From: <ron@ronnatalie.com>
To: "'Bakul Shah'" <bakul@bitblocks.com>, "'TUHS'" <tuhs@minnie.tuhs.org>
Subject: Re: [TUHS] C++ / Kernel
Date: Thu, 23 Aug 2018 18:17:06 -0400 [thread overview]
Message-ID: <151301d43b2f$07881ed0$16985c70$@ronnatalie.com> (raw)
In-Reply-To: <ACE5F352-8FCF-4C3C-B60B-7CD1C6094939@bitblocks.com>
> I haven't done much BSD kernel programming in last 15 years but this is
not my recollection. BSD used caddr_t, typedefed to char*, sort of as void
*. IIRC void*
>came in common use after BSD unix first came about. Why use a union when a
cast will do? :-) The union trick is more likely to be used for esoteric
things
> (like getting at fl.pt. bytes) or for more complex types or probably by
people with lots of programming experience in pascal and not so much in C
(in Pascal
>you *had* to use untagged variant records if you wanted to cheat!). In C,
all that void* does is allow you to avoid casts in some cases.
Your recollections are certainly wrong. I spent a lot of time tracing
down why the Kernel crashed doing I/O and traced it to this and spent a
while undoing it as I stated.
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.
This isn't the only place it occurs.
Void* came out with the V7 compiler, if I recall properly. The BSD kernel
looks as if it requires such a later compiler (it uses bit fields which the
earlier compilers didn't support).
But it doesn't matter. You are right char* (or caddr_t) would work just
fine for this albeit with some explicit casting.
next prev parent reply other threads:[~2018-08-23 22:17 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-23 14:42 ron
2018-08-23 17:24 ` Clem Cole
2018-08-23 20:21 ` Bakul Shah
2018-08-23 22:17 ` ron [this message]
2018-08-23 22:28 ` Nevin Liber
2018-08-23 22:48 ` Clem Cole
2018-08-23 23:14 ` Steffen Nurpmeso
2018-08-24 14:13 ` Steffen Nurpmeso
2018-08-24 14:32 ` ron
2018-08-24 18:15 ` Steffen Nurpmeso
2018-08-26 16:34 ` Paul Winalski
2018-08-27 16:31 ` Steffen Nurpmeso
2018-08-24 1:41 ` Bakul Shah
2018-08-24 10:41 ` Pete Turnbull
2018-08-24 12:17 ` ron
2018-08-24 18:36 ` Bakul Shah
2018-08-24 18:38 ` ron
2018-08-24 1:58 ` Dan Cross
2018-08-24 3:04 ` Clem cole
2018-08-24 14:01 ` Dan Cross
2018-08-24 13:22 ` Derek Fawcus
2018-08-24 16:59 ` Steffen Nurpmeso
2018-08-23 23:29 Noel Chiappa
2018-08-23 23:42 ` ron
2018-08-24 0:30 ` Clem Cole
2018-08-24 2:05 ` Bakul Shah
2018-08-24 12:21 ` ron
2018-08-24 1:27 Noel Chiappa
2018-08-24 2:52 ` Clem cole
2018-08-24 7:30 Paul Ruizendaal
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