Re: N2444/Boost: fast_pthread_once and atomicity w/ regard to threads

"Chris Thomasson" <>
Sat, 26 Apr 2008 03:30:54 CST
[added comp.programming.threads; is that okay?]

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Dear C++ memory model experts,

yesterday I skimmed through the sources of the latest Boost.Thread
library, especially the POSIX Threads implementation of
boost::call_once(). It fundamentally changed in the past. The current
implementation quotes the ISO/IEC JTC1 SC22 WG21 N2444 document
and uses an algorithm by Mike Burrow published there.


AFAICT, this is not "100%" POSIX compliant. There is a rule... You cannot
read or write a memory location that might be updated by another thread
without a lock; simple. BTW, there are ways to implement a lock without
using a memory barrier:

You use an asymmetric model, and elect a "dominate" thread. Also, I am not
sure if this is anything all that new:

Basically, you can do it like:
<crude code-sketch>
template<typename T>
static T* once() {
  static T* g_object = NULL;
  static __thread bool g_flag = false;
  static pthread_mutex_t g_lock = PTHREAD_MUTEX_INITIALIZER;
  if (! g_flag) {
    if (! g_object) {
      T* l_object;
      try {
        l_object = new T;
      } catch(...) {
      g_object = l_object;
    g_flag = true;
  return g_object;

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