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Re: Linked-list problem - compiles but segfaults

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"christian.bau" <christian.bau@cbau.wanadoo.co.uk> writes:
> On Jul 5, 1:53 am, Andy Elvey <neptune.poseidon...@gmail.com> wrote:
>> int main()
>> {
>> /* Declare a list */
>> List *mylist = NULL;
>> list_append(mylist, "foo");
>> puts(mylist->data);
>
> Look at the documentation of your compiler and how to turn warnings
> on. You should get a compiler warning on the last line and your
> program will crash  there; no need even to read the code for
> list_append.

You'd get a warning on the puts() call only if the compiler performs
enough dataflow analysis to infer that mylist->data will be a
null pointer at that point.  Typically that's done only when you
enable optimization (because the same analysis that allows certain
optimizations to be performed safely can also reveal such errors).

And there's no guarantee that it would crash.  There have been plenty
of systems on which dereferencing a null pointer doesn't cause any
obvious problems; for example, it might simply read whatever is at
address 0, which might be a zero byte.  (Of course on many systems
it will crash.)

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Nokia
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Linked-list problem - compiles but segfaults Andy Elvey <neptune.poseidon789@gmail.com> - 2011-07-04 17:53 -0700
  Re: Linked-list problem - compiles but segfaults "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-07-04 22:18 -0400
    Re: Linked-list problem - compiles but segfaults Andy Elvey <neptune.poseidon789@gmail.com> - 2011-07-04 19:31 -0700
  Re: Linked-list problem - compiles but segfaults gordonb.bxelr@burditt.org (Gordon Burditt) - 2011-07-04 21:39 -0500
    Re: Linked-list problem - compiles but segfaults Andy Elvey <neptune.poseidon789@gmail.com> - 2011-07-04 20:00 -0700
      Re: Linked-list problem - compiles but segfaults gordonb.tu82f@burditt.org (Gordon Burditt) - 2011-07-05 00:11 -0500
  Re: Linked-list problem - compiles but segfaults pete <pfiland@mindspring.com> - 2011-07-05 00:13 -0400
    Re: Linked-list problem - compiles but segfaults Andy Elvey <neptune.poseidon789@gmail.com> - 2011-07-04 21:20 -0700
      Re: Linked-list problem - compiles but segfaults pete <pfiland@mindspring.com> - 2011-07-05 01:04 -0400
  Re: Linked-list problem - compiles but segfaults "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-07-05 08:42 -0700
    Re: Linked-list problem - compiles but segfaults Keith Thompson <kst-u@mib.org> - 2011-07-05 09:04 -0700
  Re: Linked-list problem - compiles but segfaults Paul N <gw7rib@aol.com> - 2011-07-05 14:51 -0700
    Re: Linked-list problem - compiles but segfaults Todd Carnes <toddcarnes@gmail.com> - 2011-07-06 03:20 +0000
      Re: Linked-list problem - compiles but segfaults Shao Miller <sha0.miller@gmail.com> - 2011-07-05 23:37 -0400
        Re: Linked-list problem - compiles but segfaults Shao Miller <sha0.miller@gmail.com> - 2011-07-05 23:41 -0400
          Re: Linked-list problem - compiles but segfaults Shao Miller <sha0.miller@gmail.com> - 2011-07-05 23:44 -0400
          Re: Linked-list problem - compiles but segfaults Shao Miller <sha0.miller@gmail.com> - 2011-07-05 23:54 -0400
        Re: Linked-list problem - compiles but segfaults "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-07-06 07:51 -0400
          Re: Linked-list problem - compiles but segfaults Shao Miller <sha0.miller@gmail.com> - 2011-07-06 09:50 -0400

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