Re: [TUHS] C++ / Kernel

[Steffen Nurpmeso <steffen@sdaoden.eu>]

ron@ronnatalie.com wrote in <151301d43b2f$07881ed0$16985c70$@ronnatalie.com>:
  ...
 |This union was right in the middle of the buf struct:
 |
 | union {
 |     caddr_t b_addr;          /* low order core address */
 |     int      *b_words;               /* words for clearing */
 |     struct filsys *b_filsys; /* superblocks */
 |     struct dinode *b_dino;   /* ilist */
 |     daddr_t *b_daddr;                /* indirect block */
 |} b_un;
 |There were a number of other places that did the same thing.   It's
 |OFFICIALLY now in undefined behavior by the standard (though of course that
 |didn't exist in the BSD days) ,
 |to store in one element of the union and retrieve it via another.   This is
 |one of the reasons why.

Luckily "type punning" is possible.  Ach, what do i know;
nonetheless, i think it is a terrible direction that C goes to,
now you often need memcpy() to local storage instead of a simple
assignment (with a cast) in order to avoid undefined behaviour
that allows compilers to do bad things.  On a NetBSD list there
was a link to such a story not too long ago, the resulting
C standard compliant code was ridiculous, the last string of fun
has been screwed out of C.  Unfortunately i have not saved the
message, but Christos Zoulas has been the author.

Regarding "type punning" the standard C 2011 says (6.5.2.3)

  If the member used to read the contents of a union object is not
  the same as the member last used to store a value in the object,
  the appropriate part of the object representation of the value
  is reinterpreted as an object representation in the new type as
  described in 6.2.6 (a process sometimes called ‘‘type
  punning’’). This might be a trap representation.

This has not changed in C 2018 (though i have only version 2017
ballot).  So as long as the type sizes are identical anything
should be fine.  Just as you say.

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)