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  Filter and backend API introduction for the Common UNIX Printing System (CUPS).

  Copyright 2007 by Apple Inc.
  Copyright 1997-2006 by Easy Software Products, all rights reserved.

  These coded instructions, statements, and computer programs are the
  property of Apple Inc. and are protected by Federal copyright
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  file is missing or damaged, see the license at "http://www.cups.org/".
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<h2 class='title'>Introduction</h2>

<p>The CUPS filter and backend APIs define standard exit codes
and provide access to the backchannel data stream. They are only
used when writing backends, filters, and port monitors.</p>

<h2 class='title'>General Usage</h2>

<p>The <var>&lt;cups/backend.h&gt;</var> and
<var>&lt;cups/cups.h&gt;</var> header files must be included to
use the <tt>CUPS_BACKEND_</tt> constants and
<tt>cupsBackChannel</tt> functions, respectively.</p>

<p>The <var>&lt;cups/sidechannel.h&gt;</var> header file must be
included to use the <tt>CUPS_SC_</tt> constants and <tt>cupsSideChannel</tt> functions.</p>

<p>Programs using these functions must be linked to the CUPS
library: <var>libcups.a</var>, <var>libcups.so.2</var>,
<var>libcups.2.dylib</var>, <var>libcups_s.a</var>, or
<var>libcups2.lib</var> depending on the platform. The following
command compiles <var>myprogram.c</var> using GCC and the CUPS
library:</p>

<pre class='command'>
<kbd>gcc -o myprogram myprogram.c -lcups</kbd>
</pre>


<h2 class='title'>Compatibility</h2>

<p>The <tt>cupsBackChannel</tt> functions require CUPS 1.2 or higher. The <tt>cupsSideChannel</tt> functions require CUPS 1.3 or higher.</p>


<h2 class='title'>Using the cupsBackChannel APIs</h2>

<p>The <tt>cupsBackChannel</tt> APIs allow your filters, drivers, and port monitors to read data back from a printer and your backends to send data from a printer to the filters, drivers, and port monitors associated with the current job. Back-channel data is normally sent by the printer in response to a command sent from your program to the printer via <tt>stdout</tt>.</p>

<p>The <tt>cupsBackChannelRead()</tt> function reads data from the printer via the backend. You provide a timeout in seconds along with a buffer pointer and the size of that buffer. It returns the number of bytes or -1 if there was an error. The following code example shows how to poll for back-channel data in your program:</p>

<pre class='command'>
#include &lt;cups/cups.h&gt;

char buffer[8192];
ssize_t bytes;

/* Use a timeout of 0.0 seconds to poll for back-channel data */
bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);
</pre>

<p>If you are writing a backend, the <tt>cupsBackChannelWrite()</tt> function sends any back-channel data you have received from the printer to upstream filters in the print filter chain. We recommend using a timeout of 1.0 seconds:</p>

<pre class='command'>
#include &lt;cups/cups.h&gt;

char buffer[8192];
ssize_t bytes;

/* Use a timeout of 1.0 seconds to give filters a chance to read */
cupsBackChannelWrite(buffer, bytes, 1.0);
</pre>


<h2 class='title'>Using the cupsSideChannel APIs</h2>

<p>The <tt>cupsSideChannel</tt> APIs allow your filters, drivers, port monitors, and backend to send and receive the following out-of-band commands:</p>

<ul>

	<li><tt>CUPS_SC_CMD_SOFT_RESET</tt> -  Do a soft reset</li>
	<li><tt>CUPS_SC_CMD_DRAIN_OUTPUT</tt> -  Drain all pending output</li>
	<li><tt>CUPS_SC_CMD_GET_BIDI</tt> -  Return bidirectional capabilities</li>
	<li><tt>CUPS_SC_CMD_GET_DEVICE_ID</tt> -  Return the IEEE-1284 device ID</li>
	<li><tt>CUPS_SC_CMD_GET_STATE</tt> - Return the device state</li>

</ul>


<h3>Sending Commands from a Filter, Driver, or Port Monitor</h3>

<p>The <tt>cupsSideChannelDoRequest()</tt> function is used by filters, drivers, and port monitors to send a command to the backend and read back a response:</p>

<pre class='command'>
cups_sc_status_t cupsSideChannelDoRequest(cups_sc_command_t command,
                                          char *data, int *datalen,
                                          double timeout);
</pre>

<p>The <tt>CUPS_SC_CMD_SOFT_RESET</tt> and <tt>CUPS_SC_CMD_DRAIN_OUTPUT</tt> commands do not return any data values, while the others return one or more bytes. The <tt>timeout</tt> parameter allows your program to poll or wait for the command to complete - use a timeout of 30 seconds for <tt>CUPS_SC_CMD_SOFT_RESET</tt> and <tt>CUPS_SC_CMD_DRAIN_OUTPUT</tt> and a timeout of 1 second for all other commands.</p>

<p><tt>CUPS_SC_CMD_GET_BIDI</tt> returns a single <tt>char</tt> value that tells you whether the backend supports bidirectional communications:</p>

<pre class='command'>
#include &lt;cups/sidechannel.h&gt;

char data;
int datalen;
cups_sc_bidi_t bidi;
cups_sc_status_t status;

/* Tell cupsSideChannelDoRequest() how big our buffer is... */
datalen = 1;

/* Get the bidirectional capabilities, waiting for up to 1 second */
status  = cupsSideChannelDoRequest(CUPS_SC_CMD_GET_BIDI, &amp;data, &amp;datalen, 1.0);

/* Use the returned value if OK was returned and the length is still 1 */
if (status == CUPS_SC_STATUS_OK && datalen == 1)
  bidi = (cups_sc_bidi_t)data;
else
  bidi = CUPS_SC_BIDI_NOT_SUPPORTED;
</pre>

<p><tt>CUPS_SC_CMD_GET_DEVICE_ID</tt> returns a string of characters containing the IEEE-1284 device ID for the connected printer:</p>

<pre class='command'>
#include &lt;cups/sidechannel.h&gt;

char data[2049];
int datalen;
cups_sc_status_t status;

/* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for nul-termination... */
datalen = sizeof(data) - 1;

/* Get the IEEE-1284 device ID, waiting for up to 1 second */
status  = cupsSideChannelDoRequest(CUPS_SC_CMD_GET_DEVICE_ID, data, &amp;datalen, 1.0);

/* Use the returned value if OK was returned and the length is non-zero */
if (status == CUPS_SC_STATUS_OK && datalen > 0)
  data[datalen] = '\0';
else
  data[0] = '\0';
</pre>

<p><tt>CUPS_SC_CMD_GET_STATE</tt> returns a single <tt>char</tt> value that tells you the current device state:</p>

<pre class='command'>
#include &lt;cups/sidechannel.h&gt;

char data;
int datalen;
cups_sc_state_t state;
cups_sc_status_t status;

/* Tell cupsSideChannelDoRequest() how big our buffer is... */
datalen = 1;

/* Get the bidirectional capabilities, waiting for up to 1 second */
status  = cupsSideChannelDoRequest(CUPS_SC_CMD_GET_STATE, &amp;data, &amp;datalen, 1.0);

/* Use the returned value if OK was returned and the length is still 1 */
if (status == CUPS_SC_STATUS_OK && datalen == 1)
  state = (cups_sc_state_t)data;
else
  state = CUPS_SC_STATE_OFFLINE;
</pre>


<h3>Handling Commands in your Backend</h3>

<p>The <tt>cupsSideChannelRead()</tt> function is used by backends to read a command from a filter, driver, or port monitor:</p>

<pre class='command'>
int cupsSideChannelRead(cups_sc_command_t &amp;command,
                        cups_sc_status_t  &amp;status,
                        char *data, int *datalen, double timeout);
</pre>

<p>Backends can either poll for commands using a <tt>timeout</tt> of 0.0, wait indefinitely for commands using a <tt>timeout</tt> of -1.0 (probably in a separate thread for that purpose), or use <tt>select()</tt> or <tt>poll()</tt> on the <tt>CUPS_SC_FD</tt> file descriptor (4) to handle input and output on several file descriptors at the same time. Backends can pass <tt>NULL</tt> for the <tt>data</tt> and <tt>datalen</tt> parameters, since none of the commands sent by upstream filters contain any data at this time.</p>

<p>Once a command is processed, the backend uses the <tt>cupsSideChannelWrite()</tt> function to send its response:</p>

<pre class='command'>
#include &lt;cups/sidechannel.h&gt;

cups_sc_command_t command;
cups_sc_status_t status;

/* Poll for a command... */
if (!cupsSideChannelRead(&amp;command, &amp;status, NULL, NULL, 0.0))
{
  char data[2048];
  int datalen;

  switch (command)
  {
    ... handle supported commands, file data/datalen/status with values as needed ...

    default :
        status  = CUPS_SC_STATUS_NOT_IMPLEMENTED;
	datalen = 0;
	break;
  }

  /* Send a response... */
  cupsSideChannelWrite(command, status, data, datalen, 1.0);
}
</pre>