[Bug libstdc++/12855] Thread safety problems in ios_base::Init

["peturr02 at ru dot is" <gcc-bugzilla at gcc dot gnu dot org>]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12855



------- Additional Comments From peturr02 at ru dot is  2003-11-04 09:17 -------
> std::ios_base::Init should never be used outside global objects.

What about logging? Consider:

Foo.h:
============================

class Foo
{
public:
  Foo();
  ~Foo():
};

============================
foo.cc:
============================
#include <iostream>

Foo::Foo()
{
  std::clog << "Foo constructed\n";
}

Foo::~Foo()
{
  std::clog << "Foo destroyed\n";
}

============================

If we have another source file:

a.cc:
============================
#include "foo.h"

static Foo foo;
============================

This is unsafe, because foo may be constructed before the ios_base::Init
object in foo.cc, and Foo::Foo() may access clog before it is constructed.

This can be fixed by modifying foo.cc:
============================
#include <iostream>

Foo::Foo()
{
  std::ios_base::Init init;
  std::clog << "Foo constructed\n";
}

Foo::~Foo()
{
  std::ios_base::Init init;
  std::clog << "Foo destroyed\n";
}

============================
The standard guarantees that clog will be constructed before the constructor
of init finishes, so the problem has been fixed. However, unless
ios_base::Init is threadsafe, this means that Foo can no longer be used safely
in threaded programs.