[Cython] a warning on using string/buffer objects for getting tmp memory

[Martin C. Martin]

Robert Bradshaw wrote:
> On Apr 12, 2008, at 4:09 PM, Lisandro Dalcin wrote:
> 
>> I've just realized that using a string or buffer object for automatic
>> management of memory as I proposed has a pitfall: memory alignement is
>> not guaranteed.
>>
>> So perhaps the only way to go is with this trick is to use a custom
>> python object internally calling malloc/free.
> 
> It looks like strings are aligned on int boundaries (given their  
> struct).

On 64 bit machines, gcc uses 32 bits for ints, so simply aligning on int 
boundaries wouldn't get you 64 bit aligned.

> What guarantee does one have about malloc?

It depends on the implementation.  For glibc:

http://www.gnu.org/software/libc/manual/html_node/Aligned-Memory-Blocks.html#Aligned-Memory-Blocks

"The address of a block returned by malloc or realloc in the GNU system 
is always a multiple of eight (or sixteen on 64-bit systems)."

You could always do what memalign() does: if e.g. you need something 
aligned on 8 byte boundaries, but Python's string allocation only uses 4 
byte boundaries, then allocate an extra 4 bytes, and if the result isn't 
8 byte aligned, return the address + 4.

> I would imagine  
> we would have a custom object (which would be very simple) and even  
> faster than a stringobject.

Best,
Martin