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Re: List of undefined behaviour and other sneeky bugs

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John Reye <jononanon@googlemail.com> wrote:
> On May 3, 8:11 pm, (Jens Thoms Toerring) wrote:
> > > mystruct.c = 0x12;
> > > int tmp = mystruct.b;
> > > f(tmp);
> > > Do you really mean, that the code above does not guarantee that the
> > > lowest byte-address  of tmp is set to 0x12??
> >
> > That's exactly what it does - it sets the byte at the lowest
> > memory address.
> OK, so then there is no problem with reading b, right.
> I always did say: lowest byte-address.


> What you write there is a different issue:
> >The problem is only: what do you get when you
> > now read the whole int? On a little-endian machine this will
> > have modified the least significant byte while on a big-endian
> > machine the most significant byte. Thus the standard isn't able
> > to define  what value you will get when you read 'b' after you
> > have written something to 'c'.
> Well yes, because there are machines with different endianness.

> But I can get around that quite easy like this:

> #include <stdio.h>

> #define LITTLE_ENDIAN /* undef it, if big endian!! */

> #ifdef LITTLE_ENDIAN

> struct byte4 {
>   unsigned char byte0;
>   unsigned char byte1;
>   unsigned char byte2;
>   unsigned char byte3;
> };
> #else

> struct byte4 {
>   unsigned char byte3;
>   unsigned char byte2;
>   unsigned char byte1;
>   unsigned char byte0;
> };

> #endif

> int main(void)
> {
>    struct
>    {
>      char a;
>      union {
>        int  b;
>        struct byte4 b4;
>      };
>    } mystruct;
>    mystruct.b4.byte0 = 0x12; // this sets least signif. byte!
>    printf("%x\n", mystruct.b);
> }

Yes, if a) you can determine safely if your program is compiled
on a little- or big-endian machine (and there could, at least in
principle, exist machines with more weird bit-representations)
and b) you restrict yourself to machines were sizeof(int) is 4
- but in principle sizeof(int) can be any number not less than 1.

The question already asked by someone else is what you
achieve with this? There is a much simpler way to modify
the the least significant byte of an int (well, assuming
tha a byte consists of 8 bits only, which also isn't a
given, otherwise you'd have to use CHAR_BIT in some clever
way):

     b = ( b & ~ 0xFFU ) | 0x12;

This will work independent on endianess and sizeof(int)
since it doesn't make assumptions about the way the
bits and bytes are ordered - the compiler has to do all
that boring work.
                            Regards, Jens
-- 
  \   Jens Thoms Toerring  ___      jt@toerring.de
   \__________________________      http://toerring.de

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List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 10:04 -0700
  Re: List of undefined behaviour and other sneeky bugs jt@toerring.de (Jens Thoms Toerring) - 2012-05-03 17:38 +0000
    Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 10:50 -0700
      Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-03 11:09 -0700
        Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 11:18 -0700
          Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-03 11:24 -0700
            Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 11:35 -0700
              Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 11:56 -0700
              Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-03 12:16 -0700
                Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 12:20 -0700
                Re: List of undefined behaviour and other sneeky bugs Keith Thompson <kst-u@mib.org> - 2012-05-03 12:39 -0700
          Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 17:51 -0700
            Re: List of undefined behaviour and other sneeky bugs "Joachim Schmitz" <nospam.jojo@schmitz-digital.de> - 2012-05-08 09:10 +0200
              Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-08 05:31 -0700
      Re: List of undefined behaviour and other sneeky bugs jt@toerring.de (Jens Thoms Toerring) - 2012-05-03 18:11 +0000
        Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 11:29 -0700
          Re: List of undefined behaviour and other sneeky bugs jt@toerring.de (Jens Thoms Toerring) - 2012-05-03 19:19 +0000
            Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 13:31 -0700
              Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 14:18 -0700
                Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 15:20 -0700
            Re: List of undefined behaviour and other sneeky bugs Nobody <nobody@nowhere.com> - 2012-05-04 01:47 +0100
      Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 17:47 -0700
    Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 17:45 -0700
      Re: List of undefined behaviour and other sneeky bugs nick_keighley_nospam@hotmail.com - 2012-05-08 00:43 -0700
        Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-08 06:17 -0700
  Re: List of undefined behaviour and other sneeky bugs Keith Thompson <kst-u@mib.org> - 2012-05-03 12:46 -0700
    Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 13:35 -0700
      Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-03 14:05 -0700
        Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 14:11 -0700
          Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-03 15:02 -0700
            Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:10 -0700
              Re: List of undefined behaviour and other sneeky bugs nick_keighley_nospam@hotmail.com - 2012-05-08 00:49 -0700
                Re: List of undefined behaviour and other sneeky bugs James Kuyper <jameskuyper@verizon.net> - 2012-05-08 06:37 -0400
                Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-08 07:49 -0700
                Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-08 07:25 -0700
              Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-08 09:46 -0700
          Re: List of undefined behaviour and other sneeky bugs James Kuyper <jameskuyper@verizon.net> - 2012-05-03 18:24 -0400
            Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:24 -0700
      Re: List of undefined behaviour and other sneeky bugs jt@toerring.de (Jens Thoms Toerring) - 2012-05-03 21:17 +0000
        Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:28 -0700
      Re: List of undefined behaviour and other sneeky bugs Keith Thompson <kst-u@mib.org> - 2012-05-03 15:54 -0700
      Re: List of undefined behaviour and other sneeky bugs Old Wolf <oldwolf@inspire.net.nz> - 2012-05-06 22:32 -0700
        Re: List of undefined behaviour and other sneeky bugs Jens Gustedt <jens.gustedt@loria.fr> - 2012-05-07 08:26 +0200
        Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:31 -0700
    Re: List of undefined behaviour and other sneeky bugs Jens Gustedt <jens.gustedt@loria.fr> - 2012-05-03 23:18 +0200
      Re: List of undefined behaviour and other sneeky bugs Keith Thompson <kst-u@mib.org> - 2012-05-03 15:58 -0700
  Re: List of undefined behaviour and other sneeky bugs pete <pfiland@mindspring.com> - 2012-05-04 00:25 -0400
    Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 22:28 -0700
      Re: List of undefined behaviour and other sneeky bugs James Kuyper <jameskuyper@verizon.net> - 2012-05-04 07:33 -0400
        Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 19:20 -0700
      Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:50 -0700
    Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:42 -0700
      Re: List of undefined behaviour and other sneeky bugs pete <pfiland@mindspring.com> - 2012-05-07 23:45 -0400
        Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 23:01 -0700
          Re: List of undefined behaviour and other sneeky bugs pete <pfiland@mindspring.com> - 2012-05-08 06:28 -0400
            Re: List of undefined behaviour and other sneeky bugs pete <pfiland@mindspring.com> - 2012-05-08 06:37 -0400
              Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-08 06:19 -0700

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