Re: Question on strict aliasing in C.

[Georg-Johann Lay <avr at gjlay dot de>]

Andrew Haley wrote:
> On 04/07/11 17:57, Georg-Johann Lay wrote:
>> Hi,
>>
>> this it not a question on GCC, so I apologize for asking a question on
>> C strict aliasing rules here. As I know that some people reading this
>> list are much more familiar with C standard than I am, allow me to ask
>> that question, anyway.
>>
>> Suppose the following C code that tries to implement the standard
>> copysign function, i.e. copy the sign of y into x and return that
>> value. double be 64 bits wide:
>>
>>
>> #define MASK ((unsigned short) 0x8000)
>>
>> double copysign (double x, double y)
>> {
>>     unsigned short * const px = (unsigned short*)(char*)&x + 3;
>>     unsigned short * const py = (unsigned short*)(char*)&y + 3;
>>
>>     *px = *px & ~MASK | *py & MASK;
>>     return x;
>> }
>>
>>
>> While I say that this code is not correct because it breaks C's strict
>> aliasing rules (e.g C89/90, Chapter 6.3; C98/99, Chapter 6.5, Clause
>> 7), some other person very well familiar with the standard claims that
>> is correct and no problem.
>>
>> So I want to reassure me if the code is ok or not.
> 
> It's not.  Tey're wrong, you're right.
> 
> Hope this helps.  :-)
> 
> Andrew.

Yes, it does.  Thanks Andrew.