How to test C++ code when using C functions

I wrote a c++ class, this class uses the c function library written by a third party.

extern "C" {
#include "nats/nats.h"
}

class NatsConnection {
  public:
  void Connect() {
    natsConnection_Connect(&natsConnection_, natsOptions_);
    natsConnection_SubscribeSync(&natsSubscription_, natsConnection_,
                                 configuration_.subject.c_str());
    // some other c++ code.
  }
}

The above class uses functions imported from c: natsConnection_Connect, natsConnection_SubscribeSync.

Now I need to write a unit test to cover some other c++ code， I am using gtest, I know how to mock a C++ class, but once I use the C code I don’t know how to start.

how do i write the test? Is there has a best practice?