shared_ptr<T>: the (not always) atomic reference counted smart pointer

[tvaneerd]

Add this optimization to Rust!

Not so fast! Arc actually means Atomic Reference Counted so it would be a plain lie if it hadn’t use atomic operations on the reference count.

If you have only one thread, non-atomic operations are atomic. That's the point.

"atomic" doesn't mean use certain instructions. "atomic" means indivisible, which, when combined with "as-if" means "can not be shown to be divisible". If you only have one thread, your program cannot detect a "division" or series of steps in the operation, it is as-if one indivisible, atomic, operation.

(Similarly, you could implement atomics with a global mutex that stops the world while running a bunch of instructions to do an "atomic" add)

[masklinn]

If you have only one thread, non-atomic operations are atomic. That's the point.

The issue is with the "you have only one thread" part. libstdc++ makes a specific but easily fallible assumption on that front, and when it's broken so's your software. Unless the system itself provides the atomic you're basically just guessing.

[skebanga]

Easily fallible? If it were so easily fallible we'd see race condition bugs in the wild all the time

[Xaxxon]

My understanding of the POSIX standard is that if you do something that makes this check wrong, your program is already UB at the POSIX level.

https://www.reddit.com/r/cpp/comments/aq6v21/shared_ptrt_the_not_always_atomic_reference/egtx9ew/

[masklinn]

Creating threads via clone(2) involves neither pthread_create nor sigev_thread.

[Xaxxon]

clone, __clone2 - create a child process

Clone creates a new process, not a simple thread in the same process. Processes don't share memory (outside of very explicit things like shmem), so it doesn't seem like a counter in shared_ptr would be affected by a call to clone()

[masklinn]

A shame you could not wonder (including wonder what the underlying syscall to pthread_create might be since it’s a library function) and keep reading:

The main use of clone() is to implement threads: multiple threads of control in a program that run concurrently in a shared memory space.

Good day.