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Date: Tue, 13 May 2014 20:50:04 +1000
Subject: Re: Fortran (Was: The "does Python have variables?" debate)
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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Xref: csiph.com comp.lang.python:71466

On Tue, May 13, 2014 at 8:44 PM, Rustom Mody <rustompmody@gmail.com> wrote:
> On Tuesday, May 13, 2014 3:30:36 PM UTC+5:30, Chris Angelico wrote:
>> Actually, even the file system can do some of this to you. I was
>> checking lsof on one of my Linux systems a little while ago, and found
>> that I had half a dozen old versions of a program, all with the same
>> file name, all deleted. (When you unlink a running binary on Linux,
>> the program keeps running the old version, and then you can put a new
>> version in its place, with the same file name. Any new invocation will
>> pick up the new binary; existing ones keep what they have.) So a
>> program can unlink itself, write out a new version of itself, and
>> happily keep going - letting new invocations take the changed version,
>> safely. Again, not exactly self-modifying code... or is it? Again,
>> tamed by boundaries.
>
> Your lsof example looks like a rather typical case of ugly self-modifying code
> Self-modifying code has many flavours; consider:
>
> $ sudo apt-get upgrade apt-get
>
> relatively harmless but self-modifying nonetheless
>

Heh. Those are going to be exactly the same. (In actual fact, it was a
"sudo make install" that created the situation I described.) The only
difference is that you probably don't have any other instances of
apt-get running, meaning the change takes place for the next run - but
while that's running, you would see the old /usr/bin/apt-get as a
deleted and open file.

ChrisA