Last time, we looked at how the Microsoft C++ standard library implements wait and notify_* for std::atomic<std::shared_ptr<T>>. Today, we’ll look at the other library that (as of this writing) implements std::atomic<std::shared_ptr<T>>: libstdc++.
Inside STL: Waiting for a std::atomic<std::shared_ptr<T>> to change, part 2
by Raymond Chen
From the article:
The first thing to note is that the traditional “wait for a value to change” mechanism on unix is the futex, but futexes (futexen?) are limited to 4-byte values, which is insufficient for a 64-bit pointer, much less the two pointers inside a shared_ptr.
At this point, I will refer you to learn about how libstdc++ implements waits on atomic values, particularly the section on how it handles types that do not fit in a __platform_wait_t. The remainder of this discussion will treat that as an already-solved problem and focus on the shared pointer part.
Okay, back to atomic<
shared_ptr<T>>::
wait():
When you wait on a shared_ptr, the work is done by _Sp_atomic::wait: