https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70276
--- Comment #2 from Stefan Holdermans <stefan at vectorfabrics dot com> --- Note that the C++11 Standard (§27.4.1) prescribes that "Concurrent access to a synchronized (§27.5.3.4) standard iostream object’s formatted and unformatted input (§27.7.2.1) and output (§27.7.3.1) functions or a standard C stream by multiple threads shall not result in a data race (§1.10)."
