Which thread gets the signal?

Level: 201 · anyone whose Ctrl-C handler ran on the wrong thread

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: A signal sent to a process is delivered to one of its threads — any that has not blocked it — and the handler runs on that thread in the middle of whatever it was doing, which is why a handler may do almost nothing safely; the languages that hide this route the signal to a known place: Go to a channel, Python to the main thread's next bytecode, Rust to nothing unless you ask.

The question

Three threads busy in loops, and the process gets SIGUSR1. Which thread's handler runs? On Linux, whichever the kernel picks. The page records the thread id seen by the handler over a hundred signals, as Real runs, and then shows the fix each language offers: block the signal in every thread but one, and have that one wait for it — which is what Go's signal.Notify and Rust's signal-hook do underneath.

What each language is expected to say

	Expected answer, to be verified by the program
Rust	std has no handler API; signal-hook or ctrlc (link, not run); the Rust library's page on catching a signal is the reference
Go	signal.Notify(ch, syscall.SIGUSR1): the runtime catches it on any thread and sends on the channel
C	sigaction on any thread; pthread_sigmask to block it in workers; sigwait in one thread
C++	the same as C
Java	sun.misc.Signal (not standard); the JVM handles most signals itself
Python	the handler runs on the main thread only, between bytecodes; a worker thread cannot receive one

What the programs have to show

• Real runs: the thread that handled each of a hundred SIGUSR1s, in C
• the blocked-in-workers, waited-in-one version: every signal on the chosen thread
• Python's main-thread rule, and what happens to a signal while the main thread is blocked in a lock

See also

• Before this: How do two processes talk through a pipe?
• After this: When are processes the better workers?
• How do two processes talk through a pipe?
• What does a failure on a thread do when nobody is waiting for it?
• The Rust library's Catching a signal ↗
• The Python library's Ctrl-C is a signal ↗
• The Linux library's The signals you cannot catch ↗
• Concepts: Signals · Process · Thread · Inter-process communication