Chapter 12. Request and Sometimes pads

Table of Contents

Sometimes pads
Request pads

Until now, we've only dealt with pads that are always available. However, there's also pads that are only being created in some cases, or only if the application requests the pad. The first is called a sometimes; the second is called a request pad. The availability of a pad (always, sometimes or request) can be seen in a pad's template. This chapter will discuss when each of the two is useful, how they are created and when they should be disposed.

Sometimes pads

A sometimes pad is a pad that is created under certain conditions, but not in all cases. This mostly depends on stream content: demuxers will generally parse the stream header, decide what elementary (video, audio, subtitle, etc.) streams are embedded inside the system stream, and will then create a sometimes pad for each of those elementary streams. At its own choice, it can also create more than one instance of each of those per element instance. The only limitation is that each newly created pad should have a unique name. Sometimes pads are disposed when the stream data is disposed, too (i.e. when going from PAUSED to the READY state). You should not dispose the pad on EOS, because someone might re-activate the pipeline and seek back to before the end-of-stream point. The stream should still stay valid after EOS, at least until the stream data is disposed. In any case, the element is always the owner of such a pad.

The example code below will parse a text file, where the first line is a number (n). The next lines all start with a number (0 to n-1), which is the number of the source pad over which the data should be sent.

3
0: foo
1: bar
0: boo
2: bye
    

The code to parse this file and create the dynamic sometimes pads, looks like this:


typedef struct _GstMyFilter {
[..]
  gboolean firstrun;
  GList *srcpadlist;
} GstMyFilter;

static GstStaticPadTemplate src_factory =
GST_STATIC_PAD_TEMPLATE (
  "src_%u",
  GST_PAD_SRC,
  GST_PAD_SOMETIMES,
  GST_STATIC_CAPS ("ANY")
);

static void
gst_my_filter_class_init (GstMyFilterClass *klass)
{
  GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
[..]
  gst_element_class_add_pad_template (element_class,
	gst_static_pad_template_get (&src_factory));
[..]
}

static void
gst_my_filter_init (GstMyFilter *filter)
{
[..]
  filter->firstrun = TRUE;
  filter->srcpadlist = NULL;
}

/*
 * Get one line of data - without newline.
 */

static GstBuffer *
gst_my_filter_getline (GstMyFilter *filter)
{
  guint8 *data;
  gint n, num;

  /* max. line length is 512 characters - for safety */
  for (n = 0; n < 512; n++) {
    num = gst_bytestream_peek_bytes (filter->bs, &data, n + 1);
    if (num != n + 1)
      return NULL;

    /* newline? */
    if (data[n] == '\n') {
      GstBuffer *buf = gst_buffer_new_allocate (NULL, n + 1, NULL);

      gst_bytestream_peek_bytes (filter->bs, &data, n);
      gst_buffer_fill (buf, 0, data, n);
      gst_buffer_memset (buf, n, '\0', 1);
      gst_bytestream_flush_fast (filter->bs, n + 1);

      return buf;
    }
  }
}

static void
gst_my_filter_loopfunc (GstElement *element)
{
  GstMyFilter *filter = GST_MY_FILTER (element);
  GstBuffer *buf;
  GstPad *pad;
  GstMapInfo map;
  gint num, n;

  /* parse header */
  if (filter->firstrun) {
    gchar *padname;
    guint8 id;

    if (!(buf = gst_my_filter_getline (filter))) {
      gst_element_error (element, STREAM, READ, (NULL),
			 ("Stream contains no header"));
      return;
    }
    gst_buffer_extract (buf, 0, &id, 1);
    num = atoi (id);
    gst_buffer_unref (buf);

    /* for each of the streams, create a pad */
    for (n = 0; n < num; n++) {
      padname = g_strdup_printf ("src_%u", n);
      pad = gst_pad_new_from_static_template (src_factory, padname);
      g_free (padname);

      /* here, you would set _event () and _query () functions */

      /* need to activate the pad before adding */
      gst_pad_set_active (pad, TRUE);

      gst_element_add_pad (element, pad);
      filter->srcpadlist = g_list_append (filter->srcpadlist, pad);
    }
  }

  /* and now, simply parse each line and push over */
  if (!(buf = gst_my_filter_getline (filter))) {
    GstEvent *event = gst_event_new (GST_EVENT_EOS);
    GList *padlist;

    for (padlist = srcpadlist;
         padlist != NULL; padlist = g_list_next (padlist)) {
      pad = GST_PAD (padlist->data);
      gst_pad_push_event (pad, gst_event_ref (event));
    }
    gst_event_unref (event);
    /* pause the task here */
    return;
  }

  /* parse stream number and go beyond the ':' in the data */
  gst_buffer_map (buf, &map, GST_MAP_READ);
  num = atoi (map.data[0]);
  if (num >= 0 && num < g_list_length (filter->srcpadlist)) {
    pad = GST_PAD (g_list_nth_data (filter->srcpadlist, num);

    /* magic buffer parsing foo */
    for (n = 0; map.data[n] != ':' &&
                map.data[n] != '\0'; n++) ;
    if (map.data[n] != '\0') {
      GstBuffer *sub;

      /* create region copy that starts right past the space. The reason
       * that we don't just forward the data pointer is because the
       * pointer is no longer the start of an allocated block of memory,
       * but just a pointer to a position somewhere in the middle of it.
       * That cannot be freed upon disposal, so we'd either crash or have
       * a memleak. Creating a region copy is a simple way to solve that. */
      sub = gst_buffer_copy_region (buf, GST_BUFFER_COPY_ALL,
          n + 1, map.size - n - 1);
      gst_pad_push (pad, sub);
    }
  }
  gst_buffer_unmap (buf, &map);
  gst_buffer_unref (buf);
}

    

Note that we use a lot of checks everywhere to make sure that the content in the file is valid. This has two purposes: first, the file could be erroneous, in which case we prevent a crash. The second and most important reason is that - in extreme cases - the file could be used maliciously to cause undefined behaviour in the plugin, which might lead to security issues. Always assume that the file could be used to do bad things.