cups/api-filter.shtml

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   2   "$Id: api-filter.shtml 6649 2007-07-11 21:46:42Z mike $"
   3
   4   Filter and backend programming introduction for the Common UNIX Printing
   5   System (CUPS).
   6
   7   Copyright 2007-2008 by Apple Inc.
   8   Copyright 1997-2006 by Easy Software Products, all rights reserved.
   9
  10   These coded instructions, statements, and computer programs are the
  11   property of Apple Inc. and are protected by Federal copyright
  12   law.  Distribution and use rights are outlined in the file "LICENSE.txt"
  13   which should have been included with this file.  If this file is
  14   file is missing or damaged, see the license at "http://www.cups.org/".
  15 -->
  16
  17 <h2 class='title'><a name="OVERVIEW">Overview</a></h2>
  18
  19 <p>Filters, printer drivers, port monitors, and backends use a common interface
  20 for processing print jobs and communicating status information to the scheduler.
  21 Each filter is run with a standard set of command-line arguments:<p>
  22
  23 <dl class="code">
  24
  25         <dt>argv[1]</dt>
  26         <dd>The job ID</dd>
  27
  28         <dt>argv[2]</dt>
  29         <dd>The user printing the job</dd>
  30
  31         <dt>argv[3]</dt>
  32         <dd>The job name/title</dd>
  33
  34         <dt>argv[4]</dt>
  35         <dd>The number of copies to print</dd>
  36
  37         <dt>argv[5]</dt>
  38         <dd>The options that were provided when the job was submitted</dd>
  39
  40         <dt>argv[6]</dt>
  41         <dd>The file to print (first filter only)</dd>
  42 </dl>
  43
  44 <p>The scheduler runs one or more of these programs to print any given job. The
  45 first filter reads from the print file and writes to the standard output, while
  46 the remaining filters read from the standard input and write to the standard
  47 output. The backend is the last filter in the chain and writes to the
  48 device.</p>
  49
  50 <h3><a name="EXITCODES">Exit Codes</a></h3>
  51
  52 <p>Filters must exit with status 0 when they successfully generate print data
  53 or 1 when they encounter an error. Backends can return any of the
  54 <a href="#cups_backend_t"><code>cups_backend_t</code></a> constants.</p>
  55
  56 <h3><a name="ENVIRONMENT">Environment Variables</a></h3>
  57
  58 <p>The following environment variables are defined by the printing system:</p>
  59
  60 <dl class="code">
  61
  62         <dt>APPLE_LANGUAGES</dt>
  63         <dd>The Apple language identifier associated with the job
  64         (Mac OS X only).</dd>
  65
  66         <dt>CHARSET</dt>
  67         <dd>The job character set, typically "utf-8".</dd>
  68
  69         <dt>CLASS</dt>
  70         <dd>When a job is submitted to a printer class, contains the name of
  71         the destination printer class. Otherwise this environment
  72         variable will not be set.</dd>
  73
  74         <dt>CONTENT_TYPE</dt>
  75         <dd>The MIME type associated with the file (e.g.
  76         application/postscript).</dd>
  77
  78         <dt>CUPS_CACHEDIR</dt>
  79         <dd>The directory where cache files can be stored.</dd>
  80
  81         <dt>CUPS_DATADIR</dt>
  82         <dd>The directory where data files can be found.</dd>
  83
  84         <dt>CUPS_SERVERROOT</dt>
  85         <dd>The root directory of the server.</dd>
  86
  87         <dt>DEVICE_URI</dt>
  88         <dd>The device-uri associated with the printer.</dd>
  89
  90         <dt>FINAL_CONTENT_TYPE</dt>
  91         <dd>The MIME type associated with the printer (e.g.
  92         application/vnd.cups-postscript).</dd>
  93
  94         <dt>LANG</dt>
  95         <dd>The language locale associated with the job.</dd>
  96
  97         <dt>PPD</dt>
  98         <dd>The full pathname of the PostScript Printer Description (PPD)
  99         file for this printer.</dd>
 100
 101         <dt>PRINTER</dt>
 102         <dd>The name of the printer.</dd>
 103
 104         <dt>RIP_CACHE</dt>
 105         <dd>The recommended amount of memory to use for Raster Image
 106         Processors (RIPs).</dd>
 107
 108 </dl>
 109
 110 <h3><a name="MESSAGES">Communicating with the Scheduler</a></h3>
 111
 112 <p>Filters and backends communicate wih the scheduler by writing messages
 113 to the standard error file. For example, the following code sets the current
 114 printer state message to "Printing page 5":</p>
 115
 116 <pre class="example">
 117 int page = 5;
 118
 119 fprintf(stderr, "INFO: Printing page %d\n", page);
 120 </pre>
 121
 122 <p>Each message is a single line of text starting with one of the following
 123 prefix strings:</p>
 124
 125 <dl class="code">
 126
 127         <dt>ALERT: message</dt>
 128         <dd>Sets the printer-state-message attribute and adds the specified
 129         message to the current error log file using the "alert" log level.</dd>
 130
 131         <dt>ATTR: attribute=value [attribute=value]</dt>
 132         <dd>Sets the named printer or job attribute(s). Typically this is used
 133         to set the <code>marker-colors</code>, <code>marker-levels</code>,
 134         <code>marker-names</code>, <code>marker-types</code>,
 135         <code>printer-alert</code>, and <code>printer-alert-description</code>
 136         printer attributes.</dd>
 137
 138         <dt>CRIT: message</dt>
 139         <dd>Sets the printer-state-message attribute and adds the specified
 140         message to the current error log file using the "critical" log
 141         level.</dd>
 142
 143         <dt>DEBUG: message</dt>
 144         <dd>Sets the printer-state-message attribute and adds the specified
 145         message to the current error log file using the "debug" log level.</dd>
 146
 147         <dt>DEBUG2: message</dt>
 148         <dd>Sets the printer-state-message attribute and adds the specified
 149         message to the current error log file using the "debug2" log level.</dd>
 150
 151         <dt>EMERG: message</dt>
 152         <dd>Sets the printer-state-message attribute and adds the specified
 153         message to the current error log file using the "emergency" log
 154         level.</dd>
 155
 156         <dt>ERROR: message</dt>
 157         <dd>Sets the printer-state-message attribute and adds the specified
 158         message to the current error log file using the "error" log level.</dd>
 159
 160         <dt>INFO: message</dt>
 161         <dd>Sets the printer-state-message attribute. If the current log level
 162         is set to "debug2", also adds the specified message to the current error
 163         log file using the "info" log level.</dd>
 164
 165         <dt>NOTICE: message</dt>
 166         <dd>Sets the printer-state-message attribute and adds the specified
 167         message to the current error log file using the "notice" log level.</dd>
 168
 169         <dt>PAGE: page-number #-copies</dt>
 170         <dt>PAGE: total #-pages</dt>
 171         <dd>Adds an entry to the current page log file. The first form adds
 172         #-copies to the job-media-sheets-completed attribute. The second
 173         form sets the job-media-sheets-completed attribute to #-pages.</dd>
 174
 175         <dt>STATE: printer-state-reason [printer-state-reason ...]</dt>
 176         <dt>STATE: + printer-state-reason [printer-state-reason ...]</dt>
 177         <dt>STATE: - printer-state-reason [printer-state-reason ...]</dt>
 178         <dd>Sets, adds, or removes printer-state-reason keywords to the
 179         current queue. Typically this is used to indicate media, ink, and
 180         toner conditions on a printer.</dd>
 181
 182         <dt>WARNING: message</dt>
 183         <dd>Sets the printer-state-message attribute and adds the specified
 184         message to the current error log file using the "warning" log
 185         level.</dd>
 186
 187 </dl>
 188
 189 <p>Messages without one of these prefixes are treated as if they began with
 190 the "DEBUG:" prefix string.</p>
 191
 192 <h3><a name="COMMUNICATING">Communicating with the Backend</a></h3>
 193
 194 <p>Filters can communicate with the backend via the
 195 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> and
 196 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
 197 functions. The
 198 <a href="#cupsBackChannelRead"><code>cupsBackChannelRead</code></a> function
 199 reads data that has been sent back from the device and is typically used to
 200 obtain status and configuration information. For example, the following code
 201 polls the backend for back-channel data:</p>
 202
 203 <pre class="example">
 204 #include &lt;cups/cups.h&gt;
 205
 206 char buffer[8192];
 207 ssize_t bytes;
 208
 209 /* Use a timeout of 0.0 seconds to poll for back-channel data */
 210 bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
 211 </pre>
 212
 213 The
 214 <a href="#cupsSideChannelDoRequest"><code>cupsSideChannelDoRequest</code></a>
 215 function allows you to get out-of-band status information and do synchronization
 216 with the device. For example, the following code gets the current IEEE-1284
 217 device ID string from the backend:</p>
 218
 219 <pre class="example">
 220 #include &lt;cups/sidechannel.h&gt;
 221
 222 char data[2049];
 223 int datalen;
 224 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
 225
 226 /* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for nul-termination... */
 227 datalen = sizeof(data) - 1;
 228
 229 /* Get the IEEE-1284 device ID, waiting for up to 1 second */
 230 status = <a href="#cupsSideChannelDoRequest">cupsSideChannelDoRequest</a>(CUPS_SC_CMD_GET_DEVICE_ID, data, &amp;datalen, 1.0);
 231
 232 /* Use the returned value if OK was returned and the length is non-zero */
 233 if (status == CUPS_SC_STATUS_OK && datalen > 0)
 234   data[datalen] = '\0';
 235 else
 236   data[0] = '\0';
 237 </pre>
 238
 239 <p>Backends communicate with filters using the reciprocal functions
 240 <a href="#cupsBackChannelWrite"><code>cupsBackChannelWrite</code></a>,
 241 <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>, and
 242 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a>. We
 243 recommend writing back-channel data using a timeout of 1.0 seconds:</p>
 244
 245 <pre class="example">
 246 #include &lt;cups/cups.h&gt;
 247
 248 char buffer[8192];
 249 ssize_t bytes;
 250
 251 /* Use a timeout of 1.0 seconds to give filters a chance to read */
 252 cupsBackChannelWrite(buffer, bytes, 1.0);
 253 </pre>
 254
 255 <p>The <a href="#cupsSideChannelRead"><code>cupsSideChannelRead</code></a>
 256 function reads a side-channel command from a filter, driver, or port monitor.
 257 Backends can either poll for commands using a <code>timeout</code> of 0.0, wait
 258 indefinitely for commands using a <code>timeout</code> of -1.0 (probably in a
 259 separate thread for that purpose), or use <code>select</code> or
 260 <code>poll</code> on the <code>CUPS_SC_FD</code> file descriptor (4) to handle
 261 input and output on several file descriptors at the same time. Backends can pass
 262 <code>NULL</code> for the <code>data</code> and <code>datalen</code> parameters
 263 since none of the commands sent by upstream filters contain any data at this
 264 time.</p>
 265
 266 <p>Once a command is processed, the backend uses the
 267 <a href="#cupsSideChannelWrite"><code>cupsSideChannelWrite</code></a> function
 268 to send its response. For example, the following code shows how to poll for a
 269 side-channel command and respond to it:</p>
 270
 271 <pre class="example">
 272 #include &lt;cups/sidechannel.h&gt;
 273
 274 <a href="#cups_sc_command_t">cups_sc_command_t</a> command;
 275 <a href="#cups_sc_status_t">cups_sc_status_t</a> status;
 276
 277 /* Poll for a command... */
 278 if (!<a href="#cupsSideChannelRead">cupsSideChannelRead</a>(&amp;command, &amp;status, NULL, NULL, 0.0))
 279 {
 280   char data[2048];
 281   int datalen;
 282
 283   switch (command)
 284   {
 285     /* handle supported commands, file data/datalen/status with values as needed */
 286
 287     default :
 288         status  = CUPS_SC_STATUS_NOT_IMPLEMENTED;
 289         datalen = 0;
 290         break;
 291   }
 292
 293   /* Send a response... */
 294   <a href="#cupsSideChannelWrite">cupsSideChannelWrite</a>(command, status, data, datalen, 1.0);
 295 }
 296 </pre>