基本操作示例

gst-launch-1.0 filesrc location="/home/e0005055/Videos/test1.mp4" ! decodebin ! videoconvert ! autovideosink

代码：

#include <gst/gst.h>

static gboolean
bus_call (GstBus     *bus,
      GstMessage *msg,
      gpointer    data)
{
  GMainLoop *loop = data;

  switch (GST_MESSAGE_TYPE (msg)) {
    case GST_MESSAGE_EOS:
      g_print ("End-of-stream\n");
      g_main_loop_quit (loop);
      break;
    case GST_MESSAGE_ERROR: {
      gchar *debug = NULL;
      GError *err = NULL;

      gst_message_parse_error (msg, &err, &debug);

      g_print ("Error: %s\n", err->message);
      g_error_free (err);

      if (debug) {
        g_print ("Debug details: %s\n", debug);
        g_free (debug);
      }

      g_main_loop_quit (loop);
      break;
    }
    default:
      break;
  }

  return TRUE;
}

static void
on_pad_added (GstElement *element,
              GstPad     *pad,
              gpointer    data)
{
  GstPad *sinkpad;
  GstElement *decoder = (GstElement *) data;
  /* We can now link this pad with the vorbis-decoder sink pad */
  g_print ("Dynamic pad created, linking demuxer/decoder\n");
  sinkpad = gst_element_get_static_pad (decoder, "sink");
  gst_pad_link (pad, sinkpad);
  gst_object_unref (sinkpad);
}

gint main (gint   argc,gchar *argv[])
{
  GstStateChangeReturn ret;
  GstElement *pipeline, *filesrc, *decoder, *filter, *sink;
  GstElement *video_convert, *convert2, *resample;
  GMainLoop *loop;
  GstBus *bus;
  guint watch_id;

  g_print ("in %d !\n",__LINE__);
  /* initialization */
  gst_init (&argc, &argv);
  loop = g_main_loop_new (NULL, FALSE);
  if (argc != 2) {
    g_print ("Usage: %s <mp3 filename>\n", argv[0]);
    return 01;
  }
  /* create elements */
  pipeline = gst_pipeline_new ("my_pipeline");

  /* watch for messages on the pipeline's bus (note that this will only
   * work like this when a GLib main loop is running) */
  bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
  watch_id = gst_bus_add_watch (bus, bus_call, loop);
  gst_object_unref (bus);

  filesrc  = gst_element_factory_make ("filesrc", "my_filesource");
  decoder  = gst_element_factory_make ("decodebin", "my_decoder");
  video_convert = gst_element_factory_make ("videoconvert", "my_videoconvert");
  sink     = gst_element_factory_make ("autovideosink", "my_autovideosink");

  if (!sink || !decoder) {
    g_print ("Decoder or output could not be found - check your install\n");
    return -1;
  } else if (!video_convert) {
    g_print ("Could not create audioconvert or audioresample element, "
             "check your installation\n");
    return -1;
  }

  g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);

  gst_bin_add_many (GST_BIN (pipeline), filesrc, decoder, video_convert, sink, NULL);

  /* link everything together */
  if (!gst_element_link_many (filesrc, decoder, NULL)) {
    g_print ("Failed to link one or more elements!\n");
    return -1;
  }
  if (!gst_element_link_many (video_convert, sink, NULL)) {
    g_print ("Failed to link one or more elements!\n");
    return -1;
  }
  g_signal_connect (decoder, "pad-added", G_CALLBACK (on_pad_added), video_convert);
  /* run */
  ret = gst_element_set_state (pipeline, GST_STATE_PLAYING);
  if (ret == GST_STATE_CHANGE_FAILURE) {
    GstMessage *msg;

    g_print ("Failed to start up pipeline!\n");

    /* check if there is an error message with details on the bus */
    msg = gst_bus_poll (bus, GST_MESSAGE_ERROR, 0);
    if (msg) {
      GError *err = NULL;
      gst_message_parse_error (msg, &err, NULL);
      g_print ("ERROR: %s\n", err->message);
      g_error_free (err);
      gst_message_unref (msg);
    }
   g_print ("in %d !\n",__LINE__);
   return -1;
  }
  g_main_loop_run (loop);
  /* clean up */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (pipeline);
  g_source_remove (watch_id);
  g_main_loop_unref (loop);

  return 0;
}

信号处理

通过g_signal_connect 连接信号，设定callback函数。

  g_signal_connect (decoder, "pad-added", G_CALLBACK (on_pad_added), video_convert);

其中，元素的信号可以通过gst-inspect-1.0 decodebin查看：

# gst-inspect-1.0 decodebin
Factory Details:
  Rank                     none (0)
  Long-name                Decoder Bin
  Klass                    Generic/Bin/Decoder
……
Element Signals:
  "pad-added" :  void user_function (GstElement* object,
                                     GstPad* arg0,
                                     gpointer user_data);
  "pad-removed" :  void user_function (GstElement* object,
                                       GstPad* arg0,
                                       gpointer user_data);
  "no-more-pads" :  void user_function (GstElement* object,
                                        gpointer user_data);
  "unknown-type" :  void user_function (GstElement* object,
                                        GstPad* arg0,
                                        GstCaps* arg1,
                                        gpointer user_data);

属性设定

通过g_object_set或者g_object_get读写属性。具体属性可以通过gst-inspect-1.0 [插件名] 来查看：

# gst-inspect-1.0 filesrc
Factory Details:
  Rank                     primary (256)
  Long-name                File Source
  Klass                    Source/File
......
Element Properties:
  blocksize           : Size in bytes to read per buffer (-1 = default)
                        flags: readable, writable
                        Unsigned Integer. Range: 0 - 4294967295 Default: 4096
  do-timestamp        : Apply current stream time to buffers
                        flags: readable, writable
                        Boolean. Default: false
  location            : Location of the file to read
                        flags: readable, writable, changeable only in NULL or READY state

可以看到flag选项标明了读写条件，后面是数据类型、取值范围和默认值。

消息message

消息是从bus中获取，可以注册消息，也可以轮询消息。

使用gst_bus_add_watch +callback函数关注消息：

  bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
  watch_id = gst_bus_add_watch (bus, bus_call, loop);
static gboolean bus_call (GstBus     *bus,      GstMessage *msg,      gpointer    data)
{
  GMainLoop *loop = data;
  switch (GST_MESSAGE_TYPE (msg)) {
    case GST_MESSAGE_EOS:
      break;
    case GST_MESSAGE_ERROR: 
      break;
    default:
      break;
  }
  return TRUE;
}

使用gst_bus_poll轮询消息：

    msg = gst_bus_poll (bus, GST_MESSAGE_ERROR, 0);
    if (msg) {
      GError *err = NULL;
      gst_message_parse_error (msg, &err, NULL);
      g_print ("ERROR: %s\n", err->message);
      g_error_free (err);
      gst_message_unref (msg);
    }    

状态

通过gst_element_set_state设定管道状态：

gst_element_set_state (pipeline, GST_STATE_PLAYING);
……
gst_element_set_state (pipeline, GST_STATE_NULL);

事件

事件通过管道来控制上下游的节点，通过gst_element_send_event 来设定事件：

static void
seek_to_time (GstElement *element,          guint64     time_ns)
{
  GstEvent *event;
  event = gst_event_new_seek (1.0, GST_FORMAT_TIME,
                  GST_SEEK_FLAG_NONE,
                  GST_SEEK_METHOD_SET, time_ns,
                  GST_SEEK_TYPE_NONE, G_GUINT64_CONSTANT (0));
  gst_element_send_event (element, event);
}

衬垫控制

按照“always”、“sometimes”、“request”情况来使用衬垫。

always衬垫

因为该衬垫一直存在，可以在任何时候连接。

gst_element_link_pads (source, "src", demux, "sink");

sometimes衬垫

在特定条件下才存在，所以在"pad-added"信号发出时连接。

  g_signal_connect (demux, "pad-added", G_CALLBACK (cb_new_pad), decoder );
  ……
on_pad_added (GstElement *element,
              GstPad     *pad,
              gpointer    data)
{
  GstPad *sinkpad;
  GstElement *decoder = (GstElement *) data;
  /* We can now link this pad with the vorbis-decoder sink pad */
  g_print ("Dynamic pad created, linking demuxer/decoder\n");
  sinkpad = gst_element_get_static_pad (decoder, "sink");
  gst_pad_link (pad, sinkpad);
  gst_object_unref (sinkpad);
}

request衬垫

request衬垫是按需求创建。使用gst_element_get_request_pad函数来申请衬垫：

static void some_function (GstElement * tee)
{
  GstPad *pad;
  gchar *name;

  pad = gst_element_get_request_pad(tee, "src%d");
  name = gst_pad_get_name (pad);
  g_print ("A new pad %s was created\n", name);
  g_free (name);
  //链接衬垫 
  gst_object_unref (GST_OBJECT (pad));

注意官网教程api为gst_element_request_pad_simple，应该是被废弃了不能使用。