incrementing after a cast

[Wayne Throop]

> kanner at apple.UUCP (Herbert Kanner)
> We would like to solicit some opinions on the legality of [...]
>         *((sometype *) chp)++;

Well, I think it's illegal, Kernighan thinks it's illegal, Ritchie
thinks it's illegal, Harbison thinks it's illegal, Steele thinks it's
illegal, and (I would venture to guess) all the members of the X3J11
ANSI C group think it is illegal.  Is that enough "opinions" for you?
Will you now agree that it is Not a Very Good Idea?  Go ahead!  Argue
the point!  Make my day!

More seriously, the *reason* it is illegal is that casts (at least
conceptually, and in many cases actually) *DO* *SOMETHING* to the value
they are applied to.  Would you expect ((-i)++) to work?  Would you
expect ((i+j)++) to work?  More relevant, would you expect
(((float)i)++) to work?  I would *NOT* expect them to work, and
similarly I do not expect (((some_type *)p)++) to work, either.

> Both the portable C compiler and lint happily accept this construct.

"This turns out not to be the case."

Given this input:
        1  char *p;
        2  void f(){
        3      ((int *)p)++;
        4  }
lint on our system says (among other things):
        (3)  illegal lhs of assignment operator

If your lint doesn't complain about it, it is buggy.

> The idea of the cast is to force chp to be incremented by the size of
> sometype.

Sure.  OK.  Fine.  But that isn't the way to accomplish this.  Try

        L = *(sometype *)chp;
        chp = (char *)(((sometype *)chp)+1)

It works just as well, and has the advantage of being legal C.  Still
legal but less portable is to declare chp to be a union of all the
relevant pointer types like so:

    union {sometype *stp; anothertype *atp; someothertype *sotp; } p;

and use something like

        somevalue = *p.stp++;

to do the save-value-and-increment-pointer operation.  Note that this
only works on machines where the various pointer formats are identical,
and where the compiler implements unions "the way you expect".

--
"Happily, I read English."
(Draws sword) "Then read it happily!"
                --- Exchange in the 1950s production of IVANHOE
-- 
Wayne Throop      <the-known-world>!mcnc!rti-sel!dg_rtp!throopw