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From: davidsen@steinmetz.UUCP (Davidsen)
Newsgroups: net.lang.c
Subject: Re: Casting a postdecrement operand
Message-ID: <786@steinmetz.UUCP>
Date: Thu, 12-Jun-86 12:52:03 EDT
Article-I.D.: steinmet.786
Posted: Thu Jun 12 12:52:03 1986
Date-Received: Sun, 15-Jun-86 02:22:13 EDT
References: <114@romp.UUCP> <2842@utcsri.UUCP> <1273@ulysses.UUCP>
Reply-To: davidsen@kbsvax.UUCP (Davidsen)
Organization: GE CRD, Schenectady, NY
Lines: 42

In article <1273@ulysses.UUCP> jss@ulysses.UUCP writes:
>In article <114@romp.UUCP> lwh@romp.UUCP (lwh) writes:
>>My version of pcc on the IBM RT PC allows the following expression:
>>
>>		((struct abc *)cbap)++;
>>
>>
>>to increment cbap by 500. It appears that the ANSI standard doesn't say
>>anything about the legality of this syntax.  
>
>"++" requires an lvalue. A cast produces a value but not an lvalue,
>so this is not legal in ANSI (or in K&R ) C. 
>
>Jerry Schwarz
>Bell Labs, MH

The variable 'cbap' is a pointer. The cast '(struct abc *)' is a
pointer cast. I can use '((struct abc *)cbap)' with indirects and
subscripts and anywhere that I can use an Lvalue, so I really believe
that the expression is legal.

Now, as to being *portable*. There is some language in the X3J11
proposed standard about possible loss of information and
non-portability when pointers are cast. This is why the 'void *' type
was introduced (in part) to have a pointer which could hold any valid
data address. However, there is no guarantee that when the address of
one data type, however obtaining, is cast to a pointer to another data
type that the result will be valid.

If I recall correctly the term in use in X3J11 for stuff was "valid but
non-portable". Sorry I can't remember for sure who said it. I hope this
clarifies the question somewhat (it does in my mind, anyway).
-- 
	-bill davidsen

  ihnp4!seismo!rochester!steinmetz!--\
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"Stupidity, like virtue, is its own reward"