If your program generates signals, it could generate a flood unintentionally. If you put a print statement in the handler, you can get this result.
It's not terribly surprising, but good to know.
Is it safe to call print in a Python signal handler? (iafisher.com)
29 pointsby hellerve4 days ago11 comments
jrumbut an hour ago
I think the author undersells the significance of this. I could easily picture someone writing code that boils down to the example, all it requires is a flood of signals and a print statement.
nine_k 2 hours ago
POSIX signals are broken by design, alas: https://lwn.net/Articles/414618/
Python or not, almost nothing is safe inside a signal handler.
knome an hour ago
the first line of the article points out that python isn't run in the POSIX C handler. that just sets a flag for the interpreter to act on. the python issue is an unsafe re-entrant handling strategy in the interpreter.
inigyou an hour ago
Certain signals are true user-mode interrupts, like SIGTERM and some of the ones glibc uses to implement pthreads.
The ones related to application logic must only be handled with signalfd if you want any semblance of reliability.
IgorPartola 4 hours ago
What’s really fun is mixing signal handling and threads, especially on Linux. There is a simple way to do it and about a thousand ways that include at least one gotcha.
zbentley an hour ago
What’s the simple way? Self-pipe?
megagpt3 3 hours ago
He considers it safe if it's unlikely to crash? That's also true in C. Calling printf in a C signal handler is likely to work. So why does he consider it important in C but "not a practical consideration" in Python? It's more likely to crash in Python than in C because the signal handler takes longer to execute.
lmz 22 minutes ago
The article mentions that the Python handler is run outside of the C handler context and so is not subject to the C safety restrictions. It will not crash since the interpreter only calls the Python handler when it is safe. It will however not protect against reentrancy issues in the Python handler.
The C printf function is not async signal safe and is one of the examples in the manual: https://man7.org/linux/man-pages/man7/signal-safety.7.html
charcircuit an hour ago
I don't understand how this is still a problem in 2026. Signals should just come in via a new thread and it would solve all the complexity around them. Everyone has known the current way it works is extremely limited in what you can safely do. This whole pause the execution of what's currently running and then run some extra code somewhere else turned out to not be a good idea.
inigyou an hour ago
They should come in via signalfd unless they're the moral equivalent of a non-maskable interrupt.
Uptrenda 2 hours ago
And when you combine it with event loops and multiple OSes + python versions it gets even more difficult. Don't get me wrong: I love python, but shut down / cleanup is kind of a pain in the ass. If someone built a (good) lib for this it would probably be quite popular.