百度的网站名,山东省建设项目监理协会网站,注册好域名之后怎么做个人网站,wordpress 网站排名优化前言#xff1a; pad 是每个 element实例 都有的#xff0c;是 elemenet 之间沟通的代理人#xff0c;没有 pad 的 element 没法于其他element交流。 考虑到gstreamer存在继承体系#xff0c;那么如果继承类element不在init函数里创建pad#xff0c;那么在gst_element_l…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 arent 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);... } 结论 至此结合上下文我们已经可以知道 pad 是如何在继承体系中存在的了。 pad仅仅由GstElement使用所有继承自GstElement 的 element 仅仅是为 GstElement 提供了自定义的 pad实例同时参与 pad的生命周期管理(fixme if wrong)。 所以即便 自定义 element 不创建自己的 pad那么只要其 继承树里某个父类 element 在 init 函数里通过 add pad 函数向 根节点 GstElement里添加过 pad 那么这个pad就会作为总代理。