boost_1_45_0/libs/thread/example/once.cpp

// Copyright (C) 2001-2003
// William E. Kempf
//
//  Distributed under the Boost Software License, Version 1.0. (See accompanying 
//  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

#include <boost/thread/thread.hpp>
#include <boost/thread/once.hpp>
#include <cassert>

int value=0;
boost::once_flag once = BOOST_ONCE_INIT;

void init()
{
    ++value;
}

void thread_proc()
{
    boost::call_once(&init, once);
}

int main(int argc, char* argv[])
{
    boost::thread_group threads;
    for (int i=0; i<5; ++i)
        threads.create_thread(&thread_proc);
    threads.join_all();
    assert(value == 1);
}