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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/98108] Broken Schwarz counter for iostreams
 initialization
Date: Wed, 16 Dec 2020 23:32:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98108

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Right, this actually has nothing to do with threads.

The behaviour is exactly the same as if file1.cc contained this instead:

  void threadfunc();=20=20

  struct StaticThread {
      StaticThread() { threadfunc(); }
  };=20=20

  static StaticThread thread1;

A global constructor calls threadfunc() before std::cout has been initializ=
ed.
It doesn't matter whether that happens in a separate thread or not, and in
either case the solution is to include <iostream> so that a std::ios_base::=
Init
object is constructed before your global.=