flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ?

From	"R.Wieser" <address@not.available>
Newsgroups	comp.lang.c++
Subject	flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ?
Date	2022-08-18 07:55 +0200
Organization	Aioe.org NNTP Server
Message-ID	<tdkk90$1bt7$1@gioia.aioe.org> (permalink)

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Hello all,

I just read a post on the "thedailywtf" website where someone said (and 
someone else confirmed) :

"flags & flagMask == flagMask can be true for flags != flagMask if flags is 
a superset of flagMask"

If I translate that to assembly language (what I normally work with) it 
doesn't seem to be true :

mov eax,flags
and eax,flagmask
cmp eax,flagmask

AFAIK the last eax *cannot* have any bits set which are not present in 
flagmask.  Therefore, can somebody explain that piece of (I take it) C+ for 
me (Whats the problem with it) ?

The post itself can be found here :

https://thedailywtf.com/articles/comments/flexing-bits

Regards,
Rudy Wieser

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flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? "R.Wieser" <address@not.available> - 2022-08-18 07:55 +0200
  Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Ike Naar <ike@sdf.org> - 2022-08-18 06:52 +0000
    Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? "R.Wieser" <address@not.available> - 2022-08-18 09:58 +0200
      Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Muttley@dastardlyhq.com - 2022-08-18 14:50 +0000
        Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? "R.Wieser" <address@not.available> - 2022-08-18 17:12 +0200
        Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? scott@slp53.sl.home (Scott Lurndal) - 2022-08-18 15:29 +0000
          Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-08-18 16:43 +0100
            Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? scott@slp53.sl.home (Scott Lurndal) - 2022-08-18 16:10 +0000
            Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? David Brown <david.brown@hesbynett.no> - 2022-08-18 21:19 +0200
              Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Manfred <noname@add.invalid> - 2022-08-19 02:57 +0200
                Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? David Brown <david.brown@hesbynett.no> - 2022-08-19 08:29 +0200
            Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-08-19 08:12 -0700
          Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2022-08-18 16:47 +0100
            Re: flags & flagMask == flagMask can be true for flags != flagMask if Muttley@dastardlyhq.com - 2022-08-18 15:50 +0000
            Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-08-18 10:46 -0700
              Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-08-18 11:30 -0700
          Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Muttley@dastardlyhq.com - 2022-08-18 15:47 +0000
        Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-08-19 08:53 -0700
          Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? David Brown <david.brown@hesbynett.no> - 2022-08-19 18:00 +0200
          Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Muttley@dastardlyhq.com - 2022-08-19 16:10 +0000
            Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-08-19 16:45 -0700
    Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Juha Nieminen <nospam@thanks.invalid> - 2022-08-19 06:01 +0000
      Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-08-19 16:07 +0100
      Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-08-19 16:08 +0100
  Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2022-08-18 16:14 +0100
    Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? "R.Wieser" <address@not.available> - 2022-08-18 18:10 +0200
  Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-08-18 11:56 -0700
    Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? "R.Wieser" <address@not.available> - 2022-08-18 21:31 +0200
      Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-08-18 21:24 -0700
        Re: flags & flagMask == flagMask can be true for flags != flagMask if flags is a superset of flagMask ? "R.Wieser" <address@not.available> - 2022-08-19 09:36 +0200

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