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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: naked switches
Date: Sun, 11 Jul 2021 00:39:28 -0700
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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> David Brown <david.brown@hesbynett.no> writes:
>
>> Your extension [for extracting contiguous bits from a word] is not
>> commonly used, because it only exists in your tool.  People who need
>> to do a lot of bit slicing and don't like masking and shifting (or
>> struct bit-fields) should not have trouble defining:
>>
>> #define BIT(x, top, bottom) \
>> 	(((x) >> (bottom)) & ((1llu << ((top) - (bottom) + 1)) - 1))
>>
>> Yes, the parenthesis you need for macros are ugly, but you only need to
>> get it right once, and now you can read your bit-fields.
>
> That's undefined when all the bits are wanted.  (The left shift of 1llu
> could be equal to the width.)  Maybe you wrote it for a compiler that
> documents an extension.
>
> I usually right shift -1llu by the width minus the number of bits
> wanted, but you can also just do two left shifts.

Another way, assuming the mask width desired is greater
than zero, is this

    ((1LLU << width-1) -1) *2 +1

which works great if 'width' is a compile-time constant.