Exiting the child process after os.fork()

Question

Which is the correct function to call to exit the child process after os.fork()?

The documentation for os._exit() states:

The standard way to exit is sys.exit(n).

_exit() should normally only be used in the child process after a fork().

It does not say whether it's acceptable to terminate the child process using sys.exit(). So:

1. Is it?
2. Are there any potential side effects of doing so?

Answer 1

The unix way is that if you are a child of a fork then you call _exit. The main difference between exit and _exit is that exit tidies up more - calls the atexit handlers, flushes stdio etc, whereas _exit does the minimum amount of stuff in userspace, just getting the kernel to close all its files etc.

This translates pretty directly into the python world with sys.exit doing what exit does and doing more of the python interpreter shutdown, where os._exit does the minimum possible.

If you are a child of fork and you call exit rather than _exit then you may end up calling exit handlers that the parent will call again when it exits causing undefined behaviour.

Answer 2

Part of the documentation on os._exit(n) you did not cite is

Exit the process with status n, without calling cleanup handlers, flushing stdio buffers, etc.

So, how I'm reading this, you should use os._exit() as long as you share file handlers (so they will be close()'d by another process and you take care of flushing the buffers yourself (if it matters in your case). Without shared resources (like in "files") - it doesn't matter.

So if your child processes are computation-only, and are fed raw data (not resource handlers), then it's safe to use exit().