Mihran Hovsepyan Mihran Hovsepyan - 2 months ago 14
C++ Question

Is it thread safe to reset and copy shared_ptr simultaneously?

Boost documentation describes shared pointer's behavior when accessing it from multiple threads simultaneously. Particularly they give some examples:

shared_ptr<int> p(new int(42));

//--- Example 1 ---

// thread A
shared_ptr<int> p2(p); // reads p

// thread B
shared_ptr<int> p3(p); // OK, multiple reads are safe

//--- Example 2 ---

// thread A
p.reset(new int(1912)); // writes p

// thread B
p2.reset(); // OK, writes p2

//--- Example 3 ---

// thread A
p = p3; // reads p3, writes p

// thread B
p3.reset(); // writes p3; undefined, simultaneous read/write
...


But they do not say (or I cannot see) what will happen if the same
shared_ptr
object is written and read at the same time. Say:

shared_ptr<int> p(new int(42));

//--- My Example ---

// thread A
p.reset(new int(1912));

// thread B
shared_ptr<int> p1 = p;


So my question is whether the last example is OK or not?

NOTE: Boost's third example explains that it is not safe to read and write the same object in parallel. But in their example they assign
p3
to
p
, which is a copy of
p3
. So it is not clear to me whether the safety of that example depends on the fact that
p3
gets assigned to its copy or whether something else is unsafe.

Answer

But they does not say (or I cannot see) what will happen if the same shared_ptr object will be written and read at the same time.

Yes they do:

A shared_ptr instance can be "read" (accessed using only const operations) simultaneously by multiple threads. Different shared_ptr instances can be "written to" (accessed using mutable operations such as operator= or reset) simultaneously by multiple threads (even when these instances are copies, and share the same reference count underneath.)

Any other simultaneous accesses result in undefined behavior.

(emphasis mine)

Seems pretty clear to me.

So my question is whether the last example OK or not?

No, because what you are doing is not reading simultaneously from a single instance, nor writing to separate instances. You are simultaneously reading and writing a single instance. That's a data race, and undefined behaviour.