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Re: Bus error after casting (well defined) const unsigned char * to cons

From: Paul Pluzhnikov
Subject: Re: Bus error after casting (well defined) const unsigned char * to const double *
Date: Fri, 11 Aug 2006 15:14:47 -0700
User-agent: Gnus/5.1006 (Gnus v5.10.6) XEmacs/21.4 (Jumbo Shrimp, linux)

"Earl Purple" <> writes:

> So I receive the binary representation for a double (500000 in this
> particular case, which is 0x41 0x1e 0x84 0x80 0x00 0x00 0x00 0x00)
> If I reinterpret_cast the pointer to const double * then dereference it
> I get a Bus Error (core dumped).

The buffer into which you received the data must not have been
properly aligned.

> The workaround (that works) is to copy the buffer into a double, i.e.
> take a double, reinterpret_cast its address to unsigned char * and copy
> the buffer. That works, no crash.

You have to either do that, or ensure that the data is properly
aligned in the buffer. For example:

  char buf[200]; 
  receive_data(buf, sizeof(buf));
  double *p = reinterpret_cast<double*>(buf); // BAD
  double d = *p; // SIGBUS

do this instead:

  union { char buf[200]; double d; } u;
  receive_data(u.buf, sizeof(buf));
  double *p = reinterpret_cast<double*>(u.buf); // OK
  p = &u.d; // same thing
> It worked fine until I compiled in release mode with O3 optimisation
> setting. 

So gcc decided to pack things differently on the stack, and your
buffer shifted its alignment. What else is new?

> Is this a known bug 

Yes, in your program ...

> (as far as I'm aware it is  not undefined behaviour)? 

I am sure it *is*, but am too lazy to find the exact wording.

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