From: Bakul Shah <bakul@bitblocks.com>
To: TUHS <tuhs@minnie.tuhs.org>
Subject: Re: [TUHS] C++ / Kernel
Date: Fri, 24 Aug 2018 11:36:34 -0700 [thread overview]
Message-ID: <98A5731F-11D3-4D55-B679-8DDF9E4EFFBC@bitblocks.com> (raw)
In-Reply-To: <1d5901d43ba4$7bc413b0$734c3b10$@ronnatalie.com>
On Aug 24, 2018, at 5:17 AM, ron@ronnatalie.com wrote:
>
> For instance:
> int x;
> daddr_t* y;
>
> u.b_un.b_words = &x;
> y = u.b_un.b_daddr;
One would typically *not* do this.
>
> This is syntactically correct, but the code will blow up bizarrely on
> machines where the int* and long* pointers are not in the same format.
> Now if the datatype was a void* and we did this:
>
> u.b_vp = & x;
> y = u.b_vp;
>
> Provided there weren't any alignment issues, this would work as the
> conversion to and from void* would result in the right pointer value.
For example, given
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;
You'd use it something like this:
struct b_un x, y, z, *w;
struct filesys fs;
daddr_t d;
...
x.b_words = &b;
y.b_filsys = &fs;
z.b_addr = &d;
w = &x;
These come about because in different contexts different ptrs
may be used but one would not do any type conversion. Said another
way, if you replaced the union with a struct, type punning code
would fall apart but legit code would continue working (except
in the case where the *size* of the object is significant).
next prev parent reply other threads:[~2018-08-24 18:36 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
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 [this message]
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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