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Re: array-size/malloc limit and strlen() failure

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On 04/02/2014 03:03 PM, glen herrmannsfeldt wrote:
> Keith Thompson <kst-u@mib.org> wrote:
> 
> (snip, someone wrote)
>>> However, you can write your own strlen() function which takes some of that 
>>> into account, if you're that paranoid about it (although beware that if the 
>>> address space is 64-bits, wrap-around might be impossible).
>  
>> You can't *portably* write such a strlen() function.  There is no
>> portable way to tell whether a given non-null address is valid or not.
> 
> Hmm. You can check to see if p++ is less than p. ...

That depends upon whether you evaluate 'p' before or after evaluating
p++, and the most obvious way of checking that condition leaves that
order unspecified (it also has undefined behavior, for a different but
closely related reason). You can avoid both problems with a temporary:

	char *q = p;
        if( p++ < q) ...

but in that case the normal result is p==q. How about:

        char *q = p++;
        if(q < p) ...

but in that case, q == p - 1 is the normal result. I think you didn't
express the condition properly; or perhaps you were thinking of ++p
rather than p++?

> ... If that happens before
> you find the '\0' in the string, you should probably stop looking.

That test won't help you if p wanders into a protected block of memory.
Also, when handled properly, that test will fail only after the behavior
of p++ is already undefined, and in that case it's already too late. A
test that could be performed while the code implementing the test still
has defined behavior would be better.

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array-size/malloc limit and strlen() failure jay <arnuld.mizong@gmail.com> - 2014-04-01 23:31 -0700
  Re: array-size/malloc limit and strlen() failure Xavier Roche <xroche@free.fr.NOSPAM.invalid> - 2014-04-02 08:47 +0200
  Re: array-size/malloc limit and strlen() failure Kaz Kylheku <kaz@kylheku.com> - 2014-04-02 07:36 +0000
  Re: array-size/malloc limit and strlen() failure "BartC" <bc@freeuk.com> - 2014-04-02 10:10 +0100
    Re: array-size/malloc limit and strlen() failure Keith Thompson <kst-u@mib.org> - 2014-04-02 08:11 -0700
      Re: array-size/malloc limit and strlen() failure glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-02 19:03 +0000
        Re: array-size/malloc limit and strlen() failure James Kuyper <jameskuyper@verizon.net> - 2014-04-02 15:48 -0400
          Re: array-size/malloc limit and strlen() failure Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-02 14:42 -0700
            Re: array-size/malloc limit and strlen() failure Keith Thompson <kst-u@mib.org> - 2014-04-02 15:14 -0700
        Re: array-size/malloc limit and strlen() failure Keith Thompson <kst-u@mib.org> - 2014-04-02 14:55 -0700
          Re: array-size/malloc limit and strlen() failure glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-02 23:42 +0000
  Re: array-size/malloc limit and strlen() failure Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-02 02:34 -0700
  Re: array-size/malloc limit and strlen() failure James Kuyper <jameskuyper@verizon.net> - 2014-04-02 07:40 -0400
    Re: array-size/malloc limit and strlen() failure Thomas Jahns <jahns@idontlikespam.dkrz.de> - 2014-04-02 13:52 +0200
    Re: array-size/malloc limit and strlen() failure Keith Thompson <kst-u@mib.org> - 2014-04-02 08:21 -0700
      Re: array-size/malloc limit and strlen() failure James Kuyper <jameskuyper@verizon.net> - 2014-04-02 11:30 -0400
      Re: array-size/malloc limit and strlen() failure glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-02 19:26 +0000
        Re: array-size/malloc limit and strlen() failure "BartC" <bc@freeuk.com> - 2014-04-02 20:39 +0100
        Re: array-size/malloc limit and strlen() failure Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-04-02 14:39 -0600
        Re: array-size/malloc limit and strlen() failure Keith Thompson <kst-u@mib.org> - 2014-04-02 15:03 -0700
          Re: array-size/malloc limit and strlen() failure Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-03 01:11 -0700
            Re: array-size/malloc limit and strlen() failure Ike Naar <ike@iceland.freeshell.org> - 2014-04-03 14:27 +0000
              Re: array-size/malloc limit and strlen() failure James Kuyper <jameskuyper@verizon.net> - 2014-04-03 17:56 -0400
            Re: array-size/malloc limit and strlen() failure Keith Thompson <kst-u@mib.org> - 2014-04-03 08:18 -0700
              Re: array-size/malloc limit and strlen() failure gazelle@shell.xmission.com (Kenny McCormack) - 2014-04-03 15:30 +0000
      Re: array-size/malloc limit and strlen() failure Stephen Sprunk <stephen@sprunk.org> - 2014-04-03 00:33 -0500

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