tl;dr: C++s standard streams (cin, cout, cerr, clog) may be used alongside code using the underlying libc i/o streams API and therefore, by default, synchronize with libcs own buffers. In the very least, this means you don't end up with output resulting from characters interleaved between individual stream operations via the two sources.

Its worth noting that even Cs i/o APIs are typically synchronized across threads so you can output lines to stdout without experiencing the same issue within just vanilla threaded C code.