Re: order of destruction, singletons & std::ostream

"" <>
Thu, 9 Aug 2007 05:07:03 CST

We have a class in our system which is a singleton and it has 2
std::auto_ptr<std::ostream> members.

You get hold of the instance by calling the getInstance() method which
is inlined and does
inline Fred& Fred::getInstance() { static Fred fred; return fred; }

And the program is multithreaded. Running on solaris, in case that is

The issue is that sometimes on exit the program hangs, apparently
trying to acquire a mutex in the destructor of one of the ostream
objects. This destructor is called as the result of calling exit(0)
from the main program. And one of the other threads is still running,
and it is also locked on a mutex, though as far as I can see it is a
different mutex.

I realise that threading isn't covered by the standard, but is it safe
to get hold of pointers to ostream objects like that and then let them
be cleaned up during exit processing (especially as, as far as I can
see, the object won't have been constructed till well after main())
has been entered.

I didn't understand the issue with std::auto_ptr<std::ostream>
but probably you have problems because of thread unsafe singleton

inline Fred& Fred::getInstance() { static Fred fred; return fred; }

Compiler generates code like this:

static bool fred_initialized = false;
static char fred_store[Alignment_of_Fred];

void destroy_fred()
{ static_cast<Fred*>((void*)fred_store)->~Fred(); }

inline Fred& Fred::getInstance()
     if (!fred_initialized)
         new (fred_store) Fred();
         fred_initialized = true;
     return *static_cast<Fred*>((void*)fred_store);

This code is not thread safe. It can lead to double
destructor call for Fred. It can lead to use of
uninitialized Fred object as well.

To make it thread safe one uses call_once function.

static boost::once_flag flag = BOOST_ONCE_INIT;

struct Fred
     Fred & getInstance()
         boost::call_once(&Fred::init, flag);
         return *self;
     static init() { static Fred fred; self = &fred; }
     static volatile Fred * self;

Roman Perepelitsa.

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