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Right which is why you don't actually do much of anything in the signal handler except save that it happened somewhere to be processed by your application later.

https://docs.python.org/3/library/signal.html#execution-of-p...

Here's how Python does it.

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Uh, that Examples section calls print inside the signal handler. Python's IO stack is non-reentrant. I hope nobody follows these examples and gets exceptions at runtime because of it.

"A Python signal handler does not get executed inside the low-level (C) signal handler. Instead, the low-level signal handler sets a flag which tells the virtual machine to execute the corresponding Python signal handler at a later point(for example at the next bytecode instruction)"

The low-level C handler set the flag then returns. Only some time later does the Python run-time call the associated Python handler that you see in the Examples section.

In this case that's before the next bytecode executes. The write syscall gets interrupted and returns EINTR, then cpython checks what signal was caught and executes the signal handler, before trying to do the remaining write:

https://github.com/python/cpython/blob/5f7aba938cf5007b6f954...

Here's even a test for it: https://github.com/python/cpython/blob/5f7aba938cf5007b6f954...

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