Metadata

GStreamer makes a clear distinction between two types of metadata, and has support for both types. The first is stream tags, which describe the content of a stream in a non-technical way. Examples include the author of a song, the title of that very same song or the album it is a part of. The other type of metadata is stream-info, which is a somewhat technical description of the properties of a stream. This can include video size, audio samplerate, codecs used and so on. Tags are handled using the GStreamer tagging system. Stream-info can be retrieved from a GstPad by getting the current (negotiated) GstCaps for that pad.

Metadata reading

Stream information can most easily be read by reading it from a GstPad. This has already been discussed before in Using capabilities for metadata. Therefore, we will skip it here. Note that this requires access to all pads of which you want stream information.

Tag reading is done through a bus in GStreamer, which has been discussed previously in Bus. You can listen for GST_MESSAGE_TAG messages and handle them as you wish.

Note, however, that the GST_MESSAGE_TAG message may be fired multiple times in the pipeline. It is the application's responsibility to put all those tags together and display them to the user in a nice, coherent way. Usually, using gst_tag_list_merge () is a good enough way of doing this; make sure to empty the cache when loading a new song, or after every few minutes when listening to internet radio. Also, make sure you use GST_TAG_MERGE_PREPEND as merging mode, so that a new title (which came in later) has a preference over the old one for display.

The following example will extract tags from a file and print them:

/* compile with:
 * gcc -o tags tags.c `pkg-config --cflags --libs gstreamer-1.0` */
#include <gst/gst.h>

static void
print_one_tag (const GstTagList * list, const gchar * tag, gpointer user_data)
{
  int i, num;

  num = gst_tag_list_get_tag_size (list, tag);
  for (i = 0; i < num; ++i) {
    const GValue *val;

    /* Note: when looking for specific tags, use the gst_tag_list_get_xyz() API,
     * we only use the GValue approach here because it is more generic */
    val = gst_tag_list_get_value_index (list, tag, i);
    if (G_VALUE_HOLDS_STRING (val)) {
      g_print ("\t%20s : %s\n", tag, g_value_get_string (val));
    } else if (G_VALUE_HOLDS_UINT (val)) {
      g_print ("\t%20s : %u\n", tag, g_value_get_uint (val));
    } else if (G_VALUE_HOLDS_DOUBLE (val)) {
      g_print ("\t%20s : %g\n", tag, g_value_get_double (val));
    } else if (G_VALUE_HOLDS_BOOLEAN (val)) {
      g_print ("\t%20s : %s\n", tag,
          (g_value_get_boolean (val)) ? "true" : "false");
    } else if (GST_VALUE_HOLDS_BUFFER (val)) {
      GstBuffer *buf = gst_value_get_buffer (val);
      guint buffer_size = gst_buffer_get_size (buf);

      g_print ("\t%20s : buffer of size %u\n", tag, buffer_size);
    } else if (GST_VALUE_HOLDS_DATE_TIME (val)) {
      GstDateTime *dt = g_value_get_boxed (val);
      gchar *dt_str = gst_date_time_to_iso8601_string (dt);

      g_print ("\t%20s : %s\n", tag, dt_str);
      g_free (dt_str);
    } else {
      g_print ("\t%20s : tag of type '%s'\n", tag, G_VALUE_TYPE_NAME (val));
    }
  }
}

static void
on_new_pad (GstElement * dec, GstPad * pad, GstElement * fakesink)
{
  GstPad *sinkpad;

  sinkpad = gst_element_get_static_pad (fakesink, "sink");
  if (!gst_pad_is_linked (sinkpad)) {
    if (gst_pad_link (pad, sinkpad) != GST_PAD_LINK_OK)
      g_error ("Failed to link pads!");
  }
  gst_object_unref (sinkpad);
}

int
main (int argc, char ** argv)
{
  GstElement *pipe, *dec, *sink;
  GstMessage *msg;
  gchar *uri;

  gst_init (&argc, &argv);

  if (argc < 2)
    g_error ("Usage: %s FILE or URI", argv[0]);

  if (gst_uri_is_valid (argv[1])) {
    uri = g_strdup (argv[1]);
  } else {
    uri = gst_filename_to_uri (argv[1], NULL);
  }

  pipe = gst_pipeline_new ("pipeline");

  dec = gst_element_factory_make ("uridecodebin", NULL);
  g_object_set (dec, "uri", uri, NULL);
  gst_bin_add (GST_BIN (pipe), dec);

  sink = gst_element_factory_make ("fakesink", NULL);
  gst_bin_add (GST_BIN (pipe), sink);

  g_signal_connect (dec, "pad-added", G_CALLBACK (on_new_pad), sink);

  gst_element_set_state (pipe, GST_STATE_PAUSED);

  while (TRUE) {
    GstTagList *tags = NULL;

    msg = gst_bus_timed_pop_filtered (GST_ELEMENT_BUS (pipe),
        GST_CLOCK_TIME_NONE,
        GST_MESSAGE_ASYNC_DONE | GST_MESSAGE_TAG | GST_MESSAGE_ERROR);

    if (GST_MESSAGE_TYPE (msg) != GST_MESSAGE_TAG) /* error or async_done */
      break;

    gst_message_parse_tag (msg, &tags);

    g_print ("Got tags from element %s:\n", GST_OBJECT_NAME (msg->src));
    gst_tag_list_foreach (tags, print_one_tag, NULL);
    g_print ("\n");
    gst_tag_list_unref (tags);

    gst_message_unref (msg);
  }

  if (GST_MESSAGE_TYPE (msg) == GST_MESSAGE_ERROR) {
    GError *err = NULL;

    gst_message_parse_error (msg, &err, NULL);
    g_printerr ("Got error: %s\n", err->message);
    g_error_free (err);
  }

  gst_message_unref (msg);
  gst_element_set_state (pipe, GST_STATE_NULL);
  gst_object_unref (pipe);
  g_free (uri);
  return 0;
}

Tag writing

Tag writing is done using the GstTagSetter interface. All that's required is a tag-set-supporting element in your pipeline. In order to see if any of the elements in your pipeline supports tag writing, you can use the function gst_bin_iterate_all_by_interface (pipeline, GST_TYPE_TAG_SETTER). On the resulting element, usually an encoder or muxer, you can use gst_tag_setter_merge () (with a taglist) or gst_tag_setter_add () (with individual tags) to set tags on it.

A nice extra feature in GStreamer tag support is that tags are preserved in pipelines. This means that if you transcode one file containing tags into another media type, and that new media type supports tags too, then the tags will be handled as part of the data stream and be merged into the newly written media file, too.

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