[Python-Dev] A cute new way to get an infinite loop

Tim Peters tim.peters at gmail.com
Sat Sep 25 06:41:37 CEST 2004

[George Yoshida]
>> It does not always go into an infinite loop. I was bitten by this:
>>  >>> x = []
>>  >>> x.extend(-y for y in x)
>>  Segmentation fault

[Bob Ippolito]
> No algorithm that requires infinite memory will run for an infinite
> amount of time on a finite computer.  Of course it should raise an
> exception instead of segfaulting though.. could it be blowing the
> stack?

No, its stack use is bounded (and small) no matter how long it runs. 
On Windows it eventually raises MemoryError.  My guess is that George
is using Linux.  "It's a feature" that the Linux malloc() can lie (==
malloc(n) can return a non-NULL value p even if you're going to get a
segfault if you try to write to p+i for some i in range(n)).  Linus
likens this to airlines over-selling seats, based on the likelihood
that someone will miss their flight.  Argue with him <wink>.  When
malloc() claims to return memory that can't actually be used, there's
not much Python can do about that (other than blow up when trying to
use it).

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