[GStreamer] gst_element_link 及 pad 在继承体系中如何存在

前言：

pad 是每个 element实例都有的，是 elemenet 之间沟通的代理人，没有 pad 的 element 没法于其他element交流。

考虑到gstreamer存在继承体系，那么如果继承类element不在init函数里创建pad，那么在gst_element_link的时候是否可以使用父类element实例的pad呢? 下面从源码来找答案。

gst_element_link

在 gstutils.c 找到 gst_element_link(...) 的实现。

/*** gst_element_link:* @src: (transfer none): a #GstElement containing the source pad.* @dest: (transfer none): the #GstElement containing the destination pad.** Links @src to @dest. The link must be from source to* destination; the other direction will not be tried. The function looks for* existing pads that aren't linked yet. It will request new pads if necessary.* Such pads need to be released manually when unlinking.* If multiple links are possible, only one is established.** Make sure you have added your elements to a bin or pipeline with* gst_bin_add() before trying to link them.** Returns: %TRUE if the elements could be linked, %FALSE otherwise.*/
gboolean
gst_element_link (GstElement * src, GstElement * dest)
{return gst_element_link_pads (src, NULL, dest, NULL);
}

直接调用 gst_element_link_pads(...) , 同样在 gstutils.c ，入参为上有element和下游element 。

gst_element_link_pads

实现同样在 gstutils.c 里。

/*** gst_element_link_pads:* @src: a #GstElement containing the source pad.* @srcpadname: (nullable): the name of the #GstPad in source element*     or %NULL for any pad.* @dest: (transfer none): the #GstElement containing the destination pad.* @destpadname: (nullable): the name of the #GstPad in destination element,* or %NULL for any pad.** Links the two named pads of the source and destination elements.* Side effect is that if one of the pads has no parent, it becomes a* child of the parent of the other element.  If they have different* parents, the link fails.** Returns: %TRUE if the pads could be linked, %FALSE otherwise.*/
gboolean
gst_element_link_pads (GstElement * src, const gchar * srcpadname,GstElement * dest, const gchar * destpadname)
{return gst_element_link_pads_full (src, srcpadname, dest, destpadname,GST_PAD_LINK_CHECK_DEFAULT);
}

回到gst_element_link，第二个和第四个参数为NULL，结合gst_element_link_pads，第二个和第四个参数为padname，因此gst_element_link的行为就是“链接两个element，不在两个element中指定使用哪些pad实例进行互相链接”。

gst_element_link_pads_full

这个函数有点长，我们节选，全文同样在 gstutils.c 里。

定义及注释说明，暂且略过。

/*** gst_element_link_pads_full:* @src: a #GstElement containing the source pad.* @srcpadname: (nullable): the name of the #GstPad in source element*     or %NULL for any pad.* @dest: (transfer none): the #GstElement containing the destination pad.* @destpadname: (nullable): the name of the #GstPad in destination element,* or %NULL for any pad.* @flags: the #GstPadLinkCheck to be performed when linking pads.** Links the two named pads of the source and destination elements.* Side effect is that if one of the pads has no parent, it becomes a* child of the parent of the other element.  If they have different* parents, the link fails.** Calling gst_element_link_pads_full() with @flags == %GST_PAD_LINK_CHECK_DEFAULT* is the same as calling gst_element_link_pads() and the recommended way of* linking pads with safety checks applied.** This is a convenience function for gst_pad_link_full().** Returns: %TRUE if the pads could be linked, %FALSE otherwise.*/
gboolean
gst_element_link_pads_full (GstElement * src, const gchar * srcpadname,GstElement * dest, const gchar * destpadname, GstPadLinkCheck flags)

如果指定了上游element的padname

则在上游element里面查找pad实例，如果查找过程出现问题，则return false(对应name的pad没找到 / pad不是src pad / pad已经有peer pad了，即pad已经被链接过了)。

  /* get a src pad */if (srcpadname) {/* name specified, look it up */if (!(srcpad = gst_element_get_static_pad (src, srcpadname))) {if ((srcpad = gst_element_request_pad_simple (src, srcpadname)))srcrequest = TRUE;}if (!srcpad) {GST_CAT_DEBUG (GST_CAT_ELEMENT_PADS, "no pad %s:%s",GST_ELEMENT_NAME (src), srcpadname);return FALSE;} else {if (!(GST_PAD_DIRECTION (srcpad) == GST_PAD_SRC)) {GST_CAT_DEBUG (GST_CAT_ELEMENT_PADS, "pad %s:%s is no src pad",GST_DEBUG_PAD_NAME (srcpad));release_and_unref_pad (src, srcpad, srcrequest);return FALSE;}if (GST_PAD_PEER (srcpad) != NULL) {GST_CAT_DEBUG (GST_CAT_ELEMENT_PADS,"pad %s:%s is already linked to %s:%s", GST_DEBUG_PAD_NAME (srcpad),GST_DEBUG_PAD_NAME (GST_PAD_PEER (srcpad)));/* already linked request pads look like static pads, so the request pad* was never requested a second time above, so no need to release it */gst_object_unref (srcpad);return FALSE;}}srcpads = NULL;}

如果没有指定上有element的padname

从上游element中找到所有pad，即padlist。

else {/* no name given, get the first available pad */GST_OBJECT_LOCK (src);srcpads = GST_ELEMENT_PADS (src);srcpad = srcpads ? GST_PAD_CAST (srcpads->data) : NULL;if (srcpad)gst_object_ref (srcpad);GST_OBJECT_UNLOCK (src);}

注意，这里用的是 GST_ELEMENT_PADS (src) ，是直接吧当前 element 转换为 GstElement，然后获取GstElement的 pads 成员。记住这里是 “获取 GstElement 的pads 成员”。到这里我想大家应该已经猜出来pad的继承关系了，如果还没懂，我们回忆一下自定义Element如何给自己添加pad。

在instance结构体里定义两个pad成员。

typedef struct _GstMyFilter {GstElement element;//padsGstPad* srcpad;		//src padGstPad* sinkpad;		//sink pad
} GstMyFilter;

在init函数里实例化pad，并且append 到其 parent element GstElement 的 pads 列表中。

static void gst_my_filter_init(GstMyFilter *filter)
{...//instantiates and assigns padsfilter->srcpad = gst_pad_new_from_static_template(&src_factory,"src");filter->sinkpad = gst_pad_new_from_static_template(&sink_factory, "sink");//add pads to elementgst_element_add_pad(GST_ELEMENT(filter),filter->srcpad);gst_element_add_pad(GST_ELEMENT(filter), filter->sinkpad);...
}