Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : David Hopwood
Date : Sat Feb 05 2005 02:28 am

Marcin 'Qrczak' Kowalczyk wrote:
> Giancarlo Niccolai <searchforme@ingoogle.com> writes:
> 
>>>Please show how it's broken when thread local static objects are
>>>initialized in a thread-aware way, and at the same time it works
>>>when they are initialized using an traditional guard variable.
>>
>>Thread-aware way do not mean locked; it may require locking it may
>>not require it, it may suffice or not. The BAD thing is that THIS
>>kind of blind-locking may get in the way of the CORRECT way to
>>secure your code, preventing you from really doing so. I am sorry,
>>but if you don't get this, I cannot absolutely do nothing more to
>>help you.
> 
> I'm saying that you are wrong, and this kind of locking doesn't get
> in the way - sometimes it helps, sometimes it doesn't help, but it
> never makes things worse except efficiency.
> 
> I will happily admit my error if there is indeed some non-perverse
> code which is being broken by this semantics of local static
> initialization. You have been unable to show sketch such code, and
> I can't imagine how it may look, so I claim that it doesn't exist
> until it's shown otherwise.

Just for fun, I tried to construct an artificial (and certainly perverse)
counterexample by trying to get cyclically dependent constructors to
terminate in a single-threaded program. However, this segfaults anyway,
at least with g++. Perhaps someone can "fix" it.

test.cc:
#include <iostream>
using namespace std;

int i = 10;

class Foo {
    public: Foo();
};

class Bar {
    public: Bar() { if (i > 0) { static Foo foo; } }
};

Foo::Foo() { i--; static Bar bar; }

int main(int argc, char **argv) {
    cout << "hello\n";
    Foo();
    cout << "got here\n";
    return 0;
}

$ g++ --version
g++ (GCC) 3.2 20020818 (prerelease)
Copyright (C) 2002 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ -ansi -pedantic -Wall test.cc -o test

$ ./test
hello
Segmentation fault (core dumped)

-- 
David Hopwood <david.nospam.hopwood@blueyonder.co.uk>

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