Message-ID: <3BB5D732.AD6A64DA@yahoo.com>
From: CBFalconer <cbfalconer AT yahoo DOT com>
Organization: Ched Research
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Newsgroups: comp.os.msdos.djgpp
Subject: Re: Please add strrev proposal
References: <lcq8rtk1nqua2hc6rqfhmqisbd587n8t2n AT 4ax DOT com> <Xns912A8823C1399ASINANUNUR AT 132 DOT 236 DOT 56 DOT 8> <3BB50884 DOT 347A4384 AT yahoo DOT com> <hmjart4m6h7tt0vddvvg47438dijr428ob AT 4ax DOT com>
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Date: Sat, 29 Sep 2001 15:49:58 GMT
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Radical, NetSurfer wrote:
> 
> On Sat, 29 Sep 2001 00:28:28 GMT, CBFalconer <cbfalconer AT yahoo DOT com>
> wrote:
> 
> >"A. Sinan Unur" wrote:
> >>
> >> Radical NetSurfer wrote in
> >> news:lcq8rtk1nqua2hc6rqfhmqisbd587n8t2n AT 4ax DOT com:
> >>
> >> > I would like to encourage everyone who has a need for
> >> > strrev to come forward and encourage the maintainers of
> >> > LIBC used with GCC to kindly add   strrev.
> >>
> >> Count me against this if for no other reason that the fact that I do not
> >> like extra nonstandard function which solve tiny problems. If you need the
> >> functionality, you can write one for your own situation. If it is going to
> >> be added to a library, the solution needs to be useful to more than just
> >> one person in a particular situation.
> >>
> >> Anyway, the main point of my post, however, is to point out just one of
> >> the gotchas with these kinds of functions.
> >>
> >> You give the following usage example:
> >>
> >> > Example
> >> > printf("The reverse of %s is %s\n", str,  strrev(str) );
> >>
> >> Hmmmmm  ..... let us see using the code you suggested:
> >>
> >> /* +++Date last modified: 05-Jul-1997 */
> >> /*
> >> **  STRREV.C - reverse a string in place
> >> **
> >> **  public domain by Bob Stout
> >> */
> >>
> >> #include <string.h>
> >> #include <stdlib.h>
> >> #include <stdio.h>
> >>
> >> char *strrev(char *str) {
> >>         char *p1, *p2;
> >>
> >>         if (! str || ! *str) return str;
> >>         for (p1 = str, p2 = str + strlen(str) - 1; p2 > p1; ++p1, --p2)
> >>         {
> >>                 *p1 ^= *p2;
> >>                 *p2 ^= *p1;
> >>                 *p1 ^= *p2;
> >>         }
> >>         return str;
> >> }
> >>
> >> int main(void)
> >> {
> >>         char s[] = "This is a test.";
> >>         printf("Original String: %s\nReversed String:%s\n", s, strrev(s));
> >>         return 0;
> >> }
> >>
> >> C:\var>gcc djstrrev.c -o djstrrev.exe -O2 -Wall
> >>
> >> C:\var>djstrrev
> >> Original String: .tset a si sihT
> >> Reversed String: .tset a si sihT
> 
> If your compiler actually outputs both strings as reversed,
> THEN I am very scared your compiler is quite BROKEN.
> 
> printf("format specifier1, format_specififer2", var1, func1);
> 
> does:
> display __current__ value of   var1 FIRST using format_specifier1,
> __THEN__
> display __output__ of  funct1 using format_specifier2,
> 
> If DJGPP is not doing even this trivial behavior correctly,
> Borland programs would never port correctly,
> neither would anything else for that matter.
> 
> Left to right, in the order AND STATE ENCOUNTERED!
> 
> ....however... __AFTER__ the call to   printf(), __THEN__
> 'str' will contain the result of the function call;
> but __NEVER__ from  __WITHIN__ the printf   as
> you implied.  Your post is VERY MISLEADING.

You are wrong.  The order of evaluation of parameters is NOT
specified, and even if it were, the code gives the wrong answer.

The first %s loads a pointer to the string.  Not the string.  The
second function call returns a pointer to the string, and reverses
it.  Both pointers are the same, but since all the parameters have
now been evaluated, the string proper is reversed. We can now
actually call printf.  So printf prints both reversed.

The cause is the misleading return value from strrev.

And please DO NOT send me direct e-mail when the newsgroup can
handle it all.  Especially do not fail to mark any such "POSTED
AND MAILED", or the equivalent, so I can decide whether or not to
answer directly.

-- 
Chuck F (cbfalconer AT yahoo DOT com) (cbfalconer AT XXXXworldnet DOT att DOT net)
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