lvalues and ++

[der Mouse]

In article <31da677c.809c at apollo.uucp>, mishkin at apollo.uucp (Nathaniel Mishkin) writes:
> I am having problems with the construct:
>         *((long *)p)++;
> Some C compilers (e.g. the one sent with 4.3bsd) complain with the error:
>     illegal lhs of assignment operator
> Other compilers seem to handle this correctly (incrementing by 4, by
> the way).
> Is this invalid C or is the 4.3bsd compiler broken?

It is invalid C.  A cast does not produce an lvalue.  If you want this
effect then try

p = (whatever *) (1 + (long *)p)

(if you want to use the value as well, try
	((long *)(p=(whatever *)(1+(long *)p)))[-1]
-- yech.)

As someone on comp.lang.c pointed out when this question came up a
while ago (it is one of the periodic questions), any compiler that
"handle[s] this correctly" is broken twice - once for accepting it and
again for modifying p (since "(long *)p" is a temporary, so the ++
should change the temporary, which will then get thrown away).

					der Mouse

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