Re: "byte orders" question
Re: "byte orders" question
- Subject: Re: "byte orders" question
- From: "Clark S. Cox III" <email@hidden>
- Date: Tue, 13 Dec 2011 07:34:34 -0800
On Nov 26, 2011, at 4:23 PM, Kyle Sluder wrote:
> On Sat, Nov 26, 2011 at 4:17 PM, Greg Guerin <email@hidden> wrote:
>> Since you're just doing a memcpy(), you can simply cast the bits and avoid
>> the copying. Try this:
>>
>> float f = *((float*) &res);
>>
>> Or try defining a C union:
>>
>> union foo { float f; u_int32_t u; };
>> union foo bar;
>> bar.u = CFSwapInt32HostToBig(value);
>> float f = bar.f;
>
> Neither of these is legal.
Not true. The union solution is perfectly legal, and doesn't involve pointer aliasing.
> You are not allowed to alias a pointer to
> two different types (except pointer-to-char). See Section 6.5,
> paragraph 7 of the C99 standard:
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
See footnote 82:
"If the member used to access the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called "type punning"). This might be a trap representation."
--
Clark S. Cox III
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