[Bug c/17253] New: Inconsistency in reading array indexes

[vanveghel at home dot nl]

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