Difference between libsqlclient.so and libsqlclient_r.so

What is the difference between these two (shared) libraries? Does the _r means something?
Thanks!

Correction!
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The libarries are libmysqlclient.so and libmysqlclient_r.so.

Generally "_r" suffix denote "Thread-Safe" version. It may cause some (negligible) performance impact because of locking and dynamic memory allocations with-in such libraries.