## Overview

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.

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

The IPP APIs use two opaque structures for requests (messages sent to the CUPS scheduler) and responses (messages sent back to your application from the scheduler). The `ipp_t` type holds a complete request or response and is allocated using the `ippNew` or `ippNewRequest` functions and freed using the `ippDelete` function.

The second opaque structure is called `ipp_attribute_t` and holds a single IPP attribute which consists of a group tag (`ippGetGroupTag`), a value type tag (`ippGetValueTag`), the attribute name (`ippGetName`), and 1 or more values (`ippGetCount`, `ippGetBoolean`, `ippGetCollection`, `ippGetDate`, `ippGetInteger`, `ippGetRange`, `ippGetResolution`, and `ippGetString`). Attributes are added to an `ipp_t` pointer using one of the `ippAdd` functions. For example, use `ippAddString` to add the "printer-uri" and "requesting-user-name" string attributes to a request:

    ipp_t *request = ippNewRequest(IPP_GET_JOBS);

    ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_URI, "printer-uri",
                 NULL, "ipp://localhost/printers/");
    ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "requesting-user-name",
                 NULL, cupsUser());

Once you have created an IPP request, use the `cups` functions to send the request to and read the response from the server. For example, the `cupsDoRequest` function can be used for simple query operations that do not involve files:

    #include <cups/cups.h>

    ipp_t *get_jobs(void)
    {
      ipp_t *request = ippNewRequest(IPP_GET_JOBS);

      ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_URI, "printer-uri",
                   NULL, "ipp://localhost/printers/");
      ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "requesting-user-name",
                   NULL, cupsUser());

      return (cupsDoRequest(CUPS_HTTP_DEFAULT, request, "/"));
    }

The `cupsDoRequest` function frees the request and returns an IPP response 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 `ippFindAttribute` and `ippFindNextAttribute` functions to find specific attributes, for example:

    ipp_t *response;
    ipp_attribute_t *attr;

    attr = ippFindAttribute(response, "printer-state", IPP_TAG_ENUM);

You can also walk the list of attributes with a simple `for` loop like this:

    ipp_t *response;
    ipp_attribute_t *attr;

    for (attr = ippFirstAttribute(response); attr != NULL; attr = ippNextAttribute(response))
      if (ippGetName(attr) == NULL)
        puts("--SEPARATOR--");
      else
        puts(ippGetName(attr));

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

    ipp_t *response = get_jobs();

    if (response != NULL)
    {
      ipp_attribute_t *attr;
      const char *attrname;
      int job_id = 0;
      const char *job_name = NULL;
      const char *job_originating_user_name = NULL;

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

      for (attr = ippFirstAttribute(response); attr != NULL; attr = ippNextAttribute(response))
      {
       /* Attributes without names are separators between jobs */
        attrname = ippGetName(attr);
        if (attrname == NULL)
        {
          if (job_id > 0)
          {
            if (job_name == NULL)
              job_name = "(withheld)";

            if (job_originating_user_name == NULL)
              job_originating_user_name = "(withheld)";

            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(attrname, "job-id") && ippGetValueTag(attr) == IPP_TAG_INTEGER)
          job_id = ippGetInteger(attr, 0);
        else if (!strcmp(attrname, "job-name") && ippGetValueTag(attr) == IPP_TAG_NAME)
          job_name = ippGetString(attr, 0, NULL);
        else if (!strcmp(attrname, "job-originating-user-name") &&
                 ippGetValueTag(attr) == IPP_TAG_NAME)
          job_originating_user_name = ippGetString(attr, 0, NULL);
      }

      if (job_id > 0)
      {
        if (job_name == NULL)
          job_name = "(withheld)";

        if (job_originating_user_name == NULL)
          job_originating_user_name = "(withheld)";

        printf("%5d  %-16s  %s\n", job_id, job_originating_user_name, job_name);
      }
    }

### Creating URI Strings

To ensure proper encoding, the `httpAssembleURIf` function must be used to format a "printer-uri" string for all printer-based requests:

    const char *name = "Foo";
    char uri[1024];
    ipp_t *request;

    httpAssembleURIf(HTTP_URI_CODING_ALL, uri, sizeof(uri), "ipp", NULL, cupsServer(),
                     ippPort(), "/printers/%s", name);
    ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_URI, "printer-uri", NULL, uri);

### Sending Requests with Files

The `cupsDoFileRequest` and `cupsDoIORequest` functions are used for requests involving files. The `cupsDoFileRequest` function attaches the named file to a request and is typically used when sending a print file or changing a printer's PPD file:

    const char *filename = "/usr/share/cups/data/testprint.ps";
    const char *name = "Foo";
    char uri[1024];
    char resource[1024];
    ipp_t *request = ippNewRequest(IPP_PRINT_JOB);
    ipp_t *response;

    /* Use httpAssembleURIf for the printer-uri string */
    httpAssembleURIf(HTTP_URI_CODING_ALL, uri, sizeof(uri), "ipp", NULL, cupsServer(),
                     ippPort(), "/printers/%s", name);
    ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_URI, "printer-uri", NULL, uri);
    ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "requesting-user-name",
                 NULL, cupsUser());
    ippAddString(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 = cupsDoFileRequest(CUPS_HTTP_DEFAULT, request, resource, filename);

The `cupsDoIORequest` 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 `CUPS_GET_DOCUMENT` and `CUPS_GET_PPD`. For example, the following code will download the PPD file for the sample HP LaserJet printer driver:

    char tempfile[1024];
    int tempfd;
    ipp_t *request = ippNewRequest(CUPS_GET_PPD);
    ipp_t *response;

    ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "ppd-name",
                 NULL, "laserjet.ppd");

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

    response = cupsDoIORequest(CUPS_HTTP_DEFAULT, request, "/", -1, tempfd);

The example passes `-1` 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 `cupsTempFd` function.

### Asynchronous Request Processing

The `cupsSendRequest` and `cupsGetResponse` 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 `ippDelete` function.

File data is attached to the request using the `cupsWriteRequestData` function, while file data returned from the server is read using the `cupsReadResponseData` function. We can rewrite the previous `CUPS_GET_PPD` example to use the asynchronous functions quite easily:

    char tempfile[1024];
    int tempfd;
    ipp_t *request = ippNewRequest(CUPS_GET_PPD);
    ipp_t *response;

    ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "ppd-name",
                 NULL, "laserjet.ppd");

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

    if (cupsSendRequest(CUPS_HTTP_DEFAULT, request, "/") == HTTP_CONTINUE)
    {
      response = cupsGetResponse(CUPS_HTTP_DEFAULT, "/");

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

        while ((bytes = cupsReadResponseData(CUPS_HTTP_DEFAULT, buffer, sizeof(buffer))) > 0)
          write(tempfd, buffer, bytes);
      }
    }

    /* Free the request! */
    ippDelete(request);

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

    char tempfile[1024];
    int tempfd;
    ipp_t *request = ippNewRequest(CUPS_GET_PPD);
    ipp_t *response;
    http_status_t status;

    ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_NAME, "ppd-name",
                 NULL, "laserjet.ppd");

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

    /* Loop for authentication */
    do
    {
      status = cupsSendRequest(CUPS_HTTP_DEFAULT, request, "/");

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

    if (status == HTTP_CONTINUE)
    {
      response = cupsGetResponse(CUPS_HTTP_DEFAULT, "/");

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

        while ((bytes = cupsReadResponseData(CUPS_HTTP_DEFAULT, buffer, sizeof(buffer))) > 0)
          write(tempfd, buffer, bytes);
      }
    }

    /* Free the request! */
    ippDelete(request);