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Date: Sat, 10 Mar 2012 15:18:22 -0500
From: pete <pfiland@mindspring.com>
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Subject: Re: A very simple parser with scanf & C
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Xref: csiph.com comp.lang.c:18471

Tim Rentsch wrote:
> 
> pete <pfiland@mindspring.com> writes:
> 
> > Tim Rentsch wrote:
> >>
> >> pete <pfiland@mindspring.com> writes:
> >>
> >> > Tim Rentsch wrote:
> >> >
> >> >> [snip]
> >
> > If you're going to say for {char c;}
> >
> >>>     isalpha( * (unsigned char*) &c )
> >>>
> >>> rather than
> >>>
> >>>     isalpha(   (unsigned char )  c )
> >
> > then I think we should consider a case where
> >     ( * (unsigned char*) &c )
> > is different from
> >     (   (unsigned char )  c )
> > and the only case I can think of,
> > is where (c) is signed, negative and not twos complement.
> 
> Right (ignoring padding bits in char, which no sane
> implementation has), although actually there is one other
> case, namely a two's complement trap representation.  Not
> very likely perhaps but it is allowed, so for the sake of
> completeness I think it should be mentioned.  (And now it
> has been...)

Considering that file operations are at the root of this thread:

If char is signed, and char c is assigned a value of (-1), as in
    char c;
    c = -1;

and if (c) is written to a file using fputc,
then the value written into the file will be ((unsigned char)-1).

If that same byte is read back from the same file using fgetc,
then the value returned by fgetc will be
((int)(unsigned char)-1)
which is equal to ((unsigned char)c)
regardless of the representation of negative integers.

The return value of fgetc will be unequal to 
( * (unsigned char*) &c )
when negative integers are either sign and magnitude
or one's complement.

-- 
pete