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From: "i.hamsa at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/98108] New: Broken Schwarz counter for iostreams
 initialization
Date: Wed, 02 Dec 2020 21:32:36 +0000
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X-Bugzilla-Version: 10.2.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98108

            Bug ID: 98108
           Summary: Broken Schwarz counter for iostreams initialization
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: i.hamsa at gmail dot com
  Target Milestone: ---

Iostreams initialization doesn't work in multithreaded environments.

To reproduce one needs two source files:

// file1.cc

  #include <thread>=20=20

  void threadfunc();=20=20

  struct StaticThread {
      std::thread t;=20=20
      StaticThread() : t(threadfunc) {}
      ~StaticThread() { t.join(); }
  };=20=20

  static StaticThread thread1, thread2;

//file2.cc

  #include <iostream>=20=20

  void threadfunc() {
      std::cout << "Printing\n";
  }=20=20

  int main() {}

//compile
g++ -pthread file1.cc file2.cc

It is important that <iostream> is NOT included in file1.cc.

The resulting executable always crashes on my machine (Gentoo Linux). Add
<iostream> to file1.cc, the crash disappears. Change the order of source fi=
les
in the command line, the crash disappears.

---

The culprit is ios_base::Init::_S_refcount. Basically it is an atomic nifty
counter. The problem is, the nifty counter idiom doesn't work with
multithreading. Making it atomic achieves absolutely nothing. If we have tw=
o or
more threads that can enter Init::Init() at the same time, one should proce=
ed
and the rest must WAIT. It is the same thing as initialization of a static
object in a block scope, and should be interlocked in exactly the same way.=