Re: [TUHS] C++ / Kernel

[Steffen Nurpmeso <steffen@sdaoden.eu>]

Steffen Nurpmeso wrote in <20180823231413.QeahD%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.

I have found the link in the history of my browser, the story was

  http://pzemtsov.github.io/2016/11/06/bug-story-alignment-on-x86.html

--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)