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Date: Tue, 31 Aug 2004 19:50:00 -0000
From: "vanveghel at home dot nl" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Message-ID: <20040831195044.17253.vanveghel@home.nl>
Reply-To: gcc-bugzilla@gcc.gnu.org
Subject: [Bug c/17253] New: Inconsistency in reading array indexes
X-Bugzilla-Reason: CC
X-SW-Source: 2004-08/txt/msg03303.txt.bz2
List-Id: <gcc-bugs.sourceware.org>

Consider the following two snippets:

-- snip snip --

#include <stdio.h>
#include <string.h>

int main (void) {
        char test[5][5];
        int j = 3;

        memset (&test, ' ', 5 * 5);

        test[j][j++] = '!';

        printf ("test[3][3] = %c\ntest[4][3] = %c\n",
                test[3][3], test[4][3]);

        return 0;
}

-- snip snip --

And:

-- snip snip --

#include <stdio.h>
#include <string.h>

int main (void) {
        char test[5][5];
        int j = 3;

        memset (&test, ' ', 5 * 5);

        test[(j == 4) ? 4 : 3][j++] = '!';

        printf ("test[3][3] = %c\ntest[4][3] = %c\n",
                test[3][3], test[4][3]);

        return 0;
}

-- snip snip --

Running both these snippets produces the following results with GCC 3.3.4
running on Debian GNU/Linux Unstable (x86):

$ ./test
test[3][3] =
test[4][3] = !
$ ./test2
test[3][3] = !
test[4][3] =
$

It seems that j++ is evaluated first only in the second case, making the
statement in the first bracket evaluate to 4.
This may be a result of 'undefined behavior' caused by sequence points, and
indeed, GCC does issue a warning ("warning: operation on `j' may be undefined"),
but it does seem like something that can at least be improved.

Kind Regards,

Jasper

-- 
           Summary: Inconsistency in reading array indexes
           Product: gcc
           Version: 3.3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vanveghel at home dot nl
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: 3.3.4
  GCC host triplet: 3.3.4
GCC target triplet: 3.3.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17253