What may be handed to another thread?

Level: 201 · anyone who has met Send, or ConcurrentModificationException, or a pickle error

Stub — a question and its expected answers, not yet a lesson. No program behind this page has been written or run, so nothing on it has been through the check that backs every finished page. The table is what the finished lesson is expected to find, and every cell is unverified until a program prints it. (Not machine-checked here.)

One line: Rust decides at compile time which values may move to another thread (Send) and which may be shared between threads (Sync), and refuses the rest; every other language here lets anything cross and finds out at run time — Java with a fail-fast iterator, Python with a pickling error at the process boundary, Go and C with a data race.

The question

Hand a value to a thread. A number, obviously. A reference to a local — the previous chapter's question. A reference-counted pointer that is not thread-safe? A database connection? A mutex guard? Rust's Send and Sync are the compiler's answer to each of those, and this page walks the same five values across all six languages to see which ones say anything at all.

What each language is expected to say

	Expected answer, to be verified by the program
Rust	Rc<T> is not Send, MutexGuard is not Send, a raw pointer is neither: each is a compile error with the reason named
Go	anything may cross; the race detector finds the misuse of a map, the runtime finds it best-effort
C	a void * crosses, and what it points to is nobody's business
C++	the same: std::thread takes any callable; std::shared_ptr's count is atomic but the pointee is not protected
Java	any object crosses; ArrayList iterated while modified throws ConcurrentModificationException on a best-effort basis
Python	any object crosses between threads; only picklable ones cross to a multiprocessing process

What the programs have to show

• the five values handed to thread::spawn: which compile, and the error line for each that does not
• the same five in each other language: which fail, when, and how
• a Real runs fence for the Java fail-fast iterator, which is not guaranteed to throw

See also

• Before this: Can a compare-and-swap succeed when it should have failed?
• After this: Can one thread see another's writes out of order?
• Can a thread borrow a local variable?
• Data race or race condition?
• The Rust library's Send and Sync ↗
• The Rust library's Sharing across threads: Arc ↗
• Concepts: Send and Sync · Thread safety · Data-race freedom by construction · Thread confinement · Immutability