[TUHS] C++ / Kernel

[Clem Cole]

On Thu, Aug 23, 2018 at 6:29 PM Nevin Liber <nevin at eviloverlord.com> wrote:

> It was still kinda frowned upon in K&R1:  "It is the responsibility of the
> programmer to keep track of what type is currently stored in a union; the
> results are machine dependent if something is stored as one type and
> extracted as another."
>

No.  It was not frowned upon, it was widely used.  The message was just
just Dennis and Brian giving you fair warning. Remember K&R1 came out
approx the same time as 7th edition [I saw the proofs for the book with
UNIX/TS which was a little earlier, but pretty much the same kernel].

>From V7th editions buf.h (which I show a date of May 5, 1979) and Ron is
correct void*, caddr_t, daddr_t, dev_t and the like; were all 7th
edition-isms that BSD picked up)
...
struct buf
{
        int     b_flags;                /* see defines below */
        struct  buf *b_forw;            /* headed by d_tab of conf.c */
        struct  buf *b_back;            /*  "  */
        struct  buf *av_forw;           /* position on free list, */
        struct  buf *av_back;           /*     if not BUSY*/
        dev_t   b_dev;                  /* major+minor device name */
        unsigned b_bcount;              /* transfer count */
        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 */ls -l buf.h
            daddr_t *b_daddr;           /* indirect block */
        } b_un;
        daddr_t b_blkno;                /* block # on device */
        char    b_xmem;                 /* high order core address */
        char    b_error;                /* returned after I/O */
        unsigned int b_resid;           /* words not transferred after
error */
};
...
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