Are standard output streams in C++ thread-safe (cout, cerr, clog)?

I know that there is no concept of threads in current C++, but this article is saying:

A typesafe, threadsafe, portable logging mechanism

.....

The fprintf() function is threadsafe, so even if this log is used from different threads, the output lines won't be scrambled.

What about cout, cerr and clog?

I think this question is applicable to all kind of stream types in C++ also, like fstream and stringstream.