Re: double-checked locking for singleton pattern

"Chris Thomasson" <>
Wed, 14 May 2008 18:25:53 CST
<> wrote in message


After reading the Meyers& Alexandrescu article "C++ and the Perils of
Double-Checked Locking"
( which
claims that there is no
way of implementing double-checked locking for C++ singletons, I
wonder if the following simple
solution might do the work:

Its not going to work; there is a race-condition.


The issue which was highlighted in the article is related to the fact
that the line which looks like
pInstance = new Singleton;
can not be used in the Singleton::instance() function since some
compilers may generate code which would assign a value to the
pInstance variable right after allocating memory and BEFORE the
Singleton constructor is executed. It seems that a simple modification
shown above can overcome this problem. Am I wrong? Can someone comment
on that please.

The problem is that your missing a memory barrier after you load from,
before you store to the instance pointer. Here is sketch of
DCL algorithm that has all the correct barriers in place:
template<typename T>
static T& once() {
  static T* g_this = NULL;
  static pthread_mutex_t g_lock = PTHREAD_MUTEX_INITIALIZER;
1:T* l_this = ATOMIC_LOADPTR_MBDEPENDS(&g_this);
  if (! l_this) {
    if (! (l_this = g_lock)) {
      try {
2: l_this = ATOMIC_STOREPTR_MBRELEASE(&g_this, new T);
      } catch (...) {
  return *l_this;

Line 1 atomically load the shared pointer using trailing data-dependant
load-acquire memory barrier. Line 2 atomically stores to the shared
using preceding store-release memory barrier:
  void* v;
  atomic {
    v = *p;
    MEMBAR #LoadStore | #LoadDepends;
  return v;

void* ATOMIC_STOREPTR_MBRELEASE(void** p, void* v) {
  atomic {
    MEMBAR #LoadStore | #StoreStore;
    *p = v;
  return v;

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