os._exit vs. sys.exit

[Bryan]

Quick question:

Why does os._exit called from a Python Timer kill the whole process while
sys.exit does not? On Suse.

Bryan

[Andrew Dalke]

Re: os._exit vs. sys.exit

Bryan wrote:[color=blue]
> Why does os._exit called from a Python Timer kill the whole process while
> sys.exit does not? On Suse.[/color]

os._exit calls the C function _exit() which does an immediate program
termination. See for example

http://developer.apple.com/documentation/Darwin/Reference/ManPages/man2/_exit.2.html

and note the statement "can never return".

sys.exit() is identical to "raise SystemExit()". It raises a Python
exception which may be caught at a higher level in the program stack.

Andrew
dalke@dalkescie ntific.com

[Peter Hansen]

Re: os._exit vs. sys.exit

Andrew Dalke wrote:[color=blue]
> sys.exit() is identical to "raise SystemExit()". It raises a Python
> exception which may be caught at a higher level in the program stack.[/color]

And which *is* caught at the highest levels of threading.Threa d objects
(which Timer is based on). Exceptions raised (and caught or not) in a
Thread do not have any effect on the main thread, and thus don't affect
the interpreter as a whole.

-Peter

[Bryan]

Re: os._exit vs. sys.exit


"Peter Hansen" <peter@engcorp. com> wrote in message
news:u-SdnRAzQ9KHhnffR Vn-gg@powergate.ca ...[color=blue]
> Andrew Dalke wrote:[color=green]
>> sys.exit() is identical to "raise SystemExit()". It raises a Python
>> exception which may be caught at a higher level in the program stack.[/color]
>
> And which *is* caught at the highest levels of threading.Threa d objects
> (which Timer is based on). Exceptions raised (and caught or not) in a
> Thread do not have any effect on the main thread, and thus don't affect
> the interpreter as a whole.
>
> -Peter[/color]

Thanks for the clarifications. One more question, can I catch this
exception in my main thread and then do another sys.exit() to kill the whole
process?

Apparently sys.exit() allows the program to clean up resources and exit
gracefully, while os._exit() is rather abrupt.

Bryan

[Peter Hansen]

Re: os._exit vs. sys.exit

Bryan wrote:[color=blue]
> Thanks for the clarifications. One more question, can I catch this
> exception in my main thread and then do another sys.exit() to kill the whole
> process?[/color]

Not as such. Exceptions can be caught only in the thread in which they
are raised. There are tricky techniques to change this, but they would
have to rely on things which are themselves sufficient for what you are
trying to do.
[color=blue]
> Apparently sys.exit() allows the program to clean up resources and exit
> gracefully, while os._exit() is rather abrupt.[/color]

What does the main thread do while the other thread is running? If it's
just waiting for it and other threads to finish/fail, then you need to
have some kind of loop that waits for all other threads to not respond
True to .isAlive(), and/or you need to add a threading.Event or
something like it which the main thread can wait on or poll to see
whether a thread has caught an exception, *and* you need to make all
those other threads catch their own exceptions at the top levels of
their run() method, and to set that Event object if SystemExit is
caught. Or related techniques.

If that's not enough ideas for you to figure something out, please
provide more detail and we can come up with something more specific and
appropriate. For example, do you want to exit the app only if a thread
raises SystemExit, or would other exceptions result in the same effect?

-Peter