Got it now: https://issues.dlang.org/show_bug.cgi?id=15768
writeln() creates a copy of the stdout struct in a non thread-safe way. If stdout has been assigned a File struct created from a file name this copy includes a "racy" increment/decrement of a reference count to the underlying C-library FILE*. In the case that the reference count is erroneously reaching 0, the file is closed prematurely and when Glibc tries to access internal data it results in the observable SIGSEGV. -- Marco
