How to extract private codec data from encoder?

[sonntex]

I use gstreamer 1.8 in my c++ code to transcode h264 and mjpeg to h264
using vaapi.

Pipeline looks like:

pipeline_ = gst_parse_launch("appsrc name=appsrc0 ! vaapidecodebin !
vaapiencode_h264 ! appsink name=appsink0", nullptr);
src_ = gst_bin_get_by_name(GST_BIN(impl_->pipeline_), "appsrc0");
sink_ = gst_bin_get_by_name(GST_BIN(impl_->pipeline_), "appsink0");

How can I extract private codec data from encoder? For example, I have my
own rtp stack and file muxing. So, I need to compute such attributes as
sprop parameter sets, profile level id and packetization mode.
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