How do reference-counting smart pointer's avoid or handle reference-counter overflows?

Question

In a naive reference-counting smart pointer implementation, the reference-counter could overflow. How is this overflow avoided or handled in C++ standard library implementations?

I'm not sure you could overflow the reference count. If they used size_t then they should not be able to allocate enough pointers to overflow that. — NathanOliver
– NathanOliver, Commented May 22, 2017 at 14:43
std::shared_ptr's function to return the reference counter returns a long, so I guess that is the "soft" limit. — Rakete1111
– Rakete1111, Commented May 22, 2017 at 14:45
That's a real concern. If you generate a new reference every nanosecond, you would overflow a 64-bit count in less than 600 years. — stark
– stark, Commented May 22, 2017 at 14:51
@stark and your pointers would consume more memory than can be addressed by a 64 bit pointer :) — eerorika
– eerorika, Commented May 22, 2017 at 14:52
If your program overflows a reference counter, you probably deserve it. — n. m. could be an AI
– n. m. could be an AI, Commented May 22, 2017 at 15:19

eerorika · Accepted Answer · 2017-05-22 15:09:48Z

6

Snippets from stdlibc++ headers:

typedef int _Atomic_word;

class _Sp_counted_base
    /*snip*/
    _Atomic_word  _M_use_count;
    /*snip*/
    _M_weak_add_ref()
    { __gnu_cxx::__atomic_add_dispatch(&_M_weak_count, 1); }

/*snip*/
__atomic_add_dispatch(/*snip*/)
{
    /*snip*/
    __atomic_add_single(/*snip*/);
    /*snip*/
}

__atomic_add_single(/*snip*/)
{ *__mem += __val; }

Conclusion: This particular implementation "handles" reference-counter overflow by ignoring the possibility.

answered May 22, 2017 at 15:09

eerorika

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4 Comments

NathanOliver Over a year ago

It is surprising that they use an int but I is hard, for me at least, to envision a use case for 32767 pointers to the same thing.

Bob Jansen Over a year ago

In general yes, but what if an attacker manages to produce some malicious input that causes the program to create them?

eerorika Over a year ago

@BobJansen that could be bad. Another conclusion: You should not let user input directly affect the number of shared pointers without a bound. In fact, I would extend this to anything that allocates memory (or other resources).

zett42 Over a year ago

I just had a look at the MSVC++2017 implementation of std::shared_ptr. They call _InterlockedIncrement() which overflows aswell.

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