Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ukma!rutgers!att!cbnewsh!rkl
From: rkl@cbnewsh.ATT.COM (kevin.laux)
Newsgroups: comp.lang.c
Subject: Re: When is a cast not a cast?
Message-ID: <364@cbnewsh.ATT.COM>
Date: 2 May 89 16:10:38 GMT
References: <2747@buengc.BU.EDU>
Organization: AT&T Bell Laboratories
Lines: 53

In article <2747@buengc.BU.EDU>, bph@buengc.BU.EDU (Blair P. Houghton) writes:
| Here's one that popped up last night/this morning (keep that Sanka
| away from me! %-S )
| 
| I wanted to be a 'good little programmer' and make all the types
| match up, so I was using casts wherever a promotion might be
| non-obvious.  In particular, I fell for:  (the line numbers are shown
| only for reference)
| 
| 1   main()
| 2   {
| 3	char *c;
| 4	char *p;
| 5	int i;
| 6	...
| 7	c = "somestring";  /* Nothing fancy, null-terminated. */
| 8	i = 4;  /* For example. */
| 9	...
| 10 	p = (c + (char *) i);  /* More trouble than it's worth... */
| 11	...
| 12  }
| 
| wherupon both the lint(1) and cc(1) in my Ultrix 2.2 piped-up with
| warnings that the 'operands of + have incompatible types' on line 10...
| 
| Now, who is having the more serious problem with (reduntantly?) casting
| i to be a char * before this addition: me, or the programmming tools
| under Ultrix version 2.2?

	[ stuff deleted ]

	But you want p to point into the string by an offset i.

	Line 10 should simply be

		p	= c + i;

	so that the value of pointer c is incremented by the value of integer i
times the sizeof a char.

	You don't want to add two *pointers* together and place the result in
p.  Think about the result from subtacting two pointers:

	char	c [8];
	char	*p;
	int	i;

	p	= &c [7];
	i	= p - c;

	The result of p - c is 8, an integer.

--rkl