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NNTP-Posting-Date: Mon, 23 May 2011 22:27:42 -0500
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Date: Mon, 23 May 2011 23:27:38 -0400
From: pete <pfiland@mindspring.com>
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Subject: Re: strcmp implementation with unsigned conversion
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.c:4656

jianhua wrote:
> 
> 1. Why the standard* requires that values
> of the pair of characters be
>     "both interpreted as unsigned char"
> 
> 2. Can they be both interpreted as other larger types
>     eg. int, unsigned int, long, unsigned long
> 
> 3. Does "both interpreted as unsigned char" means
> 
>     this is wrong:
> 
> int strcmp(const char *cs, const char *ct)
> {
>         while (1) {
>                 if (*cs != *ct)
>                         return *cs < *ct ? -1 : 1;
>                 if (!*cs)
>                         break;
>                 cs++, ct++;
>         }
>         return 0;
> }
> 
>     but this is right:
> 
> /*
> Copyright (C) 1991, 1992 Linus Torvalds
> */
> int strcmp(const char *cs, const char *ct)
> {
>         unsigned char c1, c2;
> 
>         while (1) {
>                 c1 = *cs++; /*gcc warning: -Wconversion*/
>                 c2 = *ct++; /*gcc warning: -Wconversion*/

I agree with Eric Sosman that a conversion to unsigned char
is different from being interpreted as unsigned char.

It would be OK this way:


                 c1 = *(unsigned char *)cs++;
                 c2 = *(unsigned char *)ct++; 

>                 if (c1 != c2)
>                         return c1 < c2 ? -1 : 1;
>                 if (!c1)
>                         break;
>         }
>         return 0;
> }
> 
> [*] 7.23.4 Comparison functions
> 1 The sign of a nonzero value returned by the comparison functions
> memcmp, strcmp,
> and strncmp is determined by the sign of the difference between the
> values of the first
> pair of characters (both interpreted as unsigned char) that differ in
> the objects being
> compared.

-- 
pete