Re: [Tinycc-devel] bug - pointer cast

[Dave Dodge]

On Sat, Jul 23, 2005 at 09:30:31AM +0300, Oded Shimon wrote:
> 09:29) address@hidden:~ $ cat -n a.c
>      1  #include <inttypes.h>
>      2  int main(void) {
>      3          uint8_t * a, b[] = { 1, 1, 0, 0 };
>      4          a = b;
>      5          printf("%u\n", *((uint32_t *)&a[0]));
>      6          printf("%u\n", *((uint32_t *)a));
>      7          return 0;
>      8  }

Note that your example code is invoking undefined behavior (besides
the failure to include <stdio.h>).  C provides no guarantee that b is
suitably aligned for use as a uint32_t.  For example on Sparc/Solaris,
which is much stricter about alignment, simply adding a few more
variables to change the storage layout causes the program to fail:

  - uint8_t * a, b[] = { 1, 1, 0, 0 };
  + uint8_t * a, c, d, b[] = { 1, 1, 0, 0 };

  $ gcc foo.c && ./a.out
  Bus Error

That said, it might still sort of be considered a bug in tcc since
i386 doesn't really require natural alignment of types; you'd
therefore expect certain results even if C won't guarantee them.

> Took me FOREVER to find that this was why my tcc compilation of
> MPlayer was misbehaving...

If MPlayer is doing this sort of cast without being sure that the
cast is a defined operation, then that's a bug in MPlayer.

                                                  -Dave Dodge