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Re: Alignment calculation or something like that

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On 2011-02-01, vijk <vijayx@gmail.com> wrote:
> /** alignment is a power of 2, it is passed in as a hard coded int **/
>
> 1. void*p1,**p2
>
> 2. int offset = alignment - 1 + sizeof(void*);
>
> 3. p2 = (void**)(((size_t)(p1) + offset) & ~(alignment - 1));
>
> 4. p2[-1] = p1;
>
>
> I am trying to understand what/how lines 2 and 3 are doing/calculated.

when alignment is a power of 2,  

  & ~(alignment - 1)
  
rounds down to the nearest multiple of alignment

it's hard to explain, run some numbers through it and 
see how it behaves. 

It strikes me as the sort of code that might come
from someone who would rather be programming in assembler.
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Re: Alignment calculation or something like that Jasen Betts <jasen@xnet.co.nz> - 2011-02-04 12:46 -0600

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