Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!mailrus!ames!ncar!umigw!steve
From: steve@umigw.MIAMI.EDU (steve emmerson)
Newsgroups: comp.lang.c
Subject: Re: Question about VMS C
Summary: apparently arr[i][j] may break
Message-ID: <157@umigw.MIAMI.EDU>
Date: 30 Aug 88 13:30:57 GMT
References: <32725@cca.CCA.COM>
Reply-To: steve@umigw.miami.edu (steve emmerson)
Followup-To: comp.lang.c
Distribution: na
Organization: University of Miami
Lines: 51

In article <32725@cca.CCA.COM> g-rh@CCA.CCA.COM (Richard Harter) writes:
>
>char p[256];
>foo () {
>  int i,j, *s[128];
>  for (i=0;i<128;i++) s[i] = (int *)malloc(128*sizeof(int));
>  for (i=0;i<128;i++) {
>    p[i] = ' ';
>    p[i+128] = ' ';
>    for (j=0;j<128;j++) s[i][j] = 0;
>    }
>  }
>
>When this code was executed sundry things were zeroed outside of s and
>the vectors pointed to by the elements of s.  When the double subscripting
>was changed to *(s[i]+j) = 0 the code executed correctly.  The double
>subscripting executes correctly on a host of UNIX machines and on PRIMOS.

I ran accross the same problem apparently.  Here's the relevant fragment:

	/*	The following two print statements should be identical. 
		They aren't!. */

	int			i, j;
	MESH_PT		**mp = transform->mesh;		/* vector of pointers */

	for (j = -1; j <= transform->nj; ++j) {
		for (i = -1; i <= transform->ni; ++i) {
			printf("&mp[%d][%d] = %ld", j, i, (long)&mp[j][i]);
			printf("(mp[%d]+%d) = %ld", j, i, (long)(mp[j]+i));
		}
	}

(I appologize for the weird "-1" indexing -- it's for pointing into a 2D
array that has 1-point "margins" where mp[0][0] is the first, "true" data 
point.)

The results of the print statements were different (i.e. incorrect) under 
the VMS C compiler.  They were identical (i.e. correct) however, under 
both the GNU C and EUNICE C compilers.

The apparently incorrect behavior, however, was context sensitive.  When I
extracted the above fragment into its own program, I started getting
correct results.

Anyone else seen this?
-- 
Steve Emmerson                     Inet: steve@umigw.miami.edu [128.116.10.1]
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"Computers are like God in the Old Testament: lots of rules and no mercy"