difference between libmysql and libmysql_r

[Terry]

Hi, I have a question. I know that the code in libmysqld is for the
<http://www.mysql.com/doc/en/libmysqld.html> Embedded MySQL Server
Library.
Would anyone mind to direct me where I can find what the folder libmysql
and libmysql_r for?
 
Thanks
 
Terry