Request and Sometimes 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:
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.
Request pads
Request
pads are similar to sometimes pads, except that
request are created on demand of something outside of the element rather
than something inside the element. This concept is often used in muxers,
where - for each elementary stream that is to be placed in the output
system stream - one sink pad will be requested. It can also be used in
elements with a variable number of input or outputs pads, such as the
tee (multi-output) or
input-selector (multi-input) elements.
To implement request pads, you need to provide a padtemplate with a
GST_PAD_REQUEST presence and implement the
request_new_pad virtual method in
GstElement.
To clean up, you will need to implement the
release_pad virtual method.
request_new_pad = gst_my_filter_request_new_pad;
element_class->release_pad = gst_my_filter_release_pad;
}
static GstPad *
gst_my_filter_request_new_pad (GstElement *element,
GstPadTemplate *templ,
const gchar *name,
const GstCaps *caps)
{
GstPad *pad;
GstMyFilterInputContext *context;
context = g_new0 (GstMyFilterInputContext, 1);
pad = gst_pad_new_from_template (templ, name);
gst_pad_set_element_private (pad, context);
/* normally, you would set _chain () and _event () functions here */
gst_element_add_pad (element, pad);
return pad;
}
static void
gst_my_filter_release_pad (GstElement *element,
GstPad *pad)
{
GstMyFilterInputContext *context;
context = gst_pad_get_element_private (pad);
g_free (context);
gst_element_remove_pad (element, pad);
}
]]>