A GstQueue is a filter element. Queues can be used to connect 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 connected element but will be buffered. It will be pushed to the next element as soon as a gst_pad_pull () is called on the queues srcpad. Queues are mostly used in conjunction with a GstThread to provide an external connection 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.

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. #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 *disksrc, *audiosink, *queue, *parse, *decode; GstElement *bin; GstElement *thread; gst_init (&argc,&argv); if (argc != 2) { g_print ("usage: %s <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 */ disksrc = gst_elementfactory_make ("disksrc", "disk_source"); g_assert (disksrc != NULL); g_object_set (G_OBJECT (disksrc), "location", argv[1], NULL); g_signal_connect (G_OBJECT (disksrc), "eos", G_CALLBACK (eos), thread); queue = gst_elementfactory_make ("queue", "queue"); /* and an audio sink */ audiosink = gst_elementfactory_make ("audiosink", "play_audio"); g_assert (audiosink != NULL); parse = gst_elementfactory_make ("mp3parse", "parse"); decode = gst_elementfactory_make ("mpg123", "decode"); /* add objects to the main bin */ gst_bin_add (GST_BIN (bin), disksrc); gst_bin_add (GST_BIN (bin), queue); gst_bin_add (GST_BIN (thread), parse); gst_bin_add (GST_BIN (thread), decode); gst_bin_add (GST_BIN (thread), audiosink); gst_pad_connect (gst_element_get_pad (disksrc,"src"), gst_element_get_pad (queue,"sink")); gst_pad_connect (gst_element_get_pad (queue, "src"), gst_element_get_pad (parse, "sink")); gst_pad_connect (gst_element_get_pad (parse, "src"), gst_element_get_pad (decode, "sink")); gst_pad_connect (gst_element_get_pad (decode, "src"), gst_element_get_pad (audiosink, "sink")); gst_bin_add (GST_BIN (bin), thread); /* 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); exit (0); }