Re: Compiled code crashes storing to read-only location

[Geoff Keating <geoffk at geoffk dot org>]

Waldek Hebisch <hebisch@math.uni.wroc.pl> writes:

> The following program:
> 
> typedef struct {int c, l; char ch[3];} pstr;
> const pstr ao = {2, 2, "OK"};
> const pstr * const a = &ao;
> 
> int main(void)
> {
>         if (a->ch[a->l]) {
>                 a->ch[a->l] = 0;
>         }
>         return 0;
> }
> 
> segfaults when compiled using `-O' (or better optimisation). Without
> optimisation the store is not executed and the program runs fine. 
> 
> I tried CVS gcc (updated DEC 14, 04:28 CET 2003) on Athlon-XP running
> Debian Woody. The problem does not appear in 20031029 snapshot, 
> but is present in 20031105 snapshot.
> 
> By the way, I have some doubts about corretness of the program, however
> my original problem is in GNU Pascal. Constant Pascal strings are stored
> in .rodata section and compiler makes sure that the string is null
> terminated. On the other hand variable strings need not be null terminated.
> When convering Pascal strings to C strings GPC generates code to null
> terminate the strings which are not already null terminated. The C
> program above tries to reproduce the code GPC produces.

This program isn't valid C, it should be 

      ((char *)a->ch)[a->l] = 0;

but with that change, it is perfectly valid.  Could you file a bug
report in bugzilla?

FYI, this doesn't happen to me on powerpc-darwin; but making the
correction *does* change the generated code (even though it should
make no difference).  Both versions still correctly execute the store
only when it's safe.

-- 
- Geoffrey Keating <geoffk@geoffk.org>