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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: does a double cast to unsigned makes sense in any circumstance?
Date: Sat, 15 Jan 2022 18:53:07 -0800
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Meredith Montgomery <mmontgomery@levado.to> writes:

> I've seen this somewhere or I wrote this some time in the past for some
> reason and I can't see how this make sense any longer.  In a procedure
> to read a numeric string and turn it into a number, we need to convert
> each char-digit to some kind of integer:
>
>   (uint64_t) (unsigned char) (s[pos] - '0');
>
> But the double cast here seems odd.  I could have written this by
> copying it from somewhere else.  Today, what I would write is just
>
>   (uint64_t) (s[pos] - '0');
>
> Am I making a mistake now?  I can't anything wrong with this.  Isn't a
> char just an int?  I'm turning an int into an unsigned integer (of a
> larger size).

The original context for the expression in question is this
function:

> [posted by Meredith Montgomery]
>
> #include <limits.h>
> #include <inttypes.h>
> 
> int scan_ulong(register char *s, register unsigned long *u)
> {
>   register unsigned int pos;
>   register unsigned long r;
>   register unsigned long c;
> 
>   pos = 0; r = 0;
> 
>   for ( ;; ) {
>     c = (unsigned long) (unsigned char) (s[pos] - '0');
>     if (c < 10) {
>       if( ((ULONG_MAX - c) / 10) >= r)
>         r = r * 10 + c;
>       else return -1; /* lack of space */
>       ++pos; continue;
>     }
>     break;
>   }
> 
>   *u = r;
>   return pos;
> }

In this context, the casts are superfluous.  Just write

    c = s[pos] - '0';

and the right thing will happen, because the assignment to 'c'
converts the value on the right hand side to 'unsigned long',
and so takes care of any negative values.