gcc does not warn about writes beyond the maximum array index
Posted by Anonymous on Fri 18th Oct 2019 09:59
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#include <stdio.h>
#include <stdlib.h>

#ifndef MY_S_IS_LOCAL
struct my_s
{
        int a[128];
        int b[128];
        int c[128];
        int d[128];
} my_s;
#endif /* MY_S_IS_LOCAL */

int main(int ac, char *av[])
{
#ifdef MY_S_IS_LOCAL
        struct my_s
        {
                int a[128];
                int b[128];
                int c[128];
                int d[128];
        } my_s;
#endif /* MY_S_IS_LOCAL */

        /* silence warnings about unused variables*/
        (void)ac;
        (void)av;
        (void)my_s;

        for(int i=0; i< 256; i++)
        {
                my_s.b[i]=0xdeadbeef;
        }

        /* prevent optimiser from eliminating |my_s| */
        (void)&my_s;

        return EXIT_SUCCESS;
}

##############################################

# warnings are generated for global variables...
$ g++ -g -gdwarf-2 -O2 -Wall -Wextra -Waggressive-loop-optimizations x.cpp
x.cpp: In function ‘int main(int, char**)’:
x.cpp:33:23: warning: iteration 128u invokes undefined behavior [-Waggressive-loop-optimizations]
   my_s.b[i]=0xdeadbeef;
                       ^
x.cpp:31:2: note: containing loop
  for(int i=0; i< 256; i++)
  ^

# ... but NO warnings are generated for local variables
$ g++ -g -gdwarf-2 -DMY_S_IS_LOCAL -O2 -Wall -Wextra -Waggressive-loop-optimizations x.cpp
$

# EOF.