Documentation 
Section: III-18 > Data Access
http://gstreamer.freedesktop.org/data/doc/gstreamer/head/manual/html/chapter-dataaccess.html

the example reads:
#include <gst/gst.h>

static gboolean
cb_have_data (GstPad    *pad,
	      GstBuffer *buffer,
	      gpointer   u_data)
{
  gint x, y;
  guint16 *data = (guint16 *) GST_BUFFER_DATA (buffer), t;

  /* invert data */
  for (y = 0; y < 288; y++) {
    for (x = 0; x < 384 / 2; x++) {
      t = data[384 - 1 - x];
      data[384 - 1 - x] = data[x];
      data[x] = t;
    }
    data += 384;
  }

  return TRUE;
}

gint
main (gint   argc,
      gchar *argv[])
{
  GMainLoop *loop;
  GstElement *pipeline, *src, *sink, *filter, *csp;
  GstCaps *caps;
  GstPad *pad;

  /* init GStreamer */
  gst_init (&argc, &argv);
  loop = g_main_loop_new (NULL, FALSE);

  /* build */
  pipeline = gst_pipeline_new ("my-pipeline");
  src = gst_element_factory_make ("videotestsrc", "src");
  if (src == NULL)
    g_error ("Could not create 'videotestsrc' element");

  filter = gst_element_factory_make ("capsfilter", "filter");
  g_assert (filer != NULL); /* should always exist */

  csp = gst_element_factory_make ("ffmpegcolorspace", "csp");
  if (csp == NULL)
    g_error ("Could not create 'ffmpegcolorspace' element");

  sink = gst_element_factory_make ("xvimagesink", "sink");
  if (sink == NULL) {
    sink = gst_element_factory_make ("ximagesink", "sink");
    if (sink == NULL)
      g_error ("Could not create neither 'xvimagesink' nor 'ximagesink' element");
  }

  gst_bin_add_many (GST_BIN (pipeline), src, filter, csp, sink, NULL);
  gst_element_link_many (src, filter, csp, sink, NULL);
  filtercaps = gst_caps_new_simple ("video/x-raw-rgb",
			   "width", G_TYPE_INT, 384,
			   "height", G_TYPE_INT, 288,
			   "framerate", GST_TYPE_FRACTION, 25, 1,
			   "bpp", G_TYPE_INT, 16,
			   "depth", G_TYPE_INT, 16,
			   "endianness", G_TYPE_INT, G_BYTE_ORDER,
			   NULL);
  g_object_set (G_OBJECT (filter), "caps", filtercaps, NULL);
  gst_caps_unref (filtercaps);

  pad = gst_element_get_pad (src, "src");
  gst_pad_add_buffer_probe (pad, G_CALLBACK (cb_have_data), NULL);
  gst_object_unref (pad);

  /* run */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);

  /* wait until it's up and running or failed */
  if (gst_element_get_state (pipeline, NULL, NULL, -1) == GST_STATE_CHANGE_FAILURE) {
    g_error ("Failed to go into PLAYING state");
  }

  g_print ("Running ...\n");
  g_main_loop_run (loop);

  /* exit */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (pipeline);

  return 0;
}




Correct version:
the example should be:

#include <gst/gst.h>

static gboolean
cb_have_data (GstPad    *pad,
	      GstBuffer *buffer,
	      gpointer   u_data)
{
  gint x, y;
  guint16 *data = (guint16 *) GST_BUFFER_DATA (buffer), t;

  /* invert data */
  for (y = 0; y < 288; y++) {
    for (x = 0; x < 384 / 2; x++) {
      t = data[384 - 1 - x];
      data[384 - 1 - x] = data[x];
      data[x] = t;
    }
    data += 384;
  }

  return TRUE;
}

gint
main (gint   argc,
      gchar *argv[])
{
  GMainLoop *loop;
  GstElement *pipeline, *src, *sink, *filter, *csp;
  GstCaps *filtercaps;
  GstPad *pad;

  /* init GStreamer */
  gst_init (&argc, &argv);
  loop = g_main_loop_new (NULL, FALSE);

  /* build */
  pipeline = gst_pipeline_new ("my-pipeline");
  src = gst_element_factory_make ("videotestsrc", "src");
  if (src == NULL)
    g_error ("Could not create 'videotestsrc' element");

  filter = gst_element_factory_make ("capsfilter", "filter");
  g_assert (filter != NULL); /* should always exist */

  csp = gst_element_factory_make ("ffmpegcolorspace", "csp");
  if (csp == NULL)
    g_error ("Could not create 'ffmpegcolorspace' element");

  sink = gst_element_factory_make ("xvimagesink", "sink");
  if (sink == NULL) {
    sink = gst_element_factory_make ("ximagesink", "sink");
    if (sink == NULL)
      g_error ("Could not create neither 'xvimagesink' nor 'ximagesink' element");
  }

  gst_bin_add_many (GST_BIN (pipeline), src, filter, csp, sink, NULL);
  gst_element_link_many (src, filter, csp, sink, NULL);
  filtercaps = gst_caps_new_simple ("video/x-raw-rgb",
			   "width", G_TYPE_INT, 384,
			   "height", G_TYPE_INT, 288,
			   "framerate", GST_TYPE_FRACTION, 25, 1,
			   "bpp", G_TYPE_INT, 16,
			   "depth", G_TYPE_INT, 16,
			   "endianness", G_TYPE_INT, G_BYTE_ORDER,
			   NULL);
  g_object_set (G_OBJECT (filter), "caps", filtercaps, NULL);
  gst_caps_unref (filtercaps);

  pad = gst_element_get_pad (src, "src");
  gst_pad_add_buffer_probe (pad, G_CALLBACK (cb_have_data), NULL);
  gst_object_unref (pad);

  /* run */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);

  /* wait until it's up and running or failed */
  if (gst_element_get_state (pipeline, NULL, NULL, -1) == GST_STATE_CHANGE_FAILURE) {
    g_error ("Failed to go into PLAYING state");
  }

  g_print ("Running ...\n");
  g_main_loop_run (loop);

  /* exit */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (pipeline);

  return 0;
}



Other information:
The example above doesn't compile ad hoc. Minor typo. The example in the next example compiles but does nothing. Need to verify the error there.