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Re: Some issues with short-circuiting arithmetic operators


From: Chet Ramey
Subject: Re: Some issues with short-circuiting arithmetic operators
Date: Sun, 09 Sep 2012 16:41:18 -0400
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On 9/5/12 10:40 PM, Dan Douglas wrote:
> This reorder function is meant to swap values of a two-element array if
> unordered. Bash and ksh produce reversed results. mksh and zsh do as expected.
        [...]
> The Ksh issue seems to be that an explicit x[0] is needed (it's a slightly
> outdated dev build), but I can't figure out why Bash is doing this. No
> parameter expansion in the arithmetic does the same:
> 
>     function reorder2 {
>         _=$x let '(x[1] < x) && (x=x[1], x[1]=_)'
>         echo "address@hidden"
>     }
> 
> Some variations crash:
> 
>     $ bash -c 'function reorder { (( x[1] < x[0] && (x=x[1], x[1]=$x) )); 
> echo "address@hidden"; }; x=(123 456); reorder; x=(456 123); reorder'
>     Segmentation fault
>     $ bash -c 'function reorder { (( (x > x[1]) && (x=${x[1]}, x[1]=$x) )); 
> echo "address@hidden"; }; x=(123 456); reorder; x=(456 123); reorder'
>     123 456
>     Segmentation fault
> 
> The second issue is that Bash tries to resolve arithmetic variables when
> evaluation should never reach them.

There is a pointer aliasing error that causes the seg fault, but all of
these errors were caused by incorrect handling of the `noeval' state.
These will all be fixed in the next version of bash.

Chet

- -- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/
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