offsetOf

From	Jack Boot <jackboot@mailinator.com>
Newsgroups	comp.lang.c
Subject	offsetOf
Date	2011-07-23 20:54 +0000
Organization	Aioe.org NNTP Server
Message-ID	<pan.2011.07.23.20.54.51@mailinator.com> (permalink)

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Hello:

This came up in a code review recently, I've never heard of it before, 
opinions welcome.

There is a struct that looks a bit like this

struct foo {
  int a[10];
  long b;
  ...
};

Now I have a function that starts thusly:

void
bar ( struct foo * f )
{
  int * ip = (int *) foo;
  ...
}

The reviewer recommended, that for maximal portability I should replace 
this by:

int * ip = ( (int *) ( ( (unsigned char *) foo ) + offsetOf(struct foo, 
a) ) );

I've never seen this construction before, also I think type punning is 
inherently non-portable so I'm not sure how much this buys you.

Cheers,
JB

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offsetOf Jack Boot <jackboot@mailinator.com> - 2011-07-23 20:54 +0000
  Re: offsetOf Ian Collins <ian-news@hotmail.com> - 2011-07-24 09:05 +1200
  Re: offsetOf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-23 23:03 +0100
    Re: offsetOf Jack Boot <jackboot@mailinator.com> - 2011-07-24 10:30 +0000
      Re: offsetOf Ian Collins <ian-news@hotmail.com> - 2011-07-24 22:41 +1200
      Re: offsetOf James Kuyper <jameskuyper@verizon.net> - 2011-07-24 08:36 -0400
      Re: offsetOf jt@toerring.de (Jens Thoms Toerring) - 2011-07-24 14:50 +0000
      Re: offsetOf Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-24 10:28 -0600
      Re: offsetOf Nobody <nobody@nowhere.com> - 2011-07-24 17:30 +0100
      Re: offsetOf gordonb.gkpc8@burditt.org (Gordon Burditt) - 2011-07-27 08:43 -0500
    Re: offsetOf Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-07-24 14:18 +0300
      Re: offsetOf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-24 12:50 +0100
        Re: offsetOf Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-07-24 18:25 +0300
  Re: offsetOf Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-07-23 20:12 -0400
  Re: offsetOf "J. J. Farrell" <jjf@bcs.org.uk> - 2011-07-24 01:57 +0100

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