Subject: Re: /etc/login.conf required to exist after user(8) changes
To: Peter Seebach <>
From: John Nemeth <>
List: current-users
Date: 07/31/2005 03:38:16
On Dec 19,  1:44am, Peter Seebach wrote:
} In message <>, John Nemeth writes:
} >On Dec 19,  1:08am, Peter Seebach wrote:
} >} To be picky, there exist C implementations where at least some, possibly
} >} all, pointers to freed space compare equal to NULL.
} >} However, there also exist platforms on which any reference to them, even
} >} a comparison to NULL, causes undefined behavior.
} >     How can this be so?  free() is a function, not an operator.
} >functions can not change the value of their paramters.
} Ahh, but:
} 1.  free() isn't a real function, but a Library Call.  They are somewhat
} magic.

     On every system with which I'm famailiar, library calls are
regular functions and have no magic.  Admittedly, my experience is
limited to general purpose computers for the most part.

} 2.  "The value of a pointer to free space is indeterminate."

     This seems like it could make life "interesting".

} For a practical answer to "how", imagine a segmented architecture, where
} pointers indicate to which segment they point, and where in the segment.
} On this architecture, there might be many possible "null pointer"
} representations, and the check for a null pointer might involve checking
} to see whether the segment was "valid".  If the only allocation in a segment
} were freed, any pointer with that segment could legitimately compare equal
} to NULL.


} On a broader variety of platforms, where the MMU is used to sanity-check
} things, loading an address into an address register checks it for validity,
} and a freed pointer may be invalid, so you get a trap right away on access
} to the value, without dereferencing it.

     Hmm...  This could definitely make life more "interesting".

} >     Assuming the above is true, then I would accept this; however, I
} >don't understand how the above can be true.  At least not without doing
} >weird things during function calls, which would break the symantecs of
} >the language.
} The semantics of the language are very careful to allow for Weird Stuff to
} happen when pointers are freed.

     That's nice of it.  It's amazing what comittees can do; although,
I'm sure there is a good reason for it.

}-- End of excerpt from Peter Seebach