A queue 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 thread bin to provide an external link for the thread's elements. You could have one thread feeding buffers into a queue and another thread repeatedly pulling 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. /* 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 */