gst-launch的-v参数

-v参数

先看一个简单的例子：

gst-launch的pipeline，增加-v参数就可以输出caps的详细信息，包括我们想要的codec_data，那么，这个-v参数是怎么输出这些的，就需要深入跟踪下代码，因为这个输出不受GST_DEBUG的控制，是直接输出到terminal的。

$ gst-launch-1.0 filesrc location=~/h264.mp4 ! qtdemux !  fakesink -v
Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
/GstPipeline:pipeline0/GstFakeSink:fakesink0.GstPad:sink: caps = video/x-h264, stream-format=(string)avc, alignment=(string)au, \
level=(string)3.1, profile=(string)high, codec_data=(buffer)0164001fffe100196764001facd9405005ba1000000300100000030320f183196001000568ebecb22c, \
width=(int)1280, height=(int)720, framerate=(fraction)25/1, pixel-aspect-ratio=(fraction)1/1
Pipeline is PREROLLED ...
Setting pipeline to PLAYING ...
Redistribute latency...
New clock: GstSystemClock
Got EOS from element "pipeline0".
Execution ended after 0:00:00.001151180
Setting pipeline to NULL ...
Freeing pipeline ...

在gst-launch的代码中，-v参数的作用是输出状态信息和属性通知，如果有verbose参数，会先增加deep_notify处理函数：

{
    "verbose", 'v', 0, G_OPTION_ARG_NONE, &verbose,
 N_("Output status information and property notifications"), NULL},


if (verbose) {
    
    deep_notify_id =
        gst_element_add_property_deep_notify_watch (pipeline, NULL, TRUE);
}

gst_element_property_deep_notify_cb

gst_element_property_deep_notify_cb在这里是deep_notify信号回调函数：

gulong
gst_element_add_property_deep_notify_watch (GstElement * element,
    const gchar * property_name, gboolean include_value)
{
    
  const gchar *sep;
  gchar *signal_name;
  gulong id;

  g_return_val_if_fail (GST_IS_ELEMENT (element), 0);

  sep = (property_name != NULL) ? "::" : NULL;
  signal_name = g_strconcat ("deep-notify", sep, property_name, NULL);
  id = g_signal_connect (element, signal_name,
      G_CALLBACK (gst_element_property_deep_notify_cb),
      GINT_TO_POINTER (include_value));
  g_free (signal_name);

  return id;
}

跟踪进入gst_element_property_deep_notify_cb函数，gst_message_new_property_notify会先创建新的property消息，然后post message。

gst_message_new_property_notify

gst_message_new_property_notify中构造MESSAGE_PROPERTY_NOTIFY类型的message，message post之后，就会在gst-launch中接收处理。

GstMessage *
gst_message_new_property_notify (GstObject * src, const gchar * property_name,
    GValue * val)
{
    
  GstStructure *structure;
  GValue name_val = G_VALUE_INIT;

  g_return_val_if_fail (property_name != NULL, NULL);

  structure = gst_structure_new_id_empty (GST_QUARK (MESSAGE_PROPERTY_NOTIFY));
  g_value_init (&name_val, G_TYPE_STRING);
  /* should already be interned, but let's make sure */
  g_value_set_static_string (&name_val, g_intern_string (property_name));
  gst_structure_id_take_value (structure, GST_QUARK (PROPERTY_NAME), &name_val);
  if (val != NULL)
    gst_structure_id_take_value (structure, GST_QUARK (PROPERTY_VALUE), val);

  return gst_message_new_custom (GST_MESSAGE_PROPERTY_NOTIFY, src, structure);
}

gst-lauch之GST_MESSAGE_PROPERTY_NOTIFY

在gst-launch的中，GST_MESSAGE_PROPERTY_NOTIFY的处理分支，会先解析message的val，然后在最后通过gst_print打印出来，这就是为什么通过增加-v参数就能获取codec_data了。

case GST_MESSAGE_PROPERTY_NOTIFY:{
    
    const GValue *val;
    const gchar *name;
    GstObject *obj;
    gchar *val_str = NULL;
    gchar **ex_prop, *obj_name;

    if (quiet)
        break;

    gst_message_parse_property_notify (message, &obj, &name, &val);

    /* Let's not print anything for excluded properties... */
    ex_prop = exclude_args;
    while (ex_prop != NULL && *ex_prop != NULL) {
    
        if (strcmp (name, *ex_prop) == 0)
            break;
        ex_prop++;
    }
    if (ex_prop != NULL && *ex_prop != NULL)
        break;

    obj_name = gst_object_get_path_string (GST_OBJECT (obj));
    if (val != NULL) {
    
        if (G_VALUE_HOLDS_STRING (val))
            val_str = g_value_dup_string (val);
        else if (G_VALUE_TYPE (val) == GST_TYPE_CAPS)
            val_str = gst_caps_to_string (g_value_get_boxed (val));
        else if (G_VALUE_TYPE (val) == GST_TYPE_TAG_LIST)
            val_str = gst_tag_list_to_string (g_value_get_boxed (val));
        else if (G_VALUE_TYPE (val) == GST_TYPE_STRUCTURE)
            val_str = gst_structure_to_string (g_value_get_boxed (val));
        else
            val_str = gst_value_serialize (val);
    } else {
    
        val_str = g_strdup ("(no value)");
    }

    gst_print ("%s: %s = %s\n", obj_name, name, val_str);
    g_free (obj_name);
    g_free (val_str);
    break;

同样的，在codec-select的代码中，有deep_notify信号回调函数gst_object_default_deep_notify：

gst-plugins-base/tests/examples/dynamic/codec-select.c

  g_signal_connect (pipeline, "deep_notify",
      G_CALLBACK (gst_object_default_deep_notify), NULL);

gst_object_default_deep_notify：

void
gst_object_default_deep_notify (GObject * object, GstObject * orig,
    GParamSpec * pspec, gchar ** excluded_props)
{
    
  GValue value = {
     0, };        /* the important thing is that value.type = 0 */
  gchar *str = NULL;
  gchar *name = NULL;

  if (pspec->flags & G_PARAM_READABLE) {
    
    /* let's not print these out for excluded properties... */
    while (excluded_props != NULL && *excluded_props != NULL) {
    
      if (strcmp (pspec->name, *excluded_props) == 0)
        return;
      excluded_props++;
    }
    g_value_init (&value, pspec->value_type);
    g_object_get_property (G_OBJECT (orig), pspec->name, &value);

    if (G_VALUE_HOLDS_STRING (&value))
      str = g_value_dup_string (&value);
    else
      str = gst_value_serialize (&value);
    name = gst_object_get_path_string (orig);
    // 打印-v参数
    g_print ("%s: %s = %s\n", name, pspec->name, str);
    g_free (name);
    g_free (str);
    g_value_unset (&value);
  } else {
    
    name = gst_object_get_path_string (orig);
    g_warning ("Parameter %s not readable in %s.", pspec->name, name);
    g_free (name);
  }
}