# Queues

A `GstQueue` is a filter element. Queues can be used to link two elements in such way that the data can be buffered.

A buffer that is sinked to a Queue will not automatically be pushed to the next linked element but will be buffered. It will be pushed to the next element as soon as a gst_pad_pull () is called on the queue's source pad.

Queues are mostly used in conjunction with a `GstThread` to provide an external link for the thread elements. You could have one thread feeding buffers into a `GstQueue` and another thread repeadedly calling gst_pad_pull () on the queue to feed its internal elements.

Below is a figure of a two-threaded decoder. We have one thread (the main execution thread) reading the data from a file, and another thread decoding the data.

## a two-threaded decoder with a queue

The standard GStreamer queue implementation has some properties that can be changed using the g_objet_set () method. To set the maximum number of buffers that can be queued to 30, do:

      g_object_set (G_OBJECT (queue), "max_level", 30, NULL);

The following MP3 player shows you how to create the above pipeline using a thread and a queue.

    /* example-begin queue.c */
    #include <stdlib.h>
    #include <gst/gst.h>

    gboolean playing;

    /* eos will be called when the src element has an end of stream */
    void
    eos (GstElement *element, gpointer data)
    {
      g_print ("have eos, quitting\n");

      playing = FALSE;
    }

    int
    main (int argc, char *argv[])
    {
      GstElement *filesrc, *audiosink, *queue, *decode;
      GstElement *bin;
      GstElement *thread;

      gst_init (&argc,&argv);

      if (argc != 2) {
        g_print ("usage: %s <mp3 filename>\n", argv[0]);
        exit (-1);
      }

      /* create a new thread to hold the elements */
      thread = gst_thread_new ("thread");
      g_assert (thread != NULL);

      /* create a new bin to hold the elements */
      bin = gst_bin_new ("bin");
      g_assert (bin != NULL);

      /* create a disk reader */
      filesrc = gst_element_factory_make ("filesrc", "disk_source");
      g_assert (filesrc != NULL);
      g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);
      g_signal_connect (G_OBJECT (filesrc), "eos",
                        G_CALLBACK (eos), thread);

      queue = gst_element_factory_make ("queue", "queue");
      g_assert (queue != NULL);

      /* and an audio sink */
      audiosink = gst_element_factory_make ("osssink", "play_audio");
      g_assert (audiosink != NULL);

      decode = gst_element_factory_make ("mad", "decode");

      /* add objects to the main bin */
      gst_bin_add_many (GST_BIN (thread), decode, audiosink, NULL);

      gst_bin_add_many (GST_BIN (bin), filesrc, queue, thread, NULL);

      gst_element_link (filesrc, queue);
      gst_element_link_many (queue, decode, audiosink, NULL);

      /* start playing */
      gst_element_set_state (GST_ELEMENT (bin), GST_STATE_PLAYING);

      playing = TRUE;

      while (playing) {
        gst_bin_iterate (GST_BIN (bin));
      }

      gst_element_set_state (GST_ELEMENT (bin), GST_STATE_NULL);

      return 0;
    }
    /* example-end queue.c */