Re: [TUHS] C++ / Kernel

[Bakul Shah <bakul@bitblocks.com>]

> On Aug 23, 2018, at 3:17 PM, <ron@ronnatalie.com> <ron@ronnatalie.com> wrote:
> 
> 
> 
>> 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.

Note that this is a legitimate use of union. That is,
unless I misunderstood what you meant by it, there is
no "conversion by union" as you call it or "cheating"
as I call it or type punning. There is no put one thing
in and take another thing out. Now it may be that
someone misused such a union. This is easy to do as,
unlike Pascal, C has no tagged variant record & it is
user's responsibility to use it right.

> 
> 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).

From what I recall {c,m,re}alloc() returned a char*, not a void *.
I don't have K&R1 handy at the moment so can't recall if void* was
mentioned in the book (if not, that could be one reason for a lack of
its use).