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From: schell@llandru.ucdavis.edu (Stephan Schell)
Newsgroups: comp.sys.hp
Subject: Array indexing problem (bug?)
Message-ID: <SCHELL.89Dec21182712@llandru.ucdavis.edu>
Date: 22 Dec 89 02:27:12 GMT
Sender: uucp@ucdavis.ucdavis.edu
Distribution: comp
Organization: University of California at Davis
Lines: 40


While attempting to port the dviselect program in the TeX distribution
over to a 9000/835 running hp-ux 3.1, I ran into a problem with array
indexing.  This problem is showcased in the code below:

#include <stdio.h>
char junk[11] = { 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i' , 'j'};
main()
{
	int	i;

	printf("Enter a number between 20 and 29, inclusive: ");
	scanf("%d", &i);
	printf("Character selected is '%c'\n", (junk-20)[i]);
	exit(0);
}

For example, typing "25" in response to the prompt yields a segmentation
fault, even though the final value of (junk-20+i) is (junk+5).
The same behavior was observed when "(junk-20)[i]" was replaced by
"(junk-20+i)".  The program operated properly if "junk[i-20]" was used,
or if a temporary pointer, "junk2 = junk-20+i" was computed and then used.
The common thing among the usages that failed was that they all used
LDBX where the base register contained a value outside the allowable range,
even though the effective (or final) address was OK.  Thus, it seems as if
the segmentation fault is based on the base register contents rather than
the effective address.

Does this behavior seem bogus to anyone except me?

Is there any way to fix this problem without modifying the source code
of each and every application that does this sort of thing?

Thanks.
--
-------------------------------------------------------------------------------
Stephan Schell                          schell@llandru.ucdavis.edu
Dept. of Electrical Engineering         {ucbvax,lll-crg}!ucdavis!llandru!schell
      &  Compter Science				
University of California, Davis         (916) 752-1326