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 452   <body>
 453 <!--
 454   Filter and backend programming header for CUPS.
 455
 456   Copyright © 2008-2016 by Apple Inc.
 457
 458   Licensed under Apache License v2.0.  See the file "LICENSE" for more
 459   information.
 460 -->
 461
 462 <h1 class='title'>Filter and Backend Programming</h1>
 463
 464 <div class='summary'><table summary='General Information'>
 465 <thead>
 466 <tr>
 467         <th>Headers</th>
 468         <th>cups/backend.h<br>
 469         cups/ppd.h<br>
 470         cups/sidechannel.h</th>
 471 </tr>
 472 </thead>
 473 <tbody>
 474 <tr>
 475         <th>Library</th>
 476         <td>-lcups</td>
 477 </tr>
 478 <tr>
 479         <th>See Also</th>
 480         <td>Programming: <a href='api-overview.html' target='_top'>Introduction to CUPS Programming</a><br>
 481         Programming: <a href='api-cups.html' target='_top'>CUPS API</a><br>
 482         Programming: <a href='api-ppd.html' target='_top'>PPD API</a><br>
 483         Programming: <a href='api-raster.html' target='_top'>Raster API</a><br>
 484         Programming: <a href='postscript-driver.html' target='_top'>Developing PostScript Printer Drivers</a><br>
 485         Programming: <a href='raster-driver.html' target='_top'>Developing Raster Printer Drivers</a><br>
 486         Specifications: <a href='spec-design.html' target='_top'>CUPS Design Description</a></td>
 487 </tr>
 488 </tbody>
 489 </table></div>
 490     <div class="contents">
 491       <h2 class="title">Contents</h2>
 492       <ul class="contents">
 493         <li><a href="#OVERVIEW">Overview</a><ul class="subcontents">
 494           <li><a href="#SECURITY">Security Considerations</a></li>
 495           <li><a href="#SIGNALS">Canceled Jobs and Signal Handling</a></li>
 496           <li><a href="#PERMISSIONS">File Permissions</a></li>
 497           <li><a href="#TEMPFILES">Temporary Files</a></li>
 498           <li><a href="#COPIES">Copy Generation</a></li>
 499           <li><a href="#EXITCODES">Exit Codes</a></li>
 500           <li><a href="#ENVIRONMENT">Environment Variables</a></li>
 501           <li><a href="#MESSAGES">Communicating with the Scheduler</a></li>
 502           <li><a href="#COMMUNICATING_BACKEND">Communicating with the Backend</a></li>
 503           <li><a href="#COMMUNICATING_FILTER">Communicating with Filters</a></li>
 504           <li><a href="#SNMP">Doing SNMP Queries with Network Printers</a></li>
 505         </ul></li>
 506         <li><a href="#SANDBOXING">Sandboxing on macOS</a></li>
 507         <li><a href="#FUNCTIONS">Functions</a><ul class="subcontents">
 508           <li><a href="#cupsBackChannelRead">cupsBackChannelRead</a></li>
 509           <li><a href="#cupsBackChannelWrite">cupsBackChannelWrite</a></li>
 510           <li><a href="#cupsBackendDeviceURI">cupsBackendDeviceURI</a></li>
 511           <li><a href="#cupsBackendReport">cupsBackendReport</a></li>
 512           <li><a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a></li>
 513           <li><a href="#cupsSideChannelRead">cupsSideChannelRead</a></li>
 514           <li><a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a></li>
 515           <li><a href="#cupsSideChannelSNMPWalk">cupsSideChannelSNMPWalk</a></li>
 516           <li><a href="#cupsSideChannelWrite">cupsSideChannelWrite</a></li>
 517         </ul></li>
 518         <li><a href="#TYPES">Data Types</a><ul class="subcontents">
 519           <li><a href="#cups_backend_t">cups_backend_t</a></li>
 520           <li><a href="#cups_sc_bidi_t">cups_sc_bidi_t</a></li>
 521           <li><a href="#cups_sc_command_t">cups_sc_command_t</a></li>
 522           <li><a href="#cups_sc_connected_t">cups_sc_connected_t</a></li>
 523           <li><a href="#cups_sc_state_t">cups_sc_state_t</a></li>
 524           <li><a href="#cups_sc_status_t">cups_sc_status_t</a></li>
 525           <li><a href="#cups_sc_walk_func_t">cups_sc_walk_func_t</a></li>
 526         </ul></li>
 527         <li><a href="#ENUMERATIONS">Enumerations</a><ul class="subcontents">
 528           <li><a href="#cups_backend_e">cups_backend_e</a></li>
 529           <li><a href="#cups_sc_bidi_e">cups_sc_bidi_e</a></li>
 530           <li><a href="#cups_sc_command_e">cups_sc_command_e</a></li>
 531           <li><a href="#cups_sc_connected_e">cups_sc_connected_e</a></li>
 532           <li><a href="#cups_sc_state_e">cups_sc_state_e</a></li>
 533           <li><a href="#cups_sc_status_e">cups_sc_status_e</a></li>
 534         </ul></li>
 535       </ul>
 536     </div>
 537     <div class="body">
 538 <!--
 539   Filter and backend programming introduction for CUPS.
 540
 541   Copyright © 2007-2016 by Apple Inc.
 542   Copyright © 1997-2006 by Easy Software Products, all rights reserved.
 543
 544   Licensed under Apache License v2.0.  See the file "LICENSE" for more
 545   information.
 546 -->
 547
 548 <h2 class='title'><a name="OVERVIEW">Overview</a></h2>
 549
 550 <p>Filters (which include printer drivers and port monitors) and backends
 551 are used to convert job files to a printable format and send that data to the
 552 printer itself. All of these programs use a common interface for processing
 553 print jobs and communicating status information to the scheduler. Each is run
 554 with a standard set of command-line arguments:<p>
 555
 556 <dl class="code">
 557
 558         <dt>argv[1]</dt>
 559         <dd>The job ID</dd>
 560
 561         <dt>argv[2]</dt>
 562         <dd>The user printing the job</dd>
 563
 564         <dt>argv[3]</dt>
 565         <dd>The job name/title</dd>
 566
 567         <dt>argv[4]</dt>
 568         <dd>The number of copies to print</dd>
 569
 570         <dt>argv[5]</dt>
 571         <dd>The options that were provided when the job was submitted</dd>
 572
 573         <dt>argv[6]</dt>
 574         <dd>The file to print (first program only)</dd>
 575 </dl>
 576
 577 <p>The scheduler runs one or more of these programs to print any given job. The
 578 first filter reads from the print file and writes to the standard output, while
 579 the remaining filters read from the standard input and write to the standard
 580 output. The backend is the last filter in the chain and writes to the
 581 device.</p>
 582
 583 <p>Filters are always run as a non-privileged user, typically "lp", with no
 584 connection to the user's desktop. Backends are run either as a non-privileged
 585 user or as root if the file permissions do not allow user or group execution.
 586 The <a href="#PERMISSIONS">file permissions</a> section talks about this in
 587 more detail.</p>
 588
 589 <h3><a name="SECURITY">Security Considerations</a></h3>
 590
 591 <p>It is always important to use security programming practices. Filters and
 592 most backends are run as a non-privileged user, so the major security
 593 consideration is resource utilization - filters should not depend on unlimited
 594 amounts of CPU, memory, or disk space, and should protect against conditions
 595 that could lead to excess usage of any resource like infinite loops and
 596 unbounded recursion. In addition, filters must <em>never</em> allow the user to
 597 specify an arbitrary file path to a separator page, template, or other file
 598 used by the filter since that can lead to an unauthorized disclosure of
 599 information. <em>Always</em> treat input as suspect and validate it!</p>
 600
 601 <p>If you are developing a backend that runs as root, make sure to check for
 602 potential buffer overflows, integer under/overflow conditions, and file
 603 accesses since these can lead to privilege escalations. When writing files,
 604 always validate the file path and <em>never</em> allow a user to determine
 605 where to store a file.</p>
 606
 607 <blockquote><b>Note:</b>
 608
 609 <p><em>Never</em> write files to a user's home directory. Aside from the
 610 security implications, CUPS is a network print service and as such the network
 611 user may not be the same as the local user and/or there may not be a local home
 612 directory to write to.</p>
 613
 614 <p>In addition, some operating systems provide additional security mechanisms
 615 that further limit file system access, even for backends running as root. On
 616 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>
 617 </blockquote>
 618
 619 <h3><a name="SIGNALS">Canceled Jobs and Signal Handling</a></h3>
 620
 621 <p>The scheduler sends <code>SIGTERM</code> when a printing job is canceled or
 622 held. Filters, backends, and port monitors <em>must</em> catch
 623 <code>SIGTERM</code> and perform any cleanup necessary to produce a valid output
 624 file or return the printer to a known good state. The recommended behavior is to
 625 end the output on the current page, preferably on the current line or object
 626 being printed.</p>
 627
 628 <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>
 629
 630 <pre class="example">
 631 #include &lt;signal.h&gt;
 632
 633 ...
 634
 635 int
 636 main(int argc, char *argv[])
 637 {
 638   signal(SIGPIPE, SIG_IGN);
 639
 640   ...
 641 }
 642 </pre>
 643
 644 <h3><a name="PERMISSIONS">File Permissions</a></h3>
 645
 646 <p>For security reasons, CUPS will only run filters and backends that are owned
 647 by root and do not have world or group write permissions. The recommended
 648 permissions for filters and backends are 0555 - read and execute but no write.
 649 Backends that must run as root should use permissions of 0500 - read and execute
 650 by root, no access for other users. Write permissions can be enabled for the
 651 root user only.</p>
 652
 653 <p>To avoid a warning message, the directory containing your filter(s) must also
 654 be owned by root and have world and group write disabled - permissions of 0755
 655 or 0555 are strongly encouraged.</p>
 656
 657 <h3><a name="TEMPFILES">Temporary Files</a></h3>
 658
 659 <p>Temporary files should be created in the directory specified by the
 660 "TMPDIR" environment variable. The
 661 <a href="#cupsTempFile2"><code>cupsTempFile2</code></a> function can be
 662 used to safely create temporary files in this directory.</p>
 663
 664 <h3><a name="COPIES">Copy Generation</a></h3>
 665
 666 <p>The <code>argv[4]</code> argument specifies the number of copies to produce
 667 of the input file. In general, you should only generate copies if the
 668 <em>filename</em> argument is supplied. The only exception to this are
 669 filters that produce device-independent PostScript output, since the PostScript
 670 filter <var>pstops</var> is responsible for generating copies of PostScript
 671 files.</p>
 672
 673 <h3><a name="EXITCODES">Exit Codes</a></h3>
 674
 675 <p>Filters must exit with status 0 when they successfully generate print data
 676 or 1 when they encounter an error. Backends can return any of the
 677 <a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p>
 678
 679 <h3><a name="ENVIRONMENT">Environment Variables</a></h3>
 680
 681 <p>The following environment variables are defined by the printing system
 682 when running print filters and backends:</p>
 683
 684 <dl class="code">
 685
 686         <dt>APPLE_LANGUAGE</dt>
 687         <dd>The Apple language identifier associated with the job
 688         (macOS only).</dd>
 689
 690         <dt>CHARSET</dt>
 691         <dd>The job character set, typically "utf-8".</dd>
 692
 693         <dt>CLASS</dt>
 694         <dd>When a job is submitted to a printer class, contains the name of
 695         the destination printer class. Otherwise this environment
 696         variable will not be set.</dd>
 697
 698         <dt>CONTENT_TYPE</dt>
 699         <dd>The MIME type associated with the file (e.g.
 700         application/postscript).</dd>
 701
 702         <dt>CUPS_CACHEDIR</dt>
 703         <dd>The directory where cache files can be stored. Cache files can be
 704         used to retain information between jobs or files in a job.</dd>
 705
 706         <dt>CUPS_DATADIR</dt>
 707         <dd>The directory where (read-only) CUPS data files can be found.</dd>
 708
 709         <dt>CUPS_FILETYPE</dt>
 710         <dd>The type of file being printed: "job-sheet" for a banner page and
 711         "document" for a regular print file.</dd>
 712
 713         <dt>CUPS_SERVERROOT</dt>
 714         <dd>The root directory of the server.</dd>
 715
 716         <dt>DEVICE_URI</dt>
 717         <dd>The device-uri associated with the printer.</dd>
 718
 719         <dt>FINAL_CONTENT_TYPE</dt>
 720         <dd>The MIME type associated with the printer (e.g.
 721         application/vnd.cups-postscript).</dd>
 722
 723         <dt>LANG</dt>
 724         <dd>The language locale associated with the job.</dd>
 725
 726         <dt>PPD</dt>
 727         <dd>The full pathname of the PostScript Printer Description (PPD)
 728         file for this printer.</dd>
 729
 730         <dt>PRINTER</dt>
 731         <dd>The queue name of the class or printer.</dd>
 732
 733         <dt>RIP_CACHE</dt>
 734         <dd>The recommended amount of memory to use for Raster Image
 735         Processors (RIPs).</dd>
 736
 737         <dt>TMPDIR</dt>
 738         <dd>The directory where temporary files should be created.</dd>
 739
 740 </dl>
 741
 742 <h3><a name="MESSAGES">Communicating with the Scheduler</a></h3>
 743
 744 <p>Filters and backends communicate with the scheduler by writing messages
 745 to the standard error file. The scheduler reads messages from all filters in
 746 a job and processes the message based on its prefix. For example, the following
 747 code sets the current printer state message to "Printing page 5":</p>
 748
 749 <pre class="example">
 750 int page = 5;
 751
 752 fprintf(stderr, "INFO: Printing page %d\n", page);
 753 </pre>
 754
 755 <p>Each message is a single line of text starting with one of the following
 756 prefix strings:</p>
 757
 758 <dl class="code">
 759
 760         <dt>ALERT: message</dt>
 761         <dd>Sets the printer-state-message attribute and adds the specified
 762         message to the current error log file using the "alert" log level.</dd>
 763
 764         <dt>ATTR: attribute=value [attribute=value]</dt>
 765         <dd>Sets the named printer or job attribute(s). Typically this is used
 766         to set the <code>marker-colors</code>, <code>marker-high-levels</code>,
 767         <code>marker-levels</code>, <code>marker-low-levels</code>,
 768         <code>marker-message</code>, <code>marker-names</code>,
 769         <code>marker-types</code>, <code>printer-alert</code>, and
 770         <code>printer-alert-description</code> printer attributes. Standard
 771         <code>marker-types</code> values are listed in <a href='#TABLE1'>Table
 772         1</a>. String values need special handling - see <a href="#ATTR_STRINGS">Reporting Attribute String Values</a> below.</dd>
 773
 774         <dt>CRIT: message</dt>
 775         <dd>Sets the printer-state-message attribute and adds the specified
 776         message to the current error log file using the "critical" log
 777         level.</dd>
 778
 779         <dt>DEBUG: message</dt>
 780         <dd>Sets the printer-state-message attribute and adds the specified
 781         message to the current error log file using the "debug" log level.</dd>
 782
 783         <dt>DEBUG2: message</dt>
 784         <dd>Sets the printer-state-message attribute and adds the specified
 785         message to the current error log file using the "debug2" log level.</dd>
 786
 787         <dt>EMERG: message</dt>
 788         <dd>Sets the printer-state-message attribute and adds the specified
 789         message to the current error log file using the "emergency" log
 790         level.</dd>
 791
 792         <dt>ERROR: message</dt>
 793         <dd>Sets the printer-state-message attribute and adds the specified
 794         message to the current error log file using the "error" log level.
 795         Use "ERROR:" messages for non-persistent processing errors.</dd>
 796
 797         <dt>INFO: message</dt>
 798         <dd>Sets the printer-state-message attribute. If the current log level
 799         is set to "debug2", also adds the specified message to the current error
 800         log file using the "info" log level.</dd>
 801
 802         <dt>NOTICE: message</dt>
 803         <dd>Sets the printer-state-message attribute and adds the specified
 804         message to the current error log file using the "notice" log level.</dd>
 805
 806         <dt>PAGE: page-number #-copies</dt>
 807         <dt>PAGE: total #-pages</dt>
 808         <dd>Adds an entry to the current page log file. The first form adds
 809         #-copies to the job-media-sheets-completed attribute. The second
 810         form sets the job-media-sheets-completed attribute to #-pages.</dd>
 811
 812         <dt>PPD: keyword=value [keyword=value ...]</dt>
 813         <dd>Changes or adds keywords to the printer's PPD file. Typically
 814         this is used to update installable options or default media settings
 815         based on the printer configuration.</dd>
 816
 817         <dt>STATE: + printer-state-reason [printer-state-reason ...]</dt>
 818         <dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
 819         <dd>Sets or clears printer-state-reason keywords for the current queue.
 820         Typically this is used to indicate persistent media, ink, toner, and
 821         configuration conditions or errors on a printer.
 822         <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> -
 823         use vendor-prefixed ("com.example.foo") keywords for custom states. See
 824         <a href="#MANAGING_STATE">Managing Printer State in a Filter</a> for more
 825         information.
 826
 827         <dt>WARNING: message</dt>
 828         <dd>Sets the printer-state-message attribute and adds the specified
 829         message to the current error log file using the "warning" log
 830         level.</dd>
 831
 832 </dl>
 833
 834 <p>Messages without one of these prefixes are treated as if they began with
 835 the "DEBUG:" prefix string.</p>
 836
 837 <div class='table'><table width='80%' summary='Table 1: Standard marker-types Values'>
 838 <caption>Table 1: <a name='TABLE1'>Standard marker-types Values</a></caption>
 839 <thead>
 840 <tr>
 841         <th>marker-type</th>
 842         <th>Description</th>
 843 </tr>
 844 </thead>
 845 <tbody>
 846 <tr>
 847         <td>developer</td>
 848         <td>Developer unit</td>
 849 </tr>
 850 <tr>
 851         <td>fuser</td>
 852         <td>Fuser unit</td>
 853 </tr>
 854 <tr>
 855         <td>fuser-cleaning-pad</td>
 856         <td>Fuser cleaning pad</td>
 857 </tr>
 858 <tr>
 859         <td>fuser-oil</td>
 860         <td>Fuser oil</td>
 861 </tr>
 862 <tr>
 863         <td>ink</td>
 864         <td>Ink supply</td>
 865 </tr>
 866 <tr>
 867         <td>opc</td>
 868         <td>Photo conductor</td>
 869 </tr>
 870 <tr>
 871         <td>solid-wax</td>
 872         <td>Wax supply</td>
 873 </tr>
 874 <tr>
 875         <td>staples</td>
 876         <td>Staple supply</td>
 877 </tr>
 878 <tr>
 879         <td>toner</td>
 880         <td>Toner supply</td>
 881 </tr>
 882 <tr>
 883         <td>transfer-unit</td>
 884         <td>Transfer unit</td>
 885 </tr>
 886 <tr>
 887         <td>waste-ink</td>
 888         <td>Waste ink tank</td>
 889 </tr>
 890 <tr>
 891         <td>waste-toner</td>
 892         <td>Waste toner tank</td>
 893 </tr>
 894 <tr>
 895         <td>waste-wax</td>
 896         <td>Waste wax tank</td>
 897 </tr>
 898 </tbody>
 899 </table></div>
 900
 901 <br>
 902
 903 <div class='table'><table width='80%' summary='Table 2: Standard State Keywords'>
 904 <caption>Table 2: <a name='TABLE2'>Standard State Keywords</a></caption>
 905 <thead>
 906 <tr>
 907         <th>Keyword</th>
 908         <th>Description</th>
 909 </tr>
 910 </thead>
 911 <tbody>
 912 <tr>
 913         <td>connecting-to-device</td>
 914         <td>Connecting to printer but not printing yet.</td>
 915 </tr>
 916 <tr>
 917         <td>cover-open</td>
 918         <td>The printer's cover is open.</td>
 919 </tr>
 920 <tr>
 921         <td>input-tray-missing</td>
 922         <td>The paper tray is missing.</td>
 923 </tr>
 924 <tr>
 925         <td>marker-supply-empty</td>
 926         <td>The printer is out of ink.</td>
 927 </tr>
 928 <tr>
 929         <td>marker-supply-low</td>
 930         <td>The printer is almost out of ink.</td>
 931 </tr>
 932 <tr>
 933         <td>marker-waste-almost-full</td>
 934         <td>The printer's waste bin is almost full.</td>
 935 </tr>
 936 <tr>
 937         <td>marker-waste-full</td>
 938         <td>The printer's waste bin is full.</td>
 939 </tr>
 940 <tr>
 941         <td>media-empty</td>
 942         <td>The paper tray (any paper tray) is empty.</td>
 943 </tr>
 944 <tr>
 945         <td>media-jam</td>
 946         <td>There is a paper jam.</td>
 947 </tr>
 948 <tr>
 949         <td>media-low</td>
 950         <td>The paper tray (any paper tray) is almost empty.</td>
 951 </tr>
 952 <tr>
 953         <td>media-needed</td>
 954         <td>The paper tray needs to be filled (for a job that is printing).</td>
 955 </tr>
 956 <tr>
 957         <td>paused</td>
 958         <td>Stop the printer.</td>
 959 </tr>
 960 <tr>
 961         <td>timed-out</td>
 962         <td>Unable to connect to printer.</td>
 963 </tr>
 964 <tr>
 965         <td>toner-empty</td>
 966         <td>The printer is out of toner.</td>
 967 </tr>
 968 <tr>
 969         <td>toner-low</td>
 970         <td>The printer is low on toner.</td>
 971 </tr>
 972 </tbody>
 973 </table></div>
 974
 975
 976 <h4><a name="ATTR_STRINGS">Reporting Attribute String Values</a></h4>
 977
 978 <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>
 979
 980 <pre class="example">
 981 name=simple
 982 name=simple,simple,...
 983 name='complex value'
 984 name="complex value"
 985 name='"complex value"','"complex value"',...
 986 </pre>
 987
 988 <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>
 989
 990 <pre class="example">
 991 int levels[4] = { 40, 50, 60, 70 }; /* CMYK */
 992
 993 fputs("ATTR: marker-colors=#00FFFF,#FF00FF,#FFFF00,#000000\n", stderr);
 994 fputs("ATTR: marker-high-levels=100,100,100,100\n", stderr);
 995 fprintf(stderr, "ATTR: marker-levels=%d,%d,%d,%d\n", levels[0], levels[1],
 996         levels[2], levels[3], levels[4]);
 997 fputs("ATTR: marker-low-levels=5,5,5,5\n", stderr);
 998 fputs("ATTR: marker-types=toner,toner,toner,toner\n", stderr);
 999 </pre>
1000
1001 <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>
1002
1003 <pre class="example">
1004 fputs("ATTR: marker-message='Levels shown are approximate.'\n", stderr);
1005 </pre>
1006
1007 <p>When multiple values are reported, each value must be enclosed by a set of single and double quotes:</p>
1008
1009 <pre class="example">
1010 fputs("ATTR: marker-names='\"Cyan Toner\"','\"Magenta Toner\"',"
1011       "'\"Yellow Toner\"','\"Black Toner\"'\n", stderr);
1012 </pre>
1013
1014 <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>
1015
1016 <pre class="example">
1017 static const char *                     /* O - Quoted string */
1018 quote_string(const char *s,             /* I - String */
1019              char       *q,             /* I - Quoted string buffer */
1020              size_t     qsize)          /* I - Size of quoted string buffer */
1021 {
1022   char  *qptr,                          /* Pointer into string buffer */
1023         *qend;                          /* End of string buffer */
1024
1025
1026   qptr = q;
1027   qend = q + qsize - 5;
1028
1029   if (qend &lt; q)
1030   {
1031     *q = '\0';
1032     return (q);
1033   }
1034
1035   *qptr++ = '\'';
1036   *qptr++ = '\"';
1037
1038   while (*s && qptr &lt; qend)
1039   {
1040     if (*s == '\\' || *s == '\"' || *s == '\'')
1041     {
1042       if (qptr &lt; (qend - 4))
1043       {
1044         *qptr++ = '\\';
1045         *qptr++ = '\\';
1046         *qptr++ = '\\';
1047       }
1048       else
1049         break;
1050     }
1051
1052     *qptr++ = *s++;
1053   }
1054
1055   *qptr++ = '\"';
1056   *qptr++ = '\'';
1057   *qptr   = '\0';
1058
1059   return (q);
1060 }
1061 </pre>
1062
1063
1064 <h4><a name="MANAGING_STATE">Managing Printer State in a Filter</a></h4>
1065
1066 <p>Filters are responsible for managing the state keywords they set using
1067 "STATE:" messages. Typically you will update <em>all</em> of the keywords that
1068 are used by the filter at startup, for example:</p>
1069
1070 <pre class="example">
1071 if (foo_condition != 0)
1072   fputs("STATE: +com.example.foo\n", stderr);
1073 else
1074   fputs("STATE: -com.example.foo\n", stderr);
1075
1076 if (bar_condition != 0)
1077   fputs("STATE: +com.example.bar\n", stderr);
1078 else
1079   fputs("STATE: -com.example.bar\n", stderr);
1080 </pre>
1081
1082 <p>Then as conditions change, your filter sends "STATE: +keyword" or "STATE:
1083 -keyword" messages as necessary to set or clear the corresponding keyword,
1084 respectively.</p>
1085
1086 <p>State keywords are often used to notify the user of issues that span across
1087 jobs, for example "media-empty-warning" that indicates one or more paper trays
1088 are empty. These keywords should not be cleared unless the corresponding issue
1089 no longer exists.</p>
1090
1091 <p>Filters should clear job-related keywords on startup and exit so that they
1092 do not remain set between jobs.  For example, "connecting-to-device" is a job
1093 sub-state and not an issue that applies when a job is not printing.</p>
1094
1095 <blockquote><b>Note:</b>
1096
1097 <p>"STATE:" messages often provide visible alerts to the user. For example,
1098 on macOS setting a printer-state-reason value with an "-error" or
1099 "-warning" suffix will cause the printer's dock item to bounce if the
1100 corresponding reason is localized with a cupsIPPReason keyword in the
1101 printer's PPD file.</p>
1102
1103 <p>When providing a vendor-prefixed keyword, <em>always</em> provide the
1104 corresponding standard keyword (if any) to allow clients to respond to the
1105 condition correctly. For example, if you provide a vendor-prefixed keyword
1106 for a low cyan ink condition ("com.example.cyan-ink-low") you must also set the
1107 "marker-supply-low-warning" keyword. In such cases you should also refrain
1108 from localizing the vendor-prefixed keyword in the PPD file - otherwise both
1109 the generic and vendor-specific keyword will be shown in the user
1110 interface.</p>
1111
1112 </blockquote>
1113
1114 <h4><a name="REPORTING_SUPPLIES">Reporting Supply Levels</a></h4>
1115
1116 <p>CUPS tracks several "marker-*" attributes for ink/toner supply level
1117 reporting. These attributes allow applications to display the current supply
1118 levels for a printer without printer-specific software. <a href="#TABLE3">Table 3</a> lists the marker attributes and what they represent.</p>
1119
1120 <p>Filters set marker attributes by sending "ATTR:" messages to stderr. For
1121 example, a filter supporting an inkjet printer with black and tri-color ink
1122 cartridges would use the following to initialize the supply attributes:</p>
1123
1124 <pre class="example">
1125 fputs("ATTR: marker-colors=#000000,#00FFFF#FF00FF#FFFF00\n", stderr);
1126 fputs("ATTR: marker-low-levels=5,10\n", stderr);
1127 fputs("ATTR: marker-names=Black,Tri-Color\n", stderr);
1128 fputs("ATTR: marker-types=ink,ink\n", stderr);
1129 </pre>
1130
1131 <p>Then periodically the filter queries the printer for its current supply
1132 levels and updates them with a separate "ATTR:" message:</p>
1133
1134 <pre class="example">
1135 int black_level, tri_level;
1136 ...
1137 fprintf(stderr, "ATTR: marker-levels=%d,%d\n", black_level, tri_level);
1138 </pre>
1139
1140 <div class='table'><table width='80%' summary='Table 3: Supply Level Attributes'>
1141 <caption>Table 3: <a name='TABLE3'>Supply Level Attributes</a></caption>
1142 <thead>
1143 <tr>
1144         <th>Attribute</th>
1145         <th>Description</th>
1146 </tr>
1147 </thead>
1148 <tbody>
1149 <tr>
1150         <td>marker-colors</td>
1151         <td>A list of comma-separated colors; each color is either "none" or one or
1152         more hex-encoded sRGB colors of the form "#RRGGBB".</td>
1153 </tr>
1154 <tr>
1155         <td>marker-high-levels</td>
1156         <td>A list of comma-separated "almost full" level values from 0 to 100; a
1157         value of 100 should be used for supplies that are consumed/emptied like ink
1158         cartridges.</td>
1159 </tr>
1160 <tr>
1161         <td>marker-levels</td>
1162         <td>A list of comma-separated level values for each supply. A value of -1
1163         indicates the level is unavailable, -2 indicates unknown, and -3 indicates
1164         the level is unknown but has not yet reached capacity. Values from 0 to 100
1165         indicate the corresponding percentage.</td>
1166 </tr>
1167 <tr>
1168         <td>marker-low-levels</td>
1169         <td>A list of comma-separated "almost empty" level values from 0 to 100; a
1170         value of 0 should be used for supplies that are filled like waste ink
1171         tanks.</td>
1172 </tr>
1173 <tr>
1174         <td>marker-message</td>
1175         <td>A human-readable supply status message for the user like "12 pages of
1176         ink remaining."</td>
1177 </tr>
1178 <tr>
1179         <td>marker-names</td>
1180         <td>A list of comma-separated supply names like "Cyan Ink", "Fuser",
1181         etc.</td>
1182 </tr>
1183 <tr>
1184         <td>marker-types</td>
1185         <td>A list of comma-separated supply types; the types are listed in
1186         <a href="#TABLE1">Table 1</a>.</td>
1187 </tr>
1188 </tbody>
1189 </table></div>
1190
1191 <h3><a name="COMMUNICATING_BACKEND">Communicating with the Backend</a></h3>
1192
1193 <p>Filters can communicate with the backend via the
1194 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and
1195 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
1196 functions. The
1197 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function
1198 reads data that has been sent back from the device and is typically used to
1199 obtain status and configuration information. For example, the following code
1200 polls the backend for back-channel data:</p>
1201
1202 <pre class="example">
1203 #include &lt;cups/cups.h&gt;
1204
1205 char buffer[8192];
1206 ssize_t bytes;
1207
1208 /* Use a timeout of 0.0 seconds to poll for back-channel data */
1209 bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
1210 </pre>
1211
1212 <p>Filters can also use <code>select()</code> or <code>poll()</code> on the
1213 back-channel file descriptor (3 or <code>CUPS_BC_FD</code>) to read data only
1214 when it is available.</p>
1215
1216 <p>The
1217 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
1218 function allows you to get out-of-band status information and do synchronization
1219 with the device. For example, the following code gets the current IEEE-1284
1220 device ID string from the backend:</p>
1221
1222 <pre class="example">
1223 #include &lt;cups/sidechannel.h&gt;
1224
1225 char data[2049];
1226 int datalen;
1227 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
1228
1229 /* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for
1230    nul-termination... */
1231 datalen = sizeof(data) - 1;
1232
1233 /* Get the IEEE-1284 device ID, waiting for up to 1 second */
1234 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &amp;datalen, 1.0);
1235
1236 /* Use the returned value if OK was returned and the length is non-zero */
1237 if (status == CUPS_SC_STATUS_OK &amp;&amp; datalen > 0)
1238   data[datalen] = '\0';
1239 else
1240   data[0] = '\0';
1241 </pre>
1242
1243 <h4><a name="DRAIN_OUTPUT">Forcing All Output to a Printer</a></h4>
1244
1245 <p>The
1246 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
1247 function allows you to tell the backend to send all pending data to the printer.
1248 This is most often needed when sending query commands to the printer. For example:</p>
1249
1250 <pre class="example">
1251 #include &lt;cups/cups.h&gt;
1252 #include &lt;cups/sidechannel.h&gt;
1253
1254 char data[1024];
1255 int datalen = sizeof(data);
1256 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
1257
1258 /* Flush pending output to stdout */
1259 fflush(stdout);
1260
1261 /* Drain output to backend, waiting for up to 30 seconds */
1262 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_DRAIN_OUTPUT, data, &amp;datalen, 30.0);
1263
1264 /* Read the response if the output was sent */
1265 if (status == CUPS_SC_STATUS_OK)
1266 {
1267   ssize_t bytes;
1268
1269   /* Wait up to 10.0 seconds for back-channel data */
1270   bytes = cupsBackChannelRead(data, sizeof(data), 10.0);
1271   /* do something with the data from the printer */
1272 }
1273 </pre>
1274
1275 <h3><a name="COMMUNICATING_FILTER">Communicating with Filters</a></h3>
1276
1277 <p>Backends communicate with filters using the reciprocal functions
1278 <a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>,
1279 <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and
1280 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We
1281 recommend writing back-channel data using a timeout of 1.0 seconds:</p>
1282
1283 <pre class="example">
1284 #include &lt;cups/cups.h&gt;
1285
1286 char buffer[8192];
1287 ssize_t bytes;
1288
1289 /* Obtain data from printer/device */
1290 ...
1291
1292 /* Use a timeout of 1.0 seconds to give filters a chance to read */
1293 cupsBackChannelWrite(buffer, bytes, 1.0);
1294 </pre>
1295
1296 <p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>
1297 function reads a side-channel command from a filter, driver, or port monitor.
1298 Backends can either poll for commands using a <code>timeout</code> of 0.0, wait
1299 indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a
1300 separate thread for that purpose), or use <code>select</code> or
1301 <code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle
1302 input and output on several file descriptors at the same time.</p>
1303
1304 <p>Once a command is processed, the backend uses the
1305 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function
1306 to send its response. For example, the following code shows how to poll for a
1307 side-channel command and respond to it:</p>
1308
1309 <pre class="example">
1310 #include &lt;cups/sidechannel.h&gt;
1311
1312 <a href="#cups_sc_command_t">cups_sc_command_t</a> command;
1313 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
1314 char data[2048];
1315 int datalen = sizeof(data);
1316
1317 /* Poll for a command... */
1318 if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&amp;command, &amp;status, data, &amp;datalen, 0.0))
1319 {
1320   switch (command)
1321   {
1322     /* handle supported commands, fill data/datalen/status with values as needed */
1323
1324     default :
1325         status  = CUPS_SC_STATUS_NOT_IMPLEMENTED;
1326         datalen = 0;
1327         break;
1328   }
1329
1330   /* Send a response... */
1331   <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0);
1332 }
1333 </pre>
1334
1335 <h3><a name="SNMP">Doing SNMP Queries with Network Printers</a></h3>
1336
1337 <p>The Simple Network Management Protocol (SNMP) allows you to get the current
1338 status, page counter, and supply levels from most network printers. Every
1339 piece of information is associated with an Object Identifier (OID), and
1340 every printer has a <em>community</em> name associated with it. OIDs can be
1341 queried directly or by "walking" over a range of OIDs with a common prefix.</p>
1342
1343 <p>The two CUPS SNMP functions provide a simple API for querying network
1344 printers through the side-channel interface. Each accepts a string containing
1345 an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID)
1346 along with a timeout for the query.</p>
1347
1348 <p>The <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>
1349 function queries a single OID and returns the value as a string in a buffer
1350 you supply:</p>
1351
1352 <pre class="example">
1353 #include &lt;cups/sidechannel.h&gt;
1354
1355 char data[512];
1356 int datalen = sizeof(data);
1357
1358 if (<a href="#cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a>(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &amp;datalen, 5.0)
1359         == CUPS_SC_STATUS_OK)
1360 {
1361   /* Do something with the value */
1362   printf("Page counter is: %s\n", data);
1363 }
1364 </pre>
1365
1366 <p>The
1367 <a href="#cupsSideChannelSNMPWalk"><code>cupsSideChannelSNMPWalk</code></a>
1368 function allows you to query a whole group of OIDs, calling a function of your
1369 choice for each OID that is found:</p>
1370
1371 <pre class="example">
1372 #include &lt;cups/sidechannel.h&gt;
1373
1374 void
1375 my_callback(const char *oid, const char *data, int datalen, void *context)
1376 {
1377   /* Do something with the value */
1378   printf("%s=%s\n", oid, data);
1379 }
1380
1381 ...
1382
1383 void *my_data;
1384
1385 <a href="#cupsSideChannelSNMPWalk">cupsSNMPSideChannelWalk</a>(".1.3.6.1.2.1.43", 5.0, my_callback, my_data);
1386 </pre>
1387
1388 <h2><a name="SANDBOXING">Sandboxing on macOS</a></h2>
1389
1390 <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>
1391
1392 <ol>
1393
1394         <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>
1395
1396         <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>
1397
1398         <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>
1399
1400         <li>Bluetooth and USB: backends can access Bluetooth and USB printers through IOKit. <em>Filters cannot access Bluetooth and USB printers directly.</em></li>
1401
1402         <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>
1403
1404         <li>Notifications: filters and backends can send notifications via the Darwin <code>notify_post()</code> API.</li>
1405
1406 </ol>
1407
1408 <blockquote><b>Note:</b>
1409
1410 <p>The sandbox profile used in CUPS still allows some actions that are not listed above - these privileges will be removed over time until the profile matches the list above.</p>
1411 </blockquote>
1412       <h2 class="title"><a id="FUNCTIONS">Functions</a></h2>
1413 <h3 class="function"><span class="info">&#160;CUPS 1.2/macOS 10.5&#160;</span><a id="cupsBackChannelRead">cupsBackChannelRead</a></h3>
1414         <p class="description">Read data from the backchannel.</p>
1415 <p class="code">
1416 ssize_t cupsBackChannelRead(char *buffer, size_t bytes, double timeout);</p>
1417 <h4 class="parameters">Parameters</h4>
1418 <table class="list"><tbody>
1419 <tr><th>buffer</th>
1420         <td class="description">Buffer to read into</td></tr>
1421 <tr><th>bytes</th>
1422         <td class="description">Bytes to read</td></tr>
1423 <tr><th>timeout</th>
1424         <td class="description">Timeout in seconds, typically 0.0 to poll</td></tr>
1425 </tbody></table>
1426 <h4 class="returnvalue">Return Value</h4>
1427         <p class="description">Bytes read or -1 on error</p>
1428 <h4 class="discussion">Discussion</h4>
1429         <p class="discussion">Reads up to &quot;bytes&quot; bytes from the backchannel/backend. The &quot;timeout&quot;
1430 parameter controls how many seconds to wait for the data - use 0.0 to
1431 return immediately if there is no data, -1.0 to wait for data indefinitely.
1432
1433 </p>
1434 <h3 class="function"><span class="info">&#160;CUPS 1.2/macOS 10.5&#160;</span><a id="cupsBackChannelWrite">cupsBackChannelWrite</a></h3>
1435         <p class="description">Write data to the backchannel.</p>
1436 <p class="code">
1437 ssize_t cupsBackChannelWrite(const char *buffer, size_t bytes, double timeout);</p>
1438 <h4 class="parameters">Parameters</h4>
1439 <table class="list"><tbody>
1440 <tr><th>buffer</th>
1441         <td class="description">Buffer to write</td></tr>
1442 <tr><th>bytes</th>
1443         <td class="description">Bytes to write</td></tr>
1444 <tr><th>timeout</th>
1445         <td class="description">Timeout in seconds, typically 1.0</td></tr>
1446 </tbody></table>
1447 <h4 class="returnvalue">Return Value</h4>
1448         <p class="description">Bytes written or -1 on error</p>
1449 <h4 class="discussion">Discussion</h4>
1450         <p class="discussion">Writes &quot;bytes&quot; bytes to the backchannel/filter. The &quot;timeout&quot; parameter
1451 controls how many seconds to wait for the data to be written - use
1452 0.0 to return immediately if the data cannot be written, -1.0 to wait
1453 indefinitely.
1454
1455 </p>
1456 <h3 class="function"><span class="info">&#160;CUPS 1.2/macOS 10.5&#160;</span><a id="cupsBackendDeviceURI">cupsBackendDeviceURI</a></h3>
1457         <p class="description">Get the device URI for a backend.</p>
1458 <p class="code">
1459 const char *cupsBackendDeviceURI(char **argv);</p>
1460 <h4 class="parameters">Parameters</h4>
1461 <table class="list"><tbody>
1462 <tr><th>argv</th>
1463         <td class="description">Command-line arguments</td></tr>
1464 </tbody></table>
1465 <h4 class="returnvalue">Return Value</h4>
1466         <p class="description">Device URI or <code>NULL</code></p>
1467 <h4 class="discussion">Discussion</h4>
1468         <p class="discussion">The &quot;argv&quot; argument is the argv argument passed to main(). This
1469 function returns the device URI passed in the DEVICE_URI environment
1470 variable or the device URI passed in argv[0], whichever is found
1471 first.
1472
1473 </p>
1474 <h3 class="function"><span class="info">&#160;CUPS 1.4/macOS 10.6&#160;</span><a id="cupsBackendReport">cupsBackendReport</a></h3>
1475         <p class="description">Write a device line from a backend.</p>
1476 <p class="code">
1477 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>
1478 <h4 class="parameters">Parameters</h4>
1479 <table class="list"><tbody>
1480 <tr><th>device_scheme</th>
1481         <td class="description">device-scheme string</td></tr>
1482 <tr><th>device_uri</th>
1483         <td class="description">device-uri string</td></tr>
1484 <tr><th>device_make_and_model</th>
1485         <td class="description">device-make-and-model string or <code>NULL</code></td></tr>
1486 <tr><th>device_info</th>
1487         <td class="description">device-info string or <code>NULL</code></td></tr>
1488 <tr><th>device_id</th>
1489         <td class="description">device-id string or <code>NULL</code></td></tr>
1490 <tr><th>device_location</th>
1491         <td class="description">device-location string or <code>NULL</code></td></tr>
1492 </tbody></table>
1493 <h4 class="discussion">Discussion</h4>
1494         <p class="discussion">This function writes a single device line to stdout for a backend.
1495 It handles quoting of special characters in the device-make-and-model,
1496 device-info, device-id, and device-location strings.
1497
1498 </p>
1499 <h3 class="function"><span class="info">&#160;CUPS 1.3/macOS 10.5&#160;</span><a id="cupsSideChannelDoRequest">cupsSideChannelDoRequest</a></h3>
1500         <p class="description">Send a side-channel command to a backend and wait for a response.</p>
1501 <p class="code">
1502 <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>
1503 <h4 class="parameters">Parameters</h4>
1504 <table class="list"><tbody>
1505 <tr><th>command</th>
1506         <td class="description">Command to send</td></tr>
1507 <tr><th>data</th>
1508         <td class="description">Response data buffer pointer</td></tr>
1509 <tr><th>datalen</th>
1510         <td class="description">Size of data buffer on entry, number of bytes in buffer on return</td></tr>
1511 <tr><th>timeout</th>
1512         <td class="description">Timeout in seconds</td></tr>
1513 </tbody></table>
1514 <h4 class="returnvalue">Return Value</h4>
1515         <p class="description">Status of command</p>
1516 <h4 class="discussion">Discussion</h4>
1517         <p class="discussion">This function is normally only called by filters, drivers, or port
1518 monitors in order to communicate with the backend used by the current
1519 printer.  Programs must be prepared to handle timeout or &quot;not
1520 implemented&quot; status codes, which indicate that the backend or device
1521 do not support the specified side-channel command.<br>
1522 <br>
1523 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
1524 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
1525 update the value to contain the number of data bytes in the buffer.
1526
1527 </p>
1528 <h3 class="function"><span class="info">&#160;CUPS 1.3/macOS 10.5&#160;</span><a id="cupsSideChannelRead">cupsSideChannelRead</a></h3>
1529         <p class="description">Read a side-channel message.</p>
1530 <p class="code">
1531 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>
1532 <h4 class="parameters">Parameters</h4>
1533 <table class="list"><tbody>
1534 <tr><th>command</th>
1535         <td class="description">Command code</td></tr>
1536 <tr><th>status</th>
1537         <td class="description">Status code</td></tr>
1538 <tr><th>data</th>
1539         <td class="description">Data buffer pointer</td></tr>
1540 <tr><th>datalen</th>
1541         <td class="description">Size of data buffer on entry, number of bytes in buffer on return</td></tr>
1542 <tr><th>timeout</th>
1543         <td class="description">Timeout in seconds</td></tr>
1544 </tbody></table>
1545 <h4 class="returnvalue">Return Value</h4>
1546         <p class="description">0 on success, -1 on error</p>
1547 <h4 class="discussion">Discussion</h4>
1548         <p class="discussion">This function is normally only called by backend programs to read
1549 commands from a filter, driver, or port monitor program.  The
1550 caller must be prepared to handle incomplete or invalid messages
1551 and return the corresponding status codes.<br>
1552 <br>
1553 The &quot;datalen&quot; parameter must be initialized to the size of the buffer
1554 pointed to by the &quot;data&quot; parameter.  cupsSideChannelDoRequest() will
1555 update the value to contain the number of data bytes in the buffer.
1556
1557 </p>
1558 <h3 class="function"><span class="info">&#160;CUPS 1.4/macOS 10.6&#160;</span><a id="cupsSideChannelSNMPGet">cupsSideChannelSNMPGet</a></h3>
1559         <p class="description">Query a SNMP OID's value.</p>
1560 <p class="code">
1561 <a href="#cups_sc_status_t">cups_sc_status_t</a> cupsSideChannelSNMPGet(const char *oid, char *data, int *datalen, double timeout);</p>
1562 <h4 class="parameters">Parameters</h4>
1563 <table class="list"><tbody>
1564 <tr><th>oid</th>
1565         <td class="description">OID to query</td></tr>
1566 <tr><th>data</th>
1567         <td class="description">Buffer for OID value</td></tr>
1568 <tr><th>datalen</th>
1569         <td class="description">Size of OID buffer on entry, size of value on return</td></tr>
1570 <tr><th>timeout</th>
1571         <td class="description">Timeout in seconds</td></tr>
1572 </tbody></table>
1573 <h4 class="returnvalue">Return Value</h4>
1574         <p class="description">Query status</p>
1575 <h4 class="discussion">Discussion</h4>
1576         <p class="discussion">This function asks the backend to do a SNMP OID query on behalf of the
1577 filter, port monitor, or backend using the default community name.<br>
1578 <br>
1579 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
1580 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
1581 supported and must be converted to their numeric forms.<br>
1582 <br>
1583 On input, &quot;data&quot; and &quot;datalen&quot; provide the location and size of the
1584 buffer to hold the OID value as a string. HEX-String (binary) values are
1585 converted to hexadecimal strings representing the binary data, while
1586 NULL-Value and unknown OID types are returned as the empty string.
1587 The returned &quot;datalen&quot; does not include the trailing nul.
1588
1589 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
1590 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
1591 the printer does not respond to the SNMP query.
1592
1593 </p>
1594 <h3 class="function"><span class="info">&#160;CUPS 1.4/macOS 10.6&#160;</span><a id="cupsSideChannelSNMPWalk">cupsSideChannelSNMPWalk</a></h3>
1595         <p class="description">Query multiple SNMP OID values.</p>
1596 <p class="code">
1597 <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>
1598 <h4 class="parameters">Parameters</h4>
1599 <table class="list"><tbody>
1600 <tr><th>oid</th>
1601         <td class="description">First numeric OID to query</td></tr>
1602 <tr><th>timeout</th>
1603         <td class="description">Timeout for each query in seconds</td></tr>
1604 <tr><th>cb</th>
1605         <td class="description">Function to call with each value</td></tr>
1606 <tr><th>context</th>
1607         <td class="description">Application-defined pointer to send to callback</td></tr>
1608 </tbody></table>
1609 <h4 class="returnvalue">Return Value</h4>
1610         <p class="description">Status of first query of <code>CUPS_SC_STATUS_OK</code> on success</p>
1611 <h4 class="discussion">Discussion</h4>
1612         <p class="discussion">This function asks the backend to do multiple SNMP OID queries on behalf
1613 of the filter, port monitor, or backend using the default community name.
1614 All OIDs under the &quot;parent&quot; OID are queried and the results are sent to
1615 the callback function you provide.<br>
1616 <br>
1617 &quot;oid&quot; contains a numeric OID consisting of integers separated by periods,
1618 for example &quot;.1.3.6.1.2.1.43&quot;.  Symbolic names from SNMP MIBs are not
1619 supported and must be converted to their numeric forms.<br>
1620 <br>
1621 &quot;timeout&quot; specifies the timeout for each OID query. The total amount of
1622 time will depend on the number of OID values found and the time required
1623 for each query.<br>
1624 <br>
1625 &quot;cb&quot; provides a function to call for every value that is found. &quot;context&quot;
1626 is an application-defined pointer that is sent to the callback function
1627 along with the OID and current data. The data passed to the callback is the
1628 same as returned by <a href="#cupsSideChannelSNMPGet"><code>cupsSideChannelSNMPGet</code></a>.
1629
1630 <code>CUPS_SC_STATUS_NOT_IMPLEMENTED</code> is returned by backends that do not
1631 support SNMP queries.  <code>CUPS_SC_STATUS_NO_RESPONSE</code> is returned when
1632 the printer does not respond to the first SNMP query.
1633
1634 </p>
1635 <h3 class="function"><span class="info">&#160;CUPS 1.3/macOS 10.5&#160;</span><a id="cupsSideChannelWrite">cupsSideChannelWrite</a></h3>
1636         <p class="description">Write a side-channel message.</p>
1637 <p class="code">
1638 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>
1639 <h4 class="parameters">Parameters</h4>
1640 <table class="list"><tbody>
1641 <tr><th>command</th>
1642         <td class="description">Command code</td></tr>
1643 <tr><th>status</th>
1644         <td class="description">Status code</td></tr>
1645 <tr><th>data</th>
1646         <td class="description">Data buffer pointer</td></tr>
1647 <tr><th>datalen</th>
1648         <td class="description">Number of bytes of data</td></tr>
1649 <tr><th>timeout</th>
1650         <td class="description">Timeout in seconds</td></tr>
1651 </tbody></table>
1652 <h4 class="returnvalue">Return Value</h4>
1653         <p class="description">0 on success, -1 on error</p>
1654 <h4 class="discussion">Discussion</h4>
1655         <p class="discussion">This function is normally only called by backend programs to send
1656 responses to a filter, driver, or port monitor program.
1657
1658 </p>
1659       <h2 class="title"><a id="TYPES">Data Types</a></h2>
1660       <h3 class="typedef"><a id="cups_backend_t">cups_backend_t</a></h3>
1661         <p class="description">Backend exit codes</p>
1662       <p class="code">
1663 typedef enum <a href="#cups_backend_e">cups_backend_e</a> cups_backend_t;
1664 </p>
1665       <h3 class="typedef"><a id="cups_sc_bidi_t">cups_sc_bidi_t</a></h3>
1666         <p class="description">Bidirectional capabilities</p>
1667       <p class="code">
1668 typedef enum <a href="#cups_sc_bidi_e">cups_sc_bidi_e</a> cups_sc_bidi_t;
1669 </p>
1670       <h3 class="typedef"><a id="cups_sc_command_t">cups_sc_command_t</a></h3>
1671         <p class="description">Request command codes</p>
1672       <p class="code">
1673 typedef enum <a href="#cups_sc_command_e">cups_sc_command_e</a> cups_sc_command_t;
1674 </p>
1675       <h3 class="typedef"><a id="cups_sc_connected_t">cups_sc_connected_t</a></h3>
1676         <p class="description">Connectivity values</p>
1677       <p class="code">
1678 typedef enum <a href="#cups_sc_connected_e">cups_sc_connected_e</a> cups_sc_connected_t;
1679 </p>
1680       <h3 class="typedef"><a id="cups_sc_state_t">cups_sc_state_t</a></h3>
1681         <p class="description">Printer state bits</p>
1682       <p class="code">
1683 typedef enum <a href="#cups_sc_state_e">cups_sc_state_e</a> cups_sc_state_t;
1684 </p>
1685       <h3 class="typedef"><a id="cups_sc_status_t">cups_sc_status_t</a></h3>
1686         <p class="description">Response status codes</p>
1687       <p class="code">
1688 typedef enum <a href="#cups_sc_status_e">cups_sc_status_e</a> cups_sc_status_t;
1689 </p>
1690       <h3 class="typedef"><a id="cups_sc_walk_func_t">cups_sc_walk_func_t</a></h3>
1691         <p class="description">SNMP walk callback</p>
1692       <p class="code">
1693 typedef void (*cups_sc_walk_func_t)(const char *oid, const char *data, int datalen, void *context);
1694 </p>
1695       <h2 class="title"><a id="ENUMERATIONS">Constants</a></h2>
1696       <h3 class="enumeration"><a id="cups_backend_e">cups_backend_e</a></h3>
1697         <p class="description">Backend exit codes</p>
1698       <h4 class="constants">Constants</h4>
1699       <table class="list"><tbody>
1700         <tr><th>CUPS_BACKEND_AUTH_REQUIRED </th>        <td class="description">Job failed, authentication required</td></tr>
1701         <tr><th>CUPS_BACKEND_CANCEL </th>        <td class="description">Job failed, cancel job</td></tr>
1702         <tr><th>CUPS_BACKEND_FAILED </th>        <td class="description">Job failed, use error-policy</td></tr>
1703         <tr><th>CUPS_BACKEND_HOLD </th>        <td class="description">Job failed, hold job</td></tr>
1704         <tr><th>CUPS_BACKEND_OK </th>        <td class="description">Job completed successfully</td></tr>
1705         <tr><th>CUPS_BACKEND_RETRY </th>        <td class="description">Job failed, retry this job later</td></tr>
1706         <tr><th>CUPS_BACKEND_RETRY_CURRENT </th>        <td class="description">Job failed, retry this job immediately</td></tr>
1707         <tr><th>CUPS_BACKEND_STOP </th>        <td class="description">Job failed, stop queue</td></tr>
1708 </tbody></table>
1709       <h3 class="enumeration"><a id="cups_sc_bidi_e">cups_sc_bidi_e</a></h3>
1710         <p class="description">Bidirectional capability values</p>
1711       <h4 class="constants">Constants</h4>
1712       <table class="list"><tbody>
1713         <tr><th>CUPS_SC_BIDI_NOT_SUPPORTED </th>        <td class="description">Bidirectional I/O is not supported</td></tr>
1714         <tr><th>CUPS_SC_BIDI_SUPPORTED </th>        <td class="description">Bidirectional I/O is supported</td></tr>
1715 </tbody></table>
1716       <h3 class="enumeration"><a id="cups_sc_command_e">cups_sc_command_e</a></h3>
1717         <p class="description">Request command codes</p>
1718       <h4 class="constants">Constants</h4>
1719       <table class="list"><tbody>
1720         <tr><th>CUPS_SC_CMD_DRAIN_OUTPUT </th>        <td class="description">Drain all pending output</td></tr>
1721         <tr><th>CUPS_SC_CMD_GET_BIDI </th>        <td class="description">Return bidirectional capabilities</td></tr>
1722         <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>
1723         <tr><th>CUPS_SC_CMD_GET_DEVICE_ID </th>        <td class="description">Return the IEEE-1284 device ID</td></tr>
1724         <tr><th>CUPS_SC_CMD_GET_STATE </th>        <td class="description">Return the device state</td></tr>
1725         <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>
1726         <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>
1727         <tr><th>CUPS_SC_CMD_SOFT_RESET </th>        <td class="description">Do a soft reset</td></tr>
1728 </tbody></table>
1729       <h3 class="enumeration"><a id="cups_sc_connected_e">cups_sc_connected_e</a></h3>
1730         <p class="description">Connectivity values</p>
1731       <h4 class="constants">Constants</h4>
1732       <table class="list"><tbody>
1733         <tr><th>CUPS_SC_CONNECTED </th>        <td class="description">Backend is &quot;connected&quot; to printer</td></tr>
1734         <tr><th>CUPS_SC_NOT_CONNECTED </th>        <td class="description">Backend is not &quot;connected&quot; to printer</td></tr>
1735 </tbody></table>
1736       <h3 class="enumeration"><a id="cups_sc_state_e">cups_sc_state_e</a></h3>
1737         <p class="description">Printer state bits</p>
1738       <h4 class="constants">Constants</h4>
1739       <table class="list"><tbody>
1740         <tr><th>CUPS_SC_STATE_BUSY </th>        <td class="description">Device is busy</td></tr>
1741         <tr><th>CUPS_SC_STATE_ERROR </th>        <td class="description">Other error condition</td></tr>
1742         <tr><th>CUPS_SC_STATE_MARKER_EMPTY </th>        <td class="description">Toner/ink out condition</td></tr>
1743         <tr><th>CUPS_SC_STATE_MARKER_LOW </th>        <td class="description">Toner/ink low condition</td></tr>
1744         <tr><th>CUPS_SC_STATE_MEDIA_EMPTY </th>        <td class="description">Paper out condition</td></tr>
1745         <tr><th>CUPS_SC_STATE_MEDIA_LOW </th>        <td class="description">Paper low condition</td></tr>
1746         <tr><th>CUPS_SC_STATE_OFFLINE </th>        <td class="description">Device is offline</td></tr>
1747         <tr><th>CUPS_SC_STATE_ONLINE </th>        <td class="description">Device is online</td></tr>
1748 </tbody></table>
1749       <h3 class="enumeration"><a id="cups_sc_status_e">cups_sc_status_e</a></h3>
1750         <p class="description">Response status codes</p>
1751       <h4 class="constants">Constants</h4>
1752       <table class="list"><tbody>
1753         <tr><th>CUPS_SC_STATUS_BAD_MESSAGE </th>        <td class="description">The command/response message was invalid</td></tr>
1754         <tr><th>CUPS_SC_STATUS_IO_ERROR </th>        <td class="description">An I/O error occurred</td></tr>
1755         <tr><th>CUPS_SC_STATUS_NONE </th>        <td class="description">No status</td></tr>
1756         <tr><th>CUPS_SC_STATUS_NOT_IMPLEMENTED </th>        <td class="description">Command not implemented</td></tr>
1757         <tr><th>CUPS_SC_STATUS_NO_RESPONSE </th>        <td class="description">The device did not respond</td></tr>
1758         <tr><th>CUPS_SC_STATUS_OK </th>        <td class="description">Operation succeeded</td></tr>
1759         <tr><th>CUPS_SC_STATUS_TIMEOUT </th>        <td class="description">The backend did not respond</td></tr>
1760         <tr><th>CUPS_SC_STATUS_TOO_BIG </th>        <td class="description">Response too big</td></tr>
1761 </tbody></table>
1762     </div>
1763   </body>
1764 </html>