Python String Immutability Broken!

Ken Seehart ken at seehart.com
Mon Aug 25 12:43:01 CEST 2008


You can also use ctypes to globally change the value of integers less 
than 101.   Personally, I don't particularly like the number 14.  I 
changed it to 9 and I am much happier now.

I love ctypes.  So cool.  It's not supposed to be safe.

    Life is either a daring adventure or nothing. Security does not
    exist in nature, nor do the children of men as a whole experience
    it. Avoiding danger is no safer in the long run than exposure.
        *Helen Keller <http://www.quotationspage.com/quotes/Helen_Keller/>*
        /US blind & deaf educator (1880 - 1968)/

Of course I would not hire anyone who believes this quote, other than 
Helen Keller, if she were still with us.

It is quite possible to write a small program that works using abused 
strings.  But my life better not depend on it.  Among other things, if 
you use the abused string as a key anywhere, you will not get correct 
results.  Trying to change the length of the string will cause 
disasters.  Lengthening a string will corrupt memory, and shortening the 
string will not shorten it but rather embed '\0' in it.

Ken

Hendrik van Rooyen wrote:
>  
> Patrick Maupin <pmau....ail.com> wrote:
>
>   
>> Very entertaining.
>>
>>     
>
> Thanks. Nice to see that there is still some sense of humour
> left somewhere - its all been so serious here lately - people
> seem to forget that hacking is fun!
>
>   
>> But let me get this straight:   Are you just complaining that if you
>> pass a string to an arbitrary C function using ctypes, that that
>> arbitrary function can modify the string?
>>
>>     
>
> Actually, I am not complaining - I am asking for advice on the side
> effects of what I am doing, which is replacing a bunch of bits
> in what is essentially an output bit field with the corresponding
> input bits at the same addresses read back from a simulated i/o
> bus structure.  And I would also like to know if there is a better
> way of doing this.
>
> The C code actually works, doing what was intended - the \xff that
> one sees appearing back comes from the pullup resistors on the
> eBox's i/o.  I can show that it is working by adding some resistance
> and capacitance (by holding the connector against my tongue) in which
> case I get a munged version of the fox back.  (- evidently my tongue
> is not such a perfect communications medium as I would like to believe.)
>
> Passing the fox is actually deceptive and misleading, as in real
> use there would be no such correlation sideways across bits, as
> they are just representations of output lines.
> (Think "coils" in PLC jargon)
>
>   
>> Because if you are, then I think you share a great deal of
>> responsibility for the death of that string -- sending the poor thing
>> to its grave through some unknown C function.
>>     
>
> This string is NOT dead - it is alive, and not even stunned -
> it just looks as if it is sleeping because of the \xff - which
> comes from the fact that there is no real hardware out there yet.
>
> The C functions are very simple ones actually - they just do
> what are essentially Linux I/O system calls - setting direction
> bits for a port (in or out) and then reading or writing the data.
>
> - Hendrik
>
>
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>
>   




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