1474 <tr><th>device_id</th>
1475         <td class="description">device-id string or <code>NULL</code></td></tr>
1476 <tr><th>device_location</th>
1477         <td class="description">device-location string or <code>NULL</code></td></tr>
1478 </tbody></table>
1479 <h4 class="discussion">Discussion</h4>
1480         <p class="discussion">This function writes a single device line to stdout for a backend.
1481 It handles quoting of special characters in the device-make-and-model,
1482 device-info, device-id, and device-location strings.
1483
1484 </p>
1485 <h3 class="function"><span class="info">&#160;CUPS 1.3/macOS 10.5&#160;</span><a id="cupsSideChannelDoRequest">cupsSideChannelDoRequest</a></h3>
1486         <p class="description">Send a side-channel command to a backend and wait for a response.</p>
1487 <p class="code">
1488 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelDoRequest(<a href="#cups_sc_command_t">cups_sc_command_t</a> command, char *data, int *datalen, double timeout);</p>
1489 <h4 class="parameters">Parameters</h4>
1490 <table class="list"><tbody>
1491 <tr><th>command</th>
1492         <td class="description">Command to send</td></tr>
1493 <tr><th>data</th>
1494         <td class="description">Response data buffer pointer</td></tr>
1495 <tr><th>datalen</th>
1496         <td class="description">Size of data buffer on entry, number of bytes in buffer on return</td></tr>
1497 <tr><th>timeout</th>
1498         <td class="description">Timeout in seconds</td></tr>
1499 </tbody></table>
1500 <h4 class="returnvalue">Return Value</h4>
1501         <p class="description">Status of command</p>
1502 <h4 class="discussion">Discussion</h4>
1503         <p class="discussion">This function is normally only called by filters, drivers, or port
1504 monitors in order to communicate with the backend used by the current
1505 printer.  Programs must be prepared to handle timeout or &quot;not
1506 implemented&quot; status codes, which indicate that the backend or device
1507 do not support the specified side-channel command.<br>
1508 <br>
1509 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
1510 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
1511 update the value to contain the number of data bytes in the buffer.
1512
1513 </p>
1514 <h3 class="function"><span class="info">&#160;CUPS 1.3/macOS 10.5&#160;</span><a id="cupsSideChannelRead">cupsSideChannelRead</a></h3>
1515         <p class="description">Read a side-channel message.</p>
1516 <p class="code">
1517 int cupsSideChannelRead(<a href="#cups_sc_command_t">cups_sc_command_t</a> *command, <a href="#cups_sc_status_t">cups_sc_status_t</a> *status, char *data, int *datalen, double timeout);</p>
1518 <h4 class="parameters">Parameters</h4>
1519 <table class="list"><tbody>
1520 <tr><th>command</th>
1521         <td class="description">Command code</td></tr>
1522 <tr><th>status</th>
1523         <td class="description">Status code</td></tr>
1524 <tr><th>data</th>
1525         <td class="description">Data buffer pointer</td></tr>
1526 <tr><th>datalen</th>
1527         <td class="description">Size of data buffer on entry, number of bytes in buffer on return</td></tr>
1528 <tr><th>timeout</th>
1529         <td class="description">Timeout in seconds</td></tr>
1530 </tbody></table>
1531 <h4 class="returnvalue">Return Value</h4>
1532         <p class="description">0 on success, -1 on error</p>
1533 <h4 class="discussion">Discussion</h4>
1534         <p class="discussion">This function is normally only called by backend programs to read
1535 commands from a filter, driver, or port monitor program.  The
1536 caller must be prepared to handle incomplete or invalid messages
1537 and return the corresponding status codes.<br>
1538 <br>
1539 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
1540 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
1541 update the value to contain the number of data bytes in the buffer.
1542
1543 </p>
1544 <h3 class="function"><span class="info">&#160;CUPS 1.4/macOS 10.6&#160;</span><a id="cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a></h3>
1545         <p class="description">Query a SNMP OID's value.</p>
1546 <p class="code">
1547 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPGet(const char *oid, char *data, int *datalen, double timeout);</p>
1548 <h4 class="parameters">Parameters</h4>
1549 <table class="list"><tbody>
1550 <tr><th>oid</th>
1551         <td class="description">OID to query</td></tr>
1552 <tr><th>data</th>
1553         <td class="description">Buffer for OID value</td></tr>
1554 <tr><th>datalen</th>
1555         <td class="description">Size of OID buffer on entry, size of value on return</td></tr>
1556 <tr><th>timeout</th>
1557         <td class="description">Timeout in seconds</td></tr>
1558 </tbody></table>
1559 <h4 class="returnvalue">Return Value</h4>
1560         <p class="description">Query status</p>
1561 <h4 class="discussion">Discussion</h4>
1562         <p class="discussion">This function asks the backend to do a SNMP OID query on behalf of the
1563 filter, port monitor, or backend using the default community name.<br>
1564 <br>
1565 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
1566 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
1567 supported and must be converted to their numeric forms.<br>
1568 <br>
1569 On input, &quot;data&quot; and &quot;datalen&quot; provide the location and size of the
1570 buffer to hold the OID value as a string. HEX-String (binary) values are
1571 converted to hexadecimal strings representing the binary data, while
1572 NULL-Value and unknown OID types are returned as the empty string.
1573 The returned &quot;datalen&quot; does not include the trailing nul.
1574
1575 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
1576 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
1577 the printer does not respond to the SNMP query.
1578
1579 </p>
1580 <h3 class="function"><span class="info">&#160;CUPS 1.4/macOS 10.6&#160;</span><a id="cupsSideChannelSNMPWalk">cupsSideChannelSNMPWalk</a></h3>
1581         <p class="description">Query multiple SNMP OID values.</p>
1582 <p class="code">
1583 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPWalk(const char *oid, double timeout, <a href="#cups_sc_walk_func_t">cups_sc_walk_func_t</a> cb, void *context);</p>
1584 <h4 class="parameters">Parameters</h4>
1585 <table class="list"><tbody>
1586 <tr><th>oid</th>
1587         <td class="description">First numeric OID to query</td></tr>
1588 <tr><th>timeout</th>
1589         <td class="description">Timeout for each query in seconds</td></tr>
1590 <tr><th>cb</th>
1591         <td class="description">Function to call with each value</td></tr>
1592 <tr><th>context</th>
1593         <td class="description">Application-defined pointer to send to callback</td></tr>
1594 </tbody></table>
1595 <h4 class="returnvalue">Return Value</h4>
1596         <p class="description">Status of first query of <code>CUPS_SC_STATUS_OK</code> on success</p>
1597 <h4 class="discussion">Discussion</h4>
1598         <p class="discussion">This function asks the backend to do multiple SNMP OID queries on behalf
1599 of the filter, port monitor, or backend using the default community name.
1600 All OIDs under the &quot;parent&quot; OID are queried and the results are sent to
1601 the callback function you provide.<br>
1602 <br>
1603 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
1604 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
1605 supported and must be converted to their numeric forms.<br>
1606 <br>
1607 &quot;timeout&quot; specifies the timeout for each OID query. The total amount of
1608 time will depend on the number of OID values found and the time required
1609 for each query.<br>
1610 <br>
1611 &quot;cb&quot; provides a function to call for every value that is found. &quot;context&quot;
1612 is an application-defined pointer that is sent to the callback function
1613 along with the OID and current data. The data passed to the callback is the
1614 same as returned by <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>.
1615
1616 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
1617 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
1618 the printer does not respond to the first SNMP query.
1619
1620 </p>
1621 <h3 class="function"><span class="info">&#160;CUPS 1.3/macOS 10.5&#160;</span><a id="cupsSideChannelWrite">cupsSideChannelWrite</a></h3>
1622         <p class="description">Write a side-channel message.</p>
1623 <p class="code">
1624 int cupsSideChannelWrite(<a href="#cups_sc_command_t">cups_sc_command_t</a> command, <a href="#cups_sc_status_t">cups_sc_status_t</a> status, const char *data, int datalen, double timeout);</p>
1625 <h4 class="parameters">Parameters</h4>
1626 <table class="list"><tbody>
1627 <tr><th>command</th>
1628         <td class="description">Command code</td></tr>
1629 <tr><th>status</th>
1630         <td class="description">Status code</td></tr>
1631 <tr><th>data</th>
1632         <td class="description">Data buffer pointer</td></tr>
1633 <tr><th>datalen</th>
1634         <td class="description">Number of bytes of data</td></tr>
1635 <tr><th>timeout</th>
1636         <td class="description">Timeout in seconds</td></tr>
1637 </tbody></table>
1638 <h4 class="returnvalue">Return Value</h4>
1639         <p class="description">0 on success, -1 on error</p>
1640 <h4 class="discussion">Discussion</h4>
1641         <p class="discussion">This function is normally only called by backend programs to send
1642 responses to a filter, driver, or port monitor program.
1643
1644 </p>
1645       <h2 class="title"><a id="TYPES">Data Types</a></h2>
1646       <h3 class="typedef"><a id="cups_backend_t">cups_backend_t</a></h3>
1647         <p class="description">Backend exit codes</p>
1648       <p class="code">
1649 typedef enum <a href="#cups_backend_e">cups_backend_e</a> cups_backend_t;
1650 </p>
1651       <h3 class="typedef"><a id="cups_sc_bidi_t">cups_sc_bidi_t</a></h3>
1652         <p class="description">Bidirectional capabilities</p>
1653       <p class="code">
1654 typedef enum <a href="#cups_sc_bidi_e">cups_sc_bidi_e</a> cups_sc_bidi_t;
1655 </p>
1656       <h3 class="typedef"><a id="cups_sc_command_t">cups_sc_command_t</a></h3>
1657         <p class="description">Request command codes</p>
1658       <p class="code">
1659 typedef enum <a href="#cups_sc_command_e">cups_sc_command_e</a> cups_sc_command_t;
1660 </p>
1661       <h3 class="typedef"><a id="cups_sc_connected_t">cups_sc_connected_t</a></h3>
1662         <p class="description">Connectivity values</p>
1663       <p class="code">
1664 typedef enum <a href="#cups_sc_connected_e">cups_sc_connected_e</a> cups_sc_connected_t;
1665 </p>
1666       <h3 class="typedef"><a id="cups_sc_state_t">cups_sc_state_t</a></h3>
1667         <p class="description">Printer state bits</p>
1668       <p class="code">
1669 typedef enum <a href="#cups_sc_state_e">cups_sc_state_e</a> cups_sc_state_t;
1670 </p>
1671       <h3 class="typedef"><a id="cups_sc_status_t">cups_sc_status_t</a></h3>
1672         <p class="description">Response status codes</p>
1673       <p class="code">
1674 typedef enum <a href="#cups_sc_status_e">cups_sc_status_e</a> cups_sc_status_t;
1675 </p>
1676       <h3 class="typedef"><a id="cups_sc_walk_func_t">cups_sc_walk_func_t</a></h3>
1677         <p class="description">SNMP walk callback</p>
1678       <p class="code">
1679 typedef void (*cups_sc_walk_func_t)(const char *oid, const char *data, int datalen, void *context);
1680 </p>
1681       <h2 class="title"><a id="ENUMERATIONS">Constants</a></h2>
1682       <h3 class="enumeration"><a id="cups_backend_e">cups_backend_e</a></h3>
1683         <p class="description">Backend exit codes</p>
1684       <h4 class="constants">Constants</h4>
1685       <table class="list"><tbody>
1686         <tr><th>CUPS_BACKEND_AUTH_REQUIRED </th>        <td class="description">Job failed, authentication required</td></tr>
1687         <tr><th>CUPS_BACKEND_CANCEL </th>        <td class="description">Job failed, cancel job</td></tr>
1688         <tr><th>CUPS_BACKEND_FAILED </th>        <td class="description">Job failed, use error-policy</td></tr>
1689         <tr><th>CUPS_BACKEND_HOLD </th>        <td class="description">Job failed, hold job</td></tr>
1690         <tr><th>CUPS_BACKEND_OK </th>        <td class="description">Job completed successfully</td></tr>
1691         <tr><th>CUPS_BACKEND_RETRY </th>        <td class="description">Job failed, retry this job later</td></tr>
1692         <tr><th>CUPS_BACKEND_RETRY_CURRENT </th>        <td class="description">Job failed, retry this job immediately</td></tr>
1693         <tr><th>CUPS_BACKEND_STOP </th>        <td class="description">Job failed, stop queue</td></tr>
1694 </tbody></table>
1695       <h3 class="enumeration"><a id="cups_sc_bidi_e">cups_sc_bidi_e</a></h3>
1696         <p class="description">Bidirectional capability values</p>
1697       <h4 class="constants">Constants</h4>
1698       <table class="list"><tbody>
1699         <tr><th>CUPS_SC_BIDI_NOT_SUPPORTED </th>        <td class="description">Bidirectional I/O is not supported</td></tr>
1700         <tr><th>CUPS_SC_BIDI_SUPPORTED </th>        <td class="description">Bidirectional I/O is supported</td></tr>
1701 </tbody></table>
1702       <h3 class="enumeration"><a id="cups_sc_command_e">cups_sc_command_e</a></h3>
1703         <p class="description">Request command codes</p>
1704       <h4 class="constants">Constants</h4>
1705       <table class="list"><tbody>
1706         <tr><th>CUPS_SC_CMD_DRAIN_OUTPUT </th>        <td class="description">Drain all pending output</td></tr>
1707         <tr><th>CUPS_SC_CMD_GET_BIDI </th>        <td class="description">Return bidirectional capabilities</td></tr>
1708         <tr><th>CUPS_SC_CMD_GET_CONNECTED <span class="info">&#160;CUPS 1.5/macOS 10.7&#160;</span></th>        <td class="description">Return whether the backend is &quot;connected&quot; to the printer </td></tr>
1709         <tr><th>CUPS_SC_CMD_GET_DEVICE_ID </th>        <td class="description">Return the IEEE-1284 device ID</td></tr>
1710         <tr><th>CUPS_SC_CMD_GET_STATE </th>        <td class="description">Return the device state</td></tr>
1711         <tr><th>CUPS_SC_CMD_SNMP_GET <span class="info">&#160;CUPS 1.4/macOS 10.6&#160;</span></th>        <td class="description">Query an SNMP OID </td></tr>
1712         <tr><th>CUPS_SC_CMD_SNMP_GET_NEXT <span class="info">&#160;CUPS 1.4/macOS 10.6&#160;</span></th>        <td class="description">Query the next SNMP OID </td></tr>
1713         <tr><th>CUPS_SC_CMD_SOFT_RESET </th>        <td class="description">Do a soft reset</td></tr>
1714 </tbody></table>
1715       <h3 class="enumeration"><a id="cups_sc_connected_e">cups_sc_connected_e</a></h3>
1716         <p class="description">Connectivity values</p>
1717       <h4 class="constants">Constants</h4>
1718       <table class="list"><tbody>
1719         <tr><th>CUPS_SC_CONNECTED </th>        <td class="description">Backend is &quot;connected&quot; to printer</td></tr>
1720         <tr><th>CUPS_SC_NOT_CONNECTED </th>        <td class="description">Backend is not &quot;connected&quot; to printer</td></tr>
1721 </tbody></table>
1722       <h3 class="enumeration"><a id="cups_sc_state_e">cups_sc_state_e</a></h3>
1723         <p class="description">Printer state bits</p>
1724       <h4 class="constants">Constants</h4>
1725       <table class="list"><tbody>
1726         <tr><th>CUPS_SC_STATE_BUSY </th>        <td class="description">Device is busy</td></tr>
1727         <tr><th>CUPS_SC_STATE_ERROR </th>        <td class="description">Other error condition</td></tr>
1728         <tr><th>CUPS_SC_STATE_MARKER_EMPTY </th>        <td class="description">Toner/ink out condition</td></tr>
1729         <tr><th>CUPS_SC_STATE_MARKER_LOW </th>        <td class="description">Toner/ink low condition</td></tr>
1730         <tr><th>CUPS_SC_STATE_MEDIA_EMPTY </th>        <td class="description">Paper out condition</td></tr>
1731         <tr><th>CUPS_SC_STATE_MEDIA_LOW </th>        <td class="description">Paper low condition</td></tr>
1732         <tr><th>CUPS_SC_STATE_OFFLINE </th>        <td class="description">Device is offline</td></tr>
1733         <tr><th>CUPS_SC_STATE_ONLINE </th>        <td class="description">Device is online</td></tr>
1734 </tbody></table>
1735       <h3 class="enumeration"><a id="cups_sc_status_e">cups_sc_status_e</a></h3>
1736         <p class="description">Response status codes</p>
1737       <h4 class="constants">Constants</h4>
1738       <table class="list"><tbody>
1739         <tr><th>CUPS_SC_STATUS_BAD_MESSAGE </th>        <td class="description">The command/response message was invalid</td></tr>
1740         <tr><th>CUPS_SC_STATUS_IO_ERROR </th>        <td class="description">An I/O error occurred</td></tr>
1741         <tr><th>CUPS_SC_STATUS_NONE </th>        <td class="description">No status</td></tr>
1742         <tr><th>CUPS_SC_STATUS_NOT_IMPLEMENTED </th>        <td class="description">Command not implemented</td></tr>
1743         <tr><th>CUPS_SC_STATUS_NO_RESPONSE </th>        <td class="description">The device did not respond</td></tr>
1744         <tr><th>CUPS_SC_STATUS_OK </th>        <td class="description">Operation succeeded</td></tr>
1745         <tr><th>CUPS_SC_STATUS_TIMEOUT </th>        <td class="description">The backend did not respond</td></tr>
1746         <tr><th>CUPS_SC_STATUS_TOO_BIG </th>        <td class="description">Response too big</td></tr>
1747 </tbody></table>
1748     </div>
1749   </body>
1750 </html>