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Date: Wed, 25 Jan 2012 11:05:58 -0500
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Subject: Re: A very simple parser with scanf & C
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.c:16200

Tim Rentsch wrote:
 
> My understanding is that a cast to (unsigned char) may not
> give exactly the right behavior.  In particular, if we have
> a variable 'char c;' then to call a function like 'isalpha()'
> we need to do
> 
>     isalpha( * (unsigned char*) &c )
> 
> rather than
> 
>     isalpha(   (unsigned char )  c )
> 
> to get exactly the same behavior as happens for getchar(), etc.
> 
> Of course, on most current machines these two methods give
> the same results, but as a matter of correctness the first
> form is the better one.  Right?


I don't think so.
If you were to output the value of (c) using fputc, 
then the return value of the function call would be
 ((int)(unsigned char)c).

Th value of (c) "converted to unsigned char" is ((unsigned char)c).

N869
4.9.7.3 The fputc function
Synopsis
         #include <stdio.h>
         int fputc(int c, FILE *stream);
Description
   The fputc function writes the character specified by c 
   (converted to an unsigned char ) ...


Also, isalpha also has to be able to work with arguments 
which may be constant expressions that don't have addresses.

Given

#define NEG_A   ('A' - 1 - (unsigned char)-1)

then isalpha ((unsigned char)NEG_A) should return nonzero

and putchar((unsigned char)NEG_A) should return ('A')

-- 
pete