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  HTTP and IPP API introduction for the Common UNIX Printing System (CUPS).

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<h2 class='title'><a name='OVERVIEW'>Overview</a></h2>

<p>The CUPS HTTP and IPP APIs provide low-level access to the HTTP and IPP
protocols and CUPS scheduler. They are typically used by monitoring and
administration programs to perform specific functions not supported by the
high-level CUPS API functions.</p>

<p>The HTTP APIs use an opaque structure called
<a href='#http_t'><code>http_t</code></a> to manage connections to
a particular HTTP or IPP server. The
<a href='#httpConnectEncrypt'><code>httpConnectEncrypt</code></a> function is
used to create an instance of this structure for a particular server.
The constant <code>CUPS_HTTP_DEFAULT</code> can be used with all of the
<code>cups</code> functions to refer to the default CUPS server - the functions
create a per-thread <a href='#http_t'><code>http_t</code></a> as needed.</p>

<p>The IPP APIs use two structures for requests (messages sent to the CUPS
scheduler) and responses (messages sent back to your application from the
scheduler). The <a href='#ipp_t'><code>ipp_t</code></a> structure holds a
complete request or response and is allocated using the
<a href='#ippNew'><code>ippNew</code></a> or
<a href='#ippNewRequest'><code>ippNewRequest</code></a> functions and
freed using the <a href='#ippDelete'><code>ippDelete</code></a> function.</p>

<p>The second structure is called
<a href='#ipp_attribute_t'><code>ipp_attribute_t</code></a> and holds a
single IPP attribute which consists of a group tag (<code>group_tag</code>), a
value type tag (<code>value_tag</code>), the attribute name (<code>name</code>),
and 1 or more values (<code>values[]</code>). Attributes are added to an
<a href='#ipp_t'><code>ipp_t</code></a> structure using one of the
<code>ippAdd</code> functions. For example, use
<a href='#ippAddString'><code>ippAddString</code></a> to add a
"requesting-user-name" string attribute to a request:</p>

<pre class='example'>
<a href='#ipp_t'>ipp_t</a> *request = <a href='#ippNewRequest'>ippNewRequest</a>(IPP_GET_JOBS);

<a href='#ippAddString'>ippAddString</a>(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "requesting-user-name",
             NULL, cupsUser());
</pre>

<p>Once you have created an IPP request, use the <code>cups</code>
functions to send the request to and read the response from the server.
For example, the <a href='#cupsDoRequest'><code>cupsDoRequest</code></a>
function can be used for simple query operations that do not involve files:</p>

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


<a href='#ipp_t'>ipp_t</a> *<a name='get_jobs'>get_jobs</a>(void)
{
  <a href='#ipp_t'>ipp_t</a> *request = <a href='#ippNewRequest'>ippNewRequest</a>(IPP_GET_JOBS);

  <a href='#ippAddString'>ippAddString</a>(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "requesting-user-name",
               NULL, cupsUser());

  return (<a href='#cupsDoRequest'>cupsDoRequest</a>(CUPS_HTTP_DEFAULT, request, "/"));
}
</pre>

<p>The <a href='#cupsDoRequest'><code>cupsDoRequest</code></a> function frees
the request structure and returns an IPP response structure or NULL pointer if
the request could not be sent to the server. Once you have a response from
the server, you can either use the
<a href='#ippFindAttribute'><code>ippFindAttribute</code></a> and
<a href='#ippFindNextAttribute'><code>ippFindNextAttribute</code></a> functions
to find specific attributes, for example:</p>

<pre class='example'>
<a href='#ipp_t'>ipp_t</a> *response;
<a href='#ipp_attribute_t'>ipp_attribute_t</a> *attr;

attr = <a href='#ippFindAttribute'>ippFindAttribute</a>(response, "printer-state", IPP_TAG_ENUM);
</pre>

<p>You can also walk the list of attributes with a simple <code>for</code> loop
like this:</p>

<pre class='example'>
<a href='#ipp_t'>ipp_t</a> *response;
<a href='#ipp_attribute_t'>ipp_attribute_t</a> *attr;

for (attr = response->attrs; attr != NULL; attr = attr->next)
  if (attr->name == NULL)
    puts("--SEPARATOR--");
  else
    puts(attr->name);
</pre>

<p>The <code>for</code> loop approach is normally used when collecting
attributes for multiple objects (jobs, printers, etc.) in a response. Attributes
with <code>NULL</code> names indicate a separator between the attributes of
each object. For example, the following code will list the jobs returned from
our previous <a href='#get_jobs'><code>get_jobs</code></a> example code:</p>

<pre class='example'>
<a href='#ipp_t'>ipp_t</a> *response = <a href='#get_jobs'>get_jobs</a>();

if (response != NULL)
{
  <a href='#ipp_attribute_t'>ipp_attribute_t</a> *attr;
  int job_id = 0;
  char *job_name = NULL;
  char *job_originating_user_name = NULL;

  puts("Job ID  Owner             Title");
  puts("------  ----------------  ---------------------------------");

  for (attr = response->attrs; attr != NULL; attr = attr->next)
  {
   /* Attributes without names are separators between jobs */
    if (attr->name == NULL)
    {
      if (job_id > 0 &amp;&amp; job_name != NULL &amp;&amp; job_originating_user_name != NULL)
        printf("%5d  %-16s  %s\n", job_id, job_originating_user_name, job_name);

      job_id = 0;
      job_name = NULL;
      job_originating_user_name = NULL;
      continue;
    }
    else if (!strcmp(attr->name, "job-id") &amp;&amp; attr->value_tag == IPP_TAG_INTEGER)
      job_id = attr->values[0].integer;
    else if (!strcmp(attr->name, "job-name") &amp;&amp; attr->value_tag == IPP_TAG_NAME)
      job_name = attr->values[0].string.text;
    else if (!strcmp(attr->name, "job-originating-user-name") &amp;&amp;
             attr->value_tag == IPP_TAG_NAME)
      job_originating_user_name = attr->values[0].string.text;
  }

  if (job_id > 0 &amp;&amp; job_name != NULL &amp;&amp; job_originating_user_name != NULL)
    printf("%5d  %-16s  %s\n", job_id, job_originating_user_name, job_name);
}
</pre>

<h3><a name='CREATING_URI_STRINGS'>Creating URI Strings</a></h3>

<p>To ensure proper encoding, the
<a href='#httpAssembleURIf'><code>httpAssembleURIf</code></a> function must be
used to format a "printer-uri" string for all printer-based requests:</p>

<pre class='example'>
const char *name = "Foo";
char uri[1024];
<a href='#ipp_t'>ipp_t</a> *request;

<a href='#httpAssembleURIf'>httpAssembleURIf</a>(HTTP_URI_CODING_ALL, uri, sizeof(uri), "ipp", NULL, cupsServer(),
                 ippPort(), "/printers/%s", name);
<a href='#ippAddString'>ippAddString</a>(request, IPP_TAG_OPERATION, IPP_TAG_URI, "printer-uri", NULL, uri);
</pre>

<h3><a name='SENDING_REQUESTS_WITH_FILES'>Sending Requests with Files</a></h3>

<p>The <a href='#cupsDoFileRequest'><code>cupsDoFileRequest</code></a> and
<a href='#cupsDoIORequest'><code>cupsDoIORequest</code></a> functions are
used for requests involving files. The
<a href='#cupsDoFileRequest'><code>cupsDoFileRequest</code></a> function
attaches the named file to a request and is typically used when sending a print
file or changing a printer's PPD file:</p>

<pre class='example'>
const char *filename = "/usr/share/cups/data/testprint.ps";
const char *name = "Foo";
char uri[1024];
char resource[1024];
<a href='#ipp_t'>ipp_t</a> *request = <a href='#ippNewRequest'>ippNewRequest</a>(IPP_PRINT_JOB);
<a href='#ipp_t'>ipp_t</a> *response;

/* Use httpAssembleURIf for the printer-uri string */
<a href='#httpAssembleURIf'>httpAssembleURIf</a>(HTTP_URI_CODING_ALL, uri, sizeof(uri), "ipp", NULL, cupsServer(),
                 ippPort(), "/printers/%s", name);
<a href='#ippAddString'>ippAddString</a>(request, IPP_TAG_OPERATION, IPP_TAG_URI, "printer-uri", NULL, uri);
<a href='#ippAddString'>ippAddString</a>(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "requesting-user-name",
             NULL, cupsUser());
<a href='#ippAddString'>ippAddString</a>(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "job-name",
             NULL, "testprint.ps");

/* Use snprintf for the resource path */
snprintf(resource, sizeof(resource), "/printers/%s", name);

response = <a href='#cupsDoFileRequest'>cupsDoFileRequest</a>(CUPS_HTTP_DEFAULT, request, resource, filename);
</pre>

<p>The <a href='#cupsDoIORequest'><code>cupsDoIORequest</code></a> function
optionally attaches a file to the request and optionally saves a file in the
response from the server. It is used when using a pipe for the request
attachment or when using a request that returns a file, currently only
<code>CUPS_GET_DOCUMENT</code> and <code>CUPS_GET_PPD</code>. For example,
the following code will download the PPD file for the sample HP LaserJet
printer driver:</p>

<pre class='example'>
char tempfile[1024];
int tempfd;
<a href='#ipp_t'>ipp_t</a> *request = <a href='#ippNewRequest'>ippNewRequest</a>(CUPS_GET_PPD);
<a href='#ipp_t'>ipp_t</a> *response;

<a href='#ippAddString'>ippAddString</a>(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "ppd-name",
             NULL, "laserjet.ppd");

tempfd = cupsTempFd(tempfile, sizeof(tempfile));

response = <a href='#cupsDoIORequest'>cupsDoIORequest</a>(CUPS_HTTP_DEFAULT, request, "/", -1, tempfd);
</pre>

<p>The example passes <code>-1</code> for the input file descriptor to specify
that no file is to be attached to the request. The PPD file attached to the
response is written to the temporary file descriptor we created using the
<code>cupsTempFd</code> function.</p>

<h3><a name='ASYNCHRONOUS_REQUEST_PROCESSING'>Asynchronous Request Processing</a></h3>

<p>The <a href='#cupsSendRequest'><code>cupsSendRequest</code></a> and
<a href='#cupsGetResponse'><code>cupsGetResponse</code></a> support
asynchronous communications with the server. Unlike the other request
functions, the IPP request is not automatically freed, so remember to
free your request with the <a href='#ippDelete'><code>ippDelete</code></a>
function.</p>

<p>File data is attached to the request using the
<a href='#cupsWriteRequestData'><code>cupsWriteRequestData</code></a>
function, while file data returned from the server is read using the
<a href='#cupsReadResponseData'><code>cupsReadResponseData</code></a>
function. We can rewrite the previous <code>CUPS_GET_PPD</code> example
to use the asynchronous functions quite easily:</p>

<pre class='example'>
char tempfile[1024];
int tempfd;
<a href='#ipp_t'>ipp_t</a> *request = <a href='#ippNewRequest'>ippNewRequest</a>(CUPS_GET_PPD);
<a href='#ipp_t'>ipp_t</a> *response;

<a href='#ippAddString'>ippAddString</a>(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "ppd-name",
             NULL, "laserjet.ppd");

tempfd = cupsTempFd(tempfile, sizeof(tempfile));

if (<a href='#cupsSendRequest'>cupsSendRequest</a>(CUPS_HTTP_DEFAULT, request, "/") == HTTP_CONTINUE)
{
  response = <a href='#cupsGetResponse'>cupsGetResponse</a>(CUPS_HTTP_DEFAULT, "/");

  if (response != NULL)
  {
    ssize_t bytes;
    char buffer[8192];

    while ((bytes = <a href='#cupsReadResponseData'>cupsReadResponseData</a>(CUPS_HTTP_DEFAULT, buffer, sizeof(buffer))) > 0)
      write(tempfd, buffer, bytes);
  }
}

/* Free the request! */
<a href='#ippDelete'>ippDelete</a>(request);
</pre>

<p>The <a href='#cupsSendRequest'><code>cupsSendRequest</code></a> function
returns the initial HTTP request status, typically either
<code>HTTP_CONTINUE</code> or <code>HTTP_UNAUTHORIZED</code>. The latter status
is returned when the request requires authentication of some sort. The
<a href='#cupsDoAuthentication'><code>cupsDoAuthentication</code></a> function
must be called when your see <code>HTTP_UNAUTHORIZED</code> and the request
re-sent. We can add authentication support to our example code by using a
<code>do ... while</code> loop:</p>

<pre class='example'>
char tempfile[1024];
int tempfd;
<a href='#ipp_t'>ipp_t</a> *request = <a href='#ippNewRequest'>ippNewRequest</a>(CUPS_GET_PPD);
<a href='#ipp_t'>ipp_t</a> *response;
http_status_t status;

<a href='#ippAddString'>ippAddString</a>(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "ppd-name",
             NULL, "laserjet.ppd");

tempfd = cupsTempFd(tempfile, sizeof(tempfile));

/* Loop for authentication */
do
{
  status = a href='#cupsSendRequest'>cupsSendRequest</a>(CUPS_HTTP_DEFAULT, request, "/");

  if (status == HTTP_UNAUTHORIZED)
  {
    /* Try to authenticate, break out of the loop if that fails */
    if (<a href='#cupsDoAuthentication'>cupsDoAuthentication</a>(CUPS_HTTP_DEFAULT, "POST", "/"))
      break;
  }
}
while (status != HTTP_CONTINUE &amp;&amp; status != HTTP_UNAUTHORIZED);

if (status == HTTP_CONTINUE)
{
  response = <a href='#cupsGetResponse'>cupsGetResponse</a>(CUPS_HTTP_DEFAULT, "/");

  if (response != NULL)
  {
    ssize_t bytes;
    char buffer[8192];

    while ((bytes = <a href='#cupsReadResponseData'>cupsReadResponseData</a>(CUPS_HTTP_DEFAULT, buffer, sizeof(buffer))) > 0)
      write(tempfd, buffer, bytes);
  }
}

/* Free the request! */
<a href='#ippDelete'>ippDelete</a>(request);
</pre>