how to set latency to 0 in playbin winthin c/c++ code

[cfd new]

I can set latency to 0 by 
gst-launch-1.0 playbin uri=*** uridecondebin0::source::latency=0
how do I do this in C/C++ code?Tried to do the following:GstElement * source{};g_object_get( my_playbin, "source", &source, nullptr);
and source is nullptr.
  Thanks for your help.
   Joe
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