Weird strict-aliasing break

[Thibault Castel <thibault dot castel at gmail dot com>]

Hi,

I have a weird warning when compiling my program with g++:
   "dereferencing pointer âNAMEâ does break strict-aliasing rules"

So, I tried to investigate a little bit and find some clues : I do a
memory mapping from a char buffer to a C struct to have access to
different datas into the buffer. Pretty casual. But, when I try to
access one of my inner field, the warning triggers... Huh...

To understand a bit more, I wrote a litte test program outside of my
project (really huge). See it in attachment. And I don't really
understand, because the warning is triggered only line 77, not on line
55. Only when I try to access the inner fields of a mapped structured
included into another structure.

I successfully avoid the warning with "__attribute__((__may_alias__))"
but this "cheat" bother me because :
 - I don't want to fake the compiler
 - I need to compile my huge project both under Linux (gcc/g++) and
Windows (Visual IDE)
 - This warning is triggered a hundred times in my huge project (old C
project slowly turning into C++ :/)

I already saw some messages on this list about this warning. But the
difference here is my program IS working. pFoo->a and pFoo->b have
correct values.

I tried different scenarios
 - grow or reduce the raw array
 - change the definition order into TData

My gcc specs :
=======================
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian
4.4.5-8' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.4 --enable-shared --enable-multiarch
--enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib
--enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-objc-gc --enable-targets=all --with-arch-32=i586
--with-tune=generic --enable-checking=release --build=i486-linux-gnu
--host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.4.5 (Debian 4.4.5-8)
=========================

Thanks a lot for any clue or explanation

PS: I'm french, and don't really speak english. I already apologize for that :/

Thibault

#include <stdlib.h>
#include <stdio.h>

typedef struct 
{
  unsigned char a;
  unsigned char b;
  unsigned char c;
} TDummy;
// } __attribute__((__may_alias__)) TDummy;

typedef struct
{
  unsigned char foo;
  unsigned char bar;
  char raw[10];
  unsigned char tmp;
  unsigned char pop;
} TData;

int main (int ac, char** av)
{
  TData data;
  char raw[10];
  TDummy* pFoo = NULL;
  char* pChar = NULL;

  // Init
  for (unsigned char i = 0; i < 10; ++i)
  {
    data.raw[i] = i;
    raw[i] = i;
  }

  // RAW ACCESS =================================================
  printf ("Raw   => ");
  for (unsigned char i = 0; i < 10; ++i)
  {
    printf ("%d", raw[i]);
    if (i != 9)
      printf (",");
  }
  printf ("\n");
      
  // No warning
  pChar = raw;
  printf ("Void  => ");
  printf ("%d,", *(pChar));
  pChar++;
  printf ("%d\n", *(pChar));
  pChar++;

  // No warning - Why ???
  pFoo = (TDummy*) (raw);
  printf ("Dummy => %d,%d\n", pFoo->a, pFoo->b);

  // DATA ACCESS =================================================
  printf ("Data  => ");
  for (unsigned char i = 0; i < 10; ++i)
  {
    printf ("%d", data.raw[i]);
    if (i != 9)
      printf (",");
  }
  printf ("\n");

  // No warning
  pChar = data.raw;
  printf ("Void  => ");
  printf ("%d,", *(pChar));
  pChar++;
  printf ("%d\n", *(pChar));
  pChar++;

  // Strict aliasing warning - Why ???
  pFoo = (TDummy*) (data.raw);
  printf ("Dummy => %d,%d\n", pFoo->a, pFoo->b);
}