Hi Daniel,
Your proposed solution looks like a variant of the Double Checked Locking.
Unfortunately, this design pattern is very subtly broken and not thread safe.
See http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf for more
information.
I've however shown this example just to illustrate a more general problem, as
most of the getenv() are (in)directly called from within class constructors.
Hence, what we're looking as is something more similar to the following.
static Mutex s_mutex;
Object & getSingletonObject()
{
ScopedLock lock(s_mutex);
static Object object;
return object;
}
Cheers,
Tanguy
-----Original Message-----
From: osg-users-boun...@lists.openscenegraph.org
[mailto:osg-users-boun...@lists.openscenegraph.org] On Behalf Of Daniel
Trstenjak
Sent: 06 August 2009 12:24
To: OpenSceneGraph Users
Subject: Re: [osg-users] Deadlock when loading osg.dll, singletons are evil
Hi Tanguy,
> static Mutex s_mutex;
>
> const char * getSingletonGlExtensionDisable()
> {
> ScopedLock lock(s_mutex);
>
> static const char * env = getenv("OSG_GL_EXTENSION_DISABLE");
>
> return env;
> }
This doesn't make sense, because 'getenv' is called only once, during
the initialization of the static variable 'env'. After that the
value of 'env' doesn't change, but for every call to
'getSingletonGlExtensionDisable' a lock is used.
If the value of 'OSG_GL_EXTENSION_DISABLE' shouldn't change during
runtime:
static Mutex s_mutex;
const char* getSingletonGlExtensionDisable()
{
static char* env = 0L;
if (env == 0L) {
ScopedLock lock(s_mutex);
env = getenv("OSG_GL_EXTENSION_DISABLE");
}
return env;
}
Greetings,
Daniel
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