[ECOS] Re: Network programming for eCos under linux

["Grant Edwards" <grante at visi dot com>]

Jonathan Larmour writes:

> "Trenton D. Adams" wrote:
>> 
>>   >
>>   > > So, how would one go about making a buffer word aligned or DWORD
>>   > aligned
>>   > > just to be safe?
>>   >
>>   > something along the lines of:
>>   >
>>   > char buffer[1024];
>>   > #define ALIGNMENT 8
>>   > #define ALIGNUP(_x_) (((char *)(_x_) + ALIGN-1) & ~(ALIGN-1))
>>   > int *foo = ALIGNUP(buffer);
>>   >
>>   > You could also use __alignof(int) to give the alignment but that's
>>   > obviously a GNU C-ism.
>>   > 
>> 
>> First of all, I assume your ALIGN is actually supposed to be ALIGNMENT!
> 
> Oops yes.
>  
>> I don't understand your macro.  Let's say _x_ happens to be memory
>> address 0x201 the macro formula would go like this 
>> 
>> = (0x201 + 7) & ~(7)
>> = 0x208 & 0xfffffff8
>> = 0x208 
>> 
>> So, now foo is pointing to an integer that is not at the beginning of
>> the buffer.  This would mean that I couldn't put anything at the
>> beginning of the buffer, correct?
> 
> If 8 byte alignment was the requirement then you certainly wouldn't want
> to. I suppose in that case you would make the buffer be 1031 bytes to
> ensure it could fit 1024 bytes post alignment.

Yup -- but that only fixes the problem at the start of the buffer or
if you're using an array of homogeneously sized data objects. 

If you're filling the buffer with a collection of packed, hetrogenous
data objects, you'll run into alignment problems somewhere else
in the buffer. 

-- 
Grant Edwards
grante@visi.com