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Re: Pointers and Sequence Point

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On 05/03/2011 02:26 AM, Keith Thompson wrote:
> James Kuyper <jameskuyper@verizon.net> writes:
>> On 05/02/2011 12:36 PM, Chad wrote:
>>> On May 2, 6:58�am, Martin Ambuhl <mamb...@earthlink.net> wrote:
>>>> On 5/2/2011 7:38 AM, arnuld wrote:
>>>>
>>>>> I have this program which is causing confusion to me. I am told that last
>>>>> printf() of main() will always print "Second", not "First". I think line:
>>>>
>>>>> � x = myAlloc(x);
>>>>
>>>> While it is possible to write evil code in which myAlloc could modify x,
>>>> in C myAlloc receives a copy of the value of x and has no clue where x
>>>> is or how to modify it. �There are not two or more instances of
>>>> modifying x between sequence points.
>>>>
>>>
>>> What is the scope of 'x' in myAlloc(x)? Is confined to myAlloc()?
>>
>> The scope of 'x' starts at the declaration of 'x' on the first line of
>> main(), and is restricted to the body of main(). It goes out of scope
>> throughout the execution of myAlloc(). It comes back into scope after
>> that function returns. It's scope ends when main() returns.
> 
> Your wording  seems to imply that 'x' goes into and out of scope
> during execution.  In fact, an entity's scope is a region of
> program text.  C99 6.2.1p2.

As the program executes, it executes code corresponding to those
different regions of text, and that's what I was trying to talk about.
However, that correspondence gets complicated to talk about accurately,
especially after optimization. I should really have emphasized that
scope is a compile-time concept, not a run-time one.

The key point is that that all of the operations that might, in some
other context, be problematic, are performed on a variable named 'c',
whose scope is restricted to myAlloc(), that happens to be initialized
with the value of 'x' at the time myAlloc() is called. As a result,
they're not at all problematic.
-- 
James Kuyper

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Pointers and Sequence Point arnuld <sunrise@invalid.address> - 2011-05-02 11:38 +0000
  Re: Pointers and Sequence Point James Kuyper <jameskuyper@verizon.net> - 2011-05-02 08:23 -0400
  Re: Pointers and Sequence Point Martin Ambuhl <mambuhl@earthlink.net> - 2011-05-02 09:58 -0400
    Re: Pointers and Sequence Point Chad <cdalten@gmail.com> - 2011-05-02 09:36 -0700
      Re: Pointers and Sequence Point Angel <angel+news@spamcop.net> - 2011-05-02 17:21 +0000
      Re: Pointers and Sequence Point James Kuyper <jameskuyper@verizon.net> - 2011-05-02 23:20 -0400
        Re: Pointers and Sequence Point Keith Thompson <kst-u@mib.org> - 2011-05-02 23:26 -0700
          Re: Pointers and Sequence Point James Kuyper <jameskuyper@verizon.net> - 2011-05-03 05:33 -0400
    Re: Pointers and Sequence Point Kenneth Brody <kenbrody@spamcop.net> - 2011-05-02 13:35 -0400
      Re: Pointers and Sequence Point Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-05-03 17:45 +0300
  Re: Pointers and Sequence Point John Bode <jfbode1029@gmail.com> - 2011-05-02 10:17 -0700
  Re: Pointers and Sequence Point Tim Rentsch <txr@alumni.caltech.edu> - 2011-05-02 10:31 -0700
  Re: Pointers and Sequence Point "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-05-02 20:55 -0400
    Re: Pointers and Sequence Point David Resnick <lndresnick@gmail.com> - 2011-05-03 08:01 -0700

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