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Groups > comp.lang.c > #158478

Re: Usage of union

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Andrey Tarasevich <andreytarasevich@hotmail.com> writes:

[context: does the following code have undefined behavior?

        union {
          uint16_t u16;
          uint8_t  u8[2];
          float    flt;
        } u;
        
        void t() {
          u.u16=0;
        
          assert(u.u8[0]==0);
          assert(u.u8[1]==0);
        
        };

(end context)]

> [In C] there's no immediate UB when reading `u8`.  In general
> case the potential for UB is still there [because of trap
> representations], but with uint8_t` everything should be fine
> since `uint8_t` is not supposed to have trap representations.
> (BTW, it seems that this is not explicitly spelled out in C
> standard.)

All of the exact-width integer types [u]intN_t, including uint8_t,
cannot have trap representations.  The C standard doesn't say this
directly, but it follows pretty immediately from the properties of
exact-width integer types and from the definitions of object
representation and trap representation.  (For signed exact-width
types intN_t, section 7.20.2.1 provides a crucial piece of that
deduction, which is easy to miss if one doesn't know to look for
it.)

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Usage of union wij <wyniijj@gmail.com> - 2021-01-17 06:09 -0800
  Re: Usage of union wij <wyniijj@gmail.com> - 2021-01-17 06:18 -0800
    Re: Usage of union James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-01-17 10:39 -0500
      Re: Usage of union wij <wyniijj@gmail.com> - 2021-01-17 08:41 -0800
        Re: Usage of union Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2021-01-17 17:07 +0000
          Re: Usage of union James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-01-17 12:29 -0500
            Re: Usage of union Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2021-01-17 17:47 +0000
            Re: Usage of union James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-01-17 14:22 -0500
          Re: Usage of union Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-01-20 07:03 -0800
            Re: Usage of union wij <wyniijj@gmail.com> - 2021-01-20 09:33 -0800
              Re: Usage of union Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-01-23 08:01 -0800
              Re: Usage of union Jorgen Grahn <grahn+nntp@snipabacken.se> - 2021-01-26 20:28 +0000
                Re: Usage of union wij <wyniijj@gmail.com> - 2021-01-27 03:15 -0800
        Re: Usage of union James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-01-17 12:19 -0500
      Re: Usage of union Bonita Montero <Bonita.Montero@gmail.com> - 2021-01-20 19:24 +0100
        Re: Usage of union Christian Hanné <the.hanne@gmail.com> - 2021-01-20 19:31 +0100
  Re: Usage of union Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-01-17 07:54 -0800
    Re: Usage of union Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-01-17 08:17 -0800
      Re: Usage of union wij <wyniijj@gmail.com> - 2021-01-17 09:07 -0800
      Re: Usage of union James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-01-17 12:32 -0500
      Re: Usage of union Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-01-20 05:56 -0800
        Re: Usage of union Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-01-20 09:52 -0800
    Re: Usage of union Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-01-20 06:09 -0800
      Re: Usage of union Bonita Montero <Bonita.Montero@gmail.com> - 2021-01-20 19:41 +0100
        Re: Usage of union scott@slp53.sl.home (Scott Lurndal) - 2021-01-20 18:49 +0000
          Re: Usage of union Bonita Montero <Bonita.Montero@gmail.com> - 2021-01-20 20:09 +0100
            Re: Usage of union scott@slp53.sl.home (Scott Lurndal) - 2021-01-20 19:25 +0000
            Re: Usage of union David Brown <david.brown@hesbynett.no> - 2021-01-20 20:29 +0100
        Re: Usage of union Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-01-20 11:36 -0800
  Re: Usage of union James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-01-17 12:14 -0500
  Re: Usage of union Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-01-17 17:17 +0000

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